File: BerkeleyDB.pod.P

package info (click to toggle)
libberkeleydb-perl 0.55-1
  • links: PTS, VCS
  • area: main
  • in suites: stretch
  • size: 1,480 kB
  • ctags: 331
  • sloc: perl: 11,228; ansic: 6,451; makefile: 56
file content (2499 lines) | stat: -rwxr-xr-x 75,300 bytes parent folder | download | duplicates (2)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
=head1 NAME

BerkeleyDB - Perl extension for Berkeley DB version 2, 3, 4 or 5

=head1 SYNOPSIS

  use BerkeleyDB;

  $env = new BerkeleyDB::Env [OPTIONS] ;

  $db  = tie %hash, 'BerkeleyDB::Hash', [OPTIONS] ;
  $db  = new BerkeleyDB::Hash [OPTIONS] ;

  $db  = tie %hash, 'BerkeleyDB::Btree', [OPTIONS] ;
  $db  = new BerkeleyDB::Btree [OPTIONS] ;

  $db  = tie @array, 'BerkeleyDB::Recno', [OPTIONS] ;
  $db  = new BerkeleyDB::Recno [OPTIONS] ;

  $db  = tie @array, 'BerkeleyDB::Queue', [OPTIONS] ;
  $db  = new BerkeleyDB::Queue [OPTIONS] ;

  $db  = new BerkeleyDB::Heap [OPTIONS] ;

  $db  = new BerkeleyDB::Unknown [OPTIONS] ;

  $status = BerkeleyDB::db_remove [OPTIONS]
  $status = BerkeleyDB::db_rename [OPTIONS]
  $status = BerkeleyDB::db_verify [OPTIONS]

  $hash{$key} = $value ;
  $value = $hash{$key} ;
  each %hash ;
  keys %hash ;
  values %hash ;

  $env = $db->Env()
  $status = $db->db_get()
  $status = $db->db_exists() ;
  $status = $db->db_put() ;
  $status = $db->db_del() ;
  $status = $db->db_sync() ;
  $status = $db->db_close() ;
  $status = $db->db_pget()
  $hash_ref = $db->db_stat() ;
  $status = $db->db_key_range();
  $type = $db->type() ;
  $status = $db->status() ;
  $boolean = $db->byteswapped() ;
  $status = $db->truncate($count) ;
  $status = $db->compact($start, $stop, $c_data, $flags, $end);
  $status = $db->get_blob_threshold($t1) ;
  $status = $db->get_blob_dir($dir) ;

  $bool = $env->cds_enabled();
  $bool = $db->cds_enabled();
  $lock = $db->cds_lock();
  $lock->cds_unlock();
  
  ($flag, $old_offset, $old_length) = $db->partial_set($offset, $length) ;
  ($flag, $old_offset, $old_length) = $db->partial_clear() ;

  $cursor = $db->db_cursor([$flags]) ;
  $newcursor = $cursor->c_dup([$flags]);
  $status = $cursor->c_get() ;
  $status = $cursor->c_put() ;
  $status = $cursor->c_del() ;
  $status = $cursor->c_count() ;
  $status = $cursor->c_pget() ;
  $status = $cursor->status() ;
  $status = $cursor->c_close() ;
  $stream = $cursor->db_stream() ;

  $cursor = $db->db_join() ;
  $status = $cursor->c_get() ;
  $status = $cursor->c_close() ;

  $status = $stream->size($S);
  $status = $stream->read($data, $offset, $size);
  $status = $stream->write($data, $offset);

  $status = $env->txn_checkpoint()
  $hash_ref = $env->txn_stat()
  $status = $env->set_mutexlocks()
  $status = $env->set_flags()
  $status = $env->set_timeout()
  $status = $env->lock_detect()
  $status = $env->lsn_reset()
  $status = $env->get_blob_threshold($t1) ;
  $status = $env->get_blob_dir($dir) ;

  $txn = $env->txn_begin() ;
  $db->Txn($txn);
  $txn->Txn($db1, $db2,...);
  $status = $txn->txn_prepare()
  $status = $txn->txn_commit()
  $status = $txn->txn_abort()
  $status = $txn->txn_id()
  $status = $txn->txn_discard()
  $status = $txn->set_timeout()

  $status = $env->set_lg_dir();
  $status = $env->set_lg_bsize();
  $status = $env->set_lg_max();

  $status = $env->set_data_dir() ;
  $status = $env->set_tmp_dir() ;
  $status = $env->set_verbose() ;
  $db_env_ptr = $env->DB_ENV() ;

  $BerkeleyDB::Error
  $BerkeleyDB::db_version

  # DBM Filters
  $old_filter = $db->filter_store_key  ( sub { ... } ) ;
  $old_filter = $db->filter_store_value( sub { ... } ) ;
  $old_filter = $db->filter_fetch_key  ( sub { ... } ) ;
  $old_filter = $db->filter_fetch_value( sub { ... } ) ;

  # deprecated, but supported
  $txn_mgr = $env->TxnMgr();
  $status = $txn_mgr->txn_checkpoint()
  $hash_ref = $txn_mgr->txn_stat()
  $txn = $txn_mgr->txn_begin() ;

=head1 DESCRIPTION

B<NOTE: This document is still under construction. Expect it to be
incomplete in places.>

This Perl module provides an interface to most of the functionality
available in Berkeley DB versions 2, 3, 5 and 6. In general it is safe to assume
that the interface provided here to be identical to the Berkeley DB
interface. The main changes have been to make the Berkeley DB API work
in a Perl way. Note that if you are using Berkeley DB 2.x, the new
features available in Berkeley DB 3.x or later are not available via
this module.

The reader is expected to be familiar with the Berkeley DB
documentation. Where the interface provided here is identical to the
Berkeley DB library and the... TODO

The B<db_appinit>, B<db_cursor>, B<db_open> and B<db_txn> man pages are
particularly relevant.

The interface to Berkeley DB is implemented with a number of Perl
classes.

=head1 The BerkeleyDB::Env Class

The B<BerkeleyDB::Env> class provides an interface to the Berkeley DB
function B<db_appinit> in Berkeley DB 2.x or B<db_env_create> and
B<DBENV-E<gt>open> in Berkeley DB 3.x (or later). Its purpose is to initialise a
number of sub-systems that can then be used in a consistent way in all
the databases you make use of in the environment.

If you don't intend using transactions, locking or logging, then you
shouldn't need to make use of B<BerkeleyDB::Env>.

Note that an environment consists of a number of files that Berkeley DB
manages behind the scenes for you. When you first use an environment, it
needs to be explicitly created. This is done by including C<DB_CREATE>
with the C<Flags> parameter, described below.

=head2 Synopsis

    $env = new BerkeleyDB::Env
             [ -Home         => $path, ]
             [ -Server       => $name, ]
             [ -CacheSize    => $number, ]
             [ -Config       => { name => value, name => value }, ]
             [ -ErrFile      => filename, ]
             [ -MsgFile      => filename, ]
             [ -ErrPrefix    => "string", ]
             [ -Flags        => number, ]
             [ -SetFlags     => bitmask, ]
             [ -LockDetect   => number, ]
             [ -TxMax        => number, ]
             [ -LogConfig    => number, ]
             [ -MaxLockers   => number, ]
             [ -MaxLocks     => number, ]
             [ -MaxObjects   => number, ]
             [ -SharedMemKey => number, ]
             [ -Verbose      => boolean, ]
             [ -BlobThreshold=> $number, ]
             [ -BlobDir      => directory, ]
             [ -Encrypt      => { Password => "string",
	                          Flags    => number }, ]

All the parameters to the BerkeleyDB::Env constructor are optional.

=over 5

=item -Home

If present, this parameter should point to an existing directory. Any
files that I<aren't> specified with an absolute path in the sub-systems
that are initialised by the BerkeleyDB::Env class will be assumed to
live in the B<Home> directory.

For example, in the code fragment below the database "fred.db" will be
opened in the directory "/home/databases" because it was specified as a
relative path, but "joe.db" will be opened in "/other" because it was
part of an absolute path.

    $env = new BerkeleyDB::Env
             -Home         => "/home/databases"
    ...

    $db1 = new BerkeleyDB::Hash
	     -Filename => "fred.db",
	     -Env => $env
    ...

    $db2 = new BerkeleyDB::Hash
	     -Filename => "/other/joe.db",
	     -Env => $env
    ...

=item -Server

If present, this parameter should be the hostname of a server that is running
the Berkeley DB RPC server. All databases will be accessed via the RPC server.

=item -Encrypt

If present, this parameter will enable encryption of  all data before
it is written to the database. This parameters must be given a hash
reference. The format is shown below.

    -Encrypt => { -Password => "abc", Flags => DB_ENCRYPT_AES }

Valid values for the Flags are 0 or C<DB_ENCRYPT_AES>.

This option requires Berkeley DB 4.1 or better.

=item -Cachesize

If present, this parameter sets the size of the environments shared memory
buffer pool.

=item -TxMax

If present, this parameter sets the number of simultaneous
transactions that are allowed.  Default 100.  This default is
definitely too low for programs using the MVCC capabilities.

=item -LogConfig

If present, this parameter is used to configure log options.

=item -MaxLockers

If present, this parameter is used to configure the maximum number of
processes doing locking on the database.  Default 1000.

=item -MaxLocks

If present, this parameter is used to configure the maximum number of
locks on the database.  Default 1000.  This is often lower than required.

=item -MaxObjects

If present, this parameter is used to configure the maximum number of
locked objects.  Default 1000.  This is often lower than required.

=item -SharedMemKey

If present, this parameter sets the base segment ID for the shared memory
region used by Berkeley DB. 

This option requires Berkeley DB 3.1 or better.

Use C<$env-E<gt>get_shm_key($id)> to find out the base segment ID used
once the environment is open.

=item -ThreadCount

If present, this parameter declares the approximate number of threads that
will be used in the database environment. This parameter is only necessary
when the $env->failchk method will be used. It does not actually set the
maximum number of threads but rather is used to determine memory sizing.

This option requires Berkeley DB 4.4 or better. It is only supported on
Unix/Linux.

=item -BlobThreshold

Sets the size threshold that will be used to decide when data is stored as
a BLOB. This option must be set for a blobs to be used.

This option requires Berkeley DB 6.0 or better. 

=item -BlobDir      

The directory where the BLOB objects are stored.

If not specified blob files are stores in the environment directoy.


This option requires Berkeley DB 6.0 or better. 

=item -Config

This is a variation on the C<-Home> parameter, but it allows finer
control of where specific types of files will be stored.

The parameter expects a reference to a hash. Valid keys are:
B<DB_DATA_DIR>, B<DB_LOG_DIR> and B<DB_TMP_DIR>

The code below shows an example of how it can be used.

    $env = new BerkeleyDB::Env
             -Config => { DB_DATA_DIR => "/home/databases",
                          DB_LOG_DIR  => "/home/logs",
                          DB_TMP_DIR  => "/home/tmp"
                        }
    ...

=item -ErrFile

Expects a filename or filehandle. Any errors generated internally by
Berkeley DB will be logged to this file. A useful debug setting is to
open environments with either

    -ErrFile => *STDOUT

or 

    -ErrFile => *STDERR

=item -ErrPrefix

Allows a prefix to be added to the error messages before they are sent
to B<-ErrFile>.

=item -Flags

The B<Flags> parameter specifies both which sub-systems to initialise,
as well as a number of environment-wide options.
See the Berkeley DB documentation for more details of these options.

Any of the following can be specified by OR'ing them:

B<DB_CREATE>

If any of the files specified do not already exist, create them.

B<DB_INIT_CDB>

Initialise the Concurrent Access Methods  

B<DB_INIT_LOCK>

Initialise the Locking sub-system.

B<DB_INIT_LOG>

Initialise the Logging sub-system.

B<DB_INIT_MPOOL>

Initialize the shared memory buffer pool subsystem. This subsystem should be used whenever an application is using any Berkeley DB access method.

B<DB_INIT_TXN>

Initialize the transaction subsystem. This subsystem should be used when recovery and atomicity of multiple operations are important. The DB_INIT_TXN flag implies the DB_INIT_LOG flag.


B<DB_MPOOL_PRIVATE>

Create a private memory pool; see memp_open. Ignored unless DB_INIT_MPOOL is also specified.


B<DB_INIT_MPOOL> is also specified.


B<DB_NOMMAP>

Do not map this database into process memory.


B<DB_RECOVER>

Run normal recovery on this environment before opening it for normal use. If this flag is set, the DB_CREATE flag must also be set since the regions will be removed and recreated.

The db_appinit function returns successfully if DB_RECOVER is specified and no log files exist, so it is necessary to ensure all necessary log files are present before running recovery.


B<DB_PRIVATE>

B<DB_RECOVER_FATAL>

Run catastrophic recovery on this environment before opening it for normal use. If this flag is set, the DB_CREATE flag must also be set since the regions will be removed and recreated.

The db_appinit function returns successfully if DB_RECOVER_FATAL is specified and no log files exist, so it is necessary to ensure all necessary log files are present before running recovery.

B<DB_THREAD>

Ensure that handles returned by the Berkeley DB subsystems are useable by multiple threads within a single process, i.e., that the system is free-threaded.

B<DB_TXN_NOSYNC>

On transaction commit, do not synchronously flush the log; see txn_open. Ignored unless DB_INIT_TXN is also specified.

B<DB_USE_ENVIRON>

The Berkeley DB process' environment may be permitted to specify information to be used when naming files; see Berkeley DB File Naming. As permitting users to specify which files are used can create security problems, environment information will be used in file naming for all users only if the DB_USE_ENVIRON flag is set.

B<DB_USE_ENVIRON_ROOT>

The Berkeley DB process' environment may be permitted to specify information to be used when naming files; see Berkeley DB File Naming. As permitting users to specify which files are used can create security problems, if the DB_USE_ENVIRON_ROOT flag is set, environment information will be used for file naming only for users with a user-ID matching that of the superuser (specifically, users for whom the getuid(2) system call returns the user-ID 0).

=item -SetFlags

Calls ENV->set_flags with the supplied bitmask. Use this when you need to make
use of DB_ENV->set_flags before DB_ENV->open is called.

Only valid when Berkeley DB 3.x or better is used.

=item -LockDetect

Specifies what to do when a lock conflict occurs. The value should be one of

B<DB_LOCK_DEFAULT> 

Use the default policy as specified by db_deadlock.

B<DB_LOCK_OLDEST>

Abort the oldest transaction.

B<DB_LOCK_RANDOM>

Abort a random transaction involved in the deadlock.

B<DB_LOCK_YOUNGEST>

Abort the youngest transaction.


=item -Verbose

Add extra debugging information to the messages sent to B<-ErrFile>.

=back

=head2 Methods

The environment class has the following methods:

=over 5

=item $env->errPrefix("string") ;

This method is identical to the B<-ErrPrefix> flag. It allows the
error prefix string to be changed dynamically.

=item $env->set_flags(bitmask, 1|0);

=item $txn = $env->TxnMgr()

Constructor for creating a B<TxnMgr> object.
See L<"TRANSACTIONS"> for more details of using transactions.

This method is deprecated. Access the transaction methods using the B<txn_>
methods below from the environment object directly.

=item $env->txn_begin()

TODO

=item $env->txn_stat()

TODO

=item $env->txn_checkpoint()

TODO

=item $env->status()

Returns the status of the last BerkeleyDB::Env method.


=item $env->DB_ENV()

Returns a pointer to the underlying DB_ENV data structure that Berkeley
DB uses.

=item $env->get_shm_key($id)

Writes the base segment ID for the shared memory region used by the
Berkeley DB environment into C<$id>. Returns 0 on success.

This option requires Berkeley DB 4.2 or better.

Use the C<-SharedMemKey> option when opening the environmet to set the
base segment ID.

=item $env->set_isalive()

Set the callback that determines if the thread of control, identified by
the pid and tid arguments, is still running.  This method should only be
used in combination with $env->failchk.

This option requires Berkeley DB 4.4 or better.

=item $env->failchk($flags)

The $env->failchk method checks for threads of control (either a true
thread or a process) that have exited while manipulating Berkeley DB
library data structures, while holding a logical database lock, or with an
unresolved transaction (that is, a transaction that was never aborted or
committed).

If $env->failchk determines a thread of control exited while holding
database read locks, it will release those locks. If $env->failchk
determines a thread of control exited with an unresolved transaction, the
transaction will be aborted.

Applications calling the $env->failchk method must have already called the
$env->set_isalive method, on the same DB environment, and must have
configured their database environment using the -ThreadCount flag. The
ThreadCount flag cannot be used on an environment that wasn't previously
initialized with it.

This option requires Berkeley DB 4.4 or better.

=item $env->stat_print

Prints statistical information. 

If the C<MsgFile> option is specified the output will be sent to the
file. Otherwise output is sent to standard output.

This option requires Berkeley DB 4.3 or better.

=item $env->lock_stat_print

Prints locking subsystem statistics.

If the C<MsgFile> option is specified the output will be sent to the
file. Otherwise output is sent to standard output.

This option requires Berkeley DB 4.3 or better.

=item $env->mutex_stat_print

Prints mutex subsystem statistics.

If the C<MsgFile> option is specified the output will be sent to the
file. Otherwise output is sent to standard output.

This option requires Berkeley DB 4.4 or better.

=item $status = $env->get_blob_threshold($t1) ;

Sets the parameter $t1 to the threshold value (in bytes) that is used to
determine when a data item is stored as a Blob.

=item $status = $env->get_blob_dir($dir) ;

Sets the $dir parameter to the directory where blob files are stored.

=item $env->set_timeout($timeout, $flags)

=item $env->status()

Returns the status of the last BerkeleyDB::Env method.

=back

=head2 Examples

TODO.

=head1 Global Classes

  $status = BerkeleyDB::db_remove [OPTIONS]
  $status = BerkeleyDB::db_rename [OPTIONS]
  $status = BerkeleyDB::db_verify [OPTIONS]

=head1 THE DATABASE CLASSES

B<BerkeleyDB> supports the following database formats:

=over 5

=item B<BerkeleyDB::Hash>

This database type allows arbitrary key/value pairs to be stored in data
files. This is equivalent to the functionality provided by other
hashing packages like DBM, NDBM, ODBM, GDBM, and SDBM. Remember though,
the files created using B<BerkeleyDB::Hash> are not compatible with any
of the other packages mentioned.

A default hashing algorithm, which will be adequate for most applications,
is built into BerkeleyDB. If you do need to use your own hashing algorithm
it is possible to write your own in Perl and have B<BerkeleyDB> use
it instead.

=item B<BerkeleyDB::Btree>

The Btree format allows arbitrary key/value pairs to be stored in a
B+tree.

As with the B<BerkeleyDB::Hash> format, it is possible to provide a
user defined Perl routine to perform the comparison of keys. By default,
though, the keys are stored in lexical order.

=item B<BerkeleyDB::Recno>

TODO.


=item B<BerkeleyDB::Queue>

TODO.

=item B<BerkeleyDB::Heap>

TODO.

=item B<BerkeleyDB::Unknown>

This isn't a database format at all. It is used when you want to open an
existing Berkeley DB database without having to know what type is it. 

=back


Each of the database formats described above is accessed via a
corresponding B<BerkeleyDB> class. These will be described in turn in
the next sections.

=head1 BerkeleyDB::Hash

Equivalent to calling B<db_open> with type B<DB_HASH> in Berkeley DB 2.x and
calling B<db_create> followed by B<DB-E<gt>open> with type B<DB_HASH> in
Berkeley DB 3.x or greater. 

Two forms of constructor are supported:

    $db = new BerkeleyDB::Hash
                [ -Filename      => "filename", ]
                [ -Subname       => "sub-database name", ]
                [ -Flags         => flags,]
                [ -Property      => flags,]
                [ -Mode          => number,]
                [ -Cachesize     => number,]
                [ -Lorder        => number,]
                [ -Pagesize      => number,]
                [ -Env           => $env,]
                [ -Txn           => $txn,]
                [ -Encrypt       => { Password => "string",
	                              Flags    => number }, ],
                [ -BlobThreshold=> $number, ]
                [ -BlobDir      => directory, ]
                # BerkeleyDB::Hash specific
                [ -Ffactor       => number,]
                [ -Nelem         => number,]
                [ -Hash          => code reference,]
                [ -DupCompare    => code reference,]

and this

    [$db =] tie %hash, 'BerkeleyDB::Hash', 
                [ -Filename      => "filename", ]
                [ -Subname       => "sub-database name", ]
                [ -Flags         => flags,]
                [ -Property      => flags,]
                [ -Mode          => number,]
                [ -Cachesize     => number,]
                [ -Lorder        => number,]
                [ -Pagesize      => number,]
                [ -Env           => $env,]
                [ -Txn           => $txn,]
                [ -Encrypt       => { Password => "string",
	                              Flags    => number }, ],
                [ -BlobThreshold=> $number, ]
                [ -BlobDir      => directory, ]
                # BerkeleyDB::Hash specific
                [ -Ffactor       => number,]
                [ -Nelem         => number,]
                [ -Hash          => code reference,]
                [ -DupCompare    => code reference,]


When the "tie" interface is used, reading from and writing to the database
is achieved via the tied hash. In this case the database operates like
a Perl associative array that happens to be stored on disk.

In addition to the high-level tied hash interface, it is possible to
make use of the underlying methods provided by Berkeley DB

=head2 Options

In addition to the standard set of options (see L<COMMON OPTIONS>)
B<BerkeleyDB::Hash> supports these options:

=over 5

=item -Property

Used to specify extra flags when opening a database. The following
flags may be specified by bitwise OR'ing together one or more of the
following values:

B<DB_DUP>

When creating a new database, this flag enables the storing of duplicate
keys in the database. If B<DB_DUPSORT> is not specified as well, the
duplicates are stored in the order they are created in the database.

B<DB_DUPSORT>

Enables the sorting of duplicate keys in the database. Ignored if
B<DB_DUP> isn't also specified.

=item -Ffactor

=item -Nelem

See the Berkeley DB documentation for details of these options.

=item -Hash

Allows you to provide a user defined hash function. If not specified, 
a default hash function is used. Here is a template for a user-defined
hash function

    sub hash
    {
        my ($data) = shift ;
        ...
        # return the hash value for $data
	return $hash ;
    }

    tie %h, "BerkeleyDB::Hash", 
        -Filename => $filename, 
    	-Hash     => \&hash,
	...

See L<""> for an example.

=item -DupCompare

Used in conjunction with the B<DB_DUPOSRT> flag. 

    sub compare
    {
	my ($key, $key2) = @_ ;
        ...
        # return  0 if $key1 eq $key2
        #        -1 if $key1 lt $key2
        #         1 if $key1 gt $key2
        return (-1 , 0 or 1) ;
    }

    tie %h, "BerkeleyDB::Hash", 
        -Filename   => $filename, 
	-Property   => DB_DUP|DB_DUPSORT,
    	-DupCompare => \&compare,
	...

=back


=head2 Methods

B<BerkeleyDB::Hash> only supports the standard database methods.
See L<COMMON DATABASE METHODS>.

=head2 A Simple Tied Hash Example

## simpleHash

here is the output:

    Banana Exists
    
    orange -> orange
    tomato -> red
    banana -> yellow

Note that the like ordinary associative arrays, the order of the keys
retrieved from a Hash database are in an apparently random order.

=head2 Another Simple Hash Example

Do the same as the previous example but not using tie.

## simpleHash2

=head2 Duplicate keys

The code below is a variation on the examples above. This time the hash has
been inverted. The key this time is colour and the value is the fruit name.
The B<DB_DUP> flag has been specified to allow duplicates.

##dupHash

here is the output:

    orange -> orange
    yellow -> banana
    red -> apple
    red -> tomato
    green -> banana
    green -> apple

=head2 Sorting Duplicate Keys

In the previous example, when there were duplicate keys, the values are
sorted in the order they are stored in. The code below is
identical to the previous example except the B<DB_DUPSORT> flag is
specified.

##dupSortHash

Notice that in the output below the duplicate values are sorted.

    orange -> orange
    yellow -> banana
    red -> apple
    red -> tomato
    green -> apple
    green -> banana

=head2 Custom Sorting Duplicate Keys

Another variation 

TODO

=head2 Changing the hash

TODO

=head2 Using db_stat

TODO

=head1 BerkeleyDB::Btree

Equivalent to calling B<db_open> with type B<DB_BTREE> in Berkeley DB 2.x and
calling B<db_create> followed by B<DB-E<gt>open> with type B<DB_BTREE> in
Berkeley DB 3.x or greater. 

Two forms of constructor are supported:


    $db = new BerkeleyDB::Btree
                [ -Filename      => "filename", ]
                [ -Subname       => "sub-database name", ]
                [ -Flags         => flags,]
                [ -Property      => flags,]
                [ -Mode          => number,]
                [ -Cachesize     => number,]
                [ -Lorder        => number,]
                [ -Pagesize      => number,]
                [ -Env           => $env,]
                [ -Txn           => $txn,]
                [ -Encrypt       => { Password => "string",
	                              Flags    => number }, ],
                [ -BlobThreshold=> $number, ]
                [ -BlobDir      => directory, ]
                # BerkeleyDB::Btree specific
                [ -Minkey        => number,]
                [ -Compare       => code reference,]
                [ -DupCompare    => code reference,]
                [ -Prefix        => code reference,]

and this

    [$db =] tie %hash, 'BerkeleyDB::Btree', 
                [ -Filename      => "filename", ]
                [ -Subname       => "sub-database name", ]
                [ -Flags         => flags,]
                [ -Property      => flags,]
                [ -Mode          => number,]
                [ -Cachesize     => number,]
                [ -Lorder        => number,]
                [ -Pagesize      => number,]
                [ -Env           => $env,]
                [ -Txn           => $txn,]
                [ -Encrypt       => { Password => "string",
	                              Flags    => number }, ],
                [ -BlobThreshold=> $number, ]
                [ -BlobDir      => directory, ]
                # BerkeleyDB::Btree specific
                [ -Minkey        => number,]
                [ -Compare       => code reference,]
                [ -DupCompare    => code reference,]
                [ -Prefix        => code reference,]

=head2 Options

In addition to the standard set of options (see L<COMMON OPTIONS>)
B<BerkeleyDB::Btree> supports these options:

=over 5

=item -Property

Used to specify extra flags when opening a database. The following
flags may be specified by bitwise OR'ing together one or more of the
following values:

B<DB_DUP>

When creating a new database, this flag enables the storing of duplicate
keys in the database. If B<DB_DUPSORT> is not specified as well, the
duplicates are stored in the order they are created in the database.

B<DB_DUPSORT>

Enables the sorting of duplicate keys in the database. Ignored if
B<DB_DUP> isn't also specified.

=item Minkey

TODO

=item Compare

Allow you to override the default sort order used in the database. See
L<"Changing the sort order"> for an example.

    sub compare
    {
	my ($key, $key2) = @_ ;
        ...
        # return  0 if $key1 eq $key2
        #        -1 if $key1 lt $key2
        #         1 if $key1 gt $key2
        return (-1 , 0 or 1) ;
    }

    tie %h, "BerkeleyDB::Hash", 
        -Filename   => $filename, 
    	-Compare    => \&compare,
	...

=item Prefix

    sub prefix
    {
	my ($key, $key2) = @_ ;
        ...
        # return number of bytes of $key2 which are 
        # necessary to determine that it is greater than $key1
        return $bytes ;
    }

    tie %h, "BerkeleyDB::Hash", 
        -Filename   => $filename, 
    	-Prefix     => \&prefix,
	...
=item DupCompare

    sub compare
    {
	my ($key, $key2) = @_ ;
        ...
        # return  0 if $key1 eq $key2
        #        -1 if $key1 lt $key2
        #         1 if $key1 gt $key2
        return (-1 , 0 or 1) ;
    }

    tie %h, "BerkeleyDB::Hash", 
        -Filename   => $filename, 
    	-DupCompare => \&compare,
	...

=item set_bt_compress

Enabled compression of the btree data. The callback interface is not
supported at present. Need Berkeley DB 4.8 or better.

=back

=head2 Methods

B<BerkeleyDB::Btree> supports the following database methods.
See also L<COMMON DATABASE METHODS>.

All the methods below return 0 to indicate success.

=over 5

=item $status = $db->db_key_range($key, $less, $equal, $greater [, $flags])

Given a key, C<$key>, this method returns the proportion of keys less than 
C<$key> in C<$less>, the proportion equal to C<$key> in C<$equal> and the
proportion greater than C<$key> in C<$greater>.

The proportion is returned as a double in the range 0.0 to 1.0.

=back

=head2 A Simple Btree Example

The code below is a simple example of using a btree database.

## btreeSimple

Here is the output from the code above. The keys have been sorted using
Berkeley DB's default sorting algorithm.

    Smith
    Wall
    mouse


=head2 Changing the sort order

It is possible to supply your own sorting algorithm if the one that Berkeley
DB used isn't suitable. The code below is identical to the previous example
except for the case insensitive compare function.

## btreeSortOrder

Here is the output from the code above.

    mouse
    Smith
    Wall

There are a few point to bear in mind if you want to change the
ordering in a BTREE database:

=over 5

=item 1.

The new compare function must be specified when you create the database.

=item 2.

You cannot change the ordering once the database has been created. Thus
you must use the same compare function every time you access the
database.

=back 

=head2 Using db_stat

TODO

=head1 BerkeleyDB::Recno

Equivalent to calling B<db_open> with type B<DB_RECNO> in Berkeley DB 2.x and
calling B<db_create> followed by B<DB-E<gt>open> with type B<DB_RECNO> in
Berkeley DB 3.x or greater. 

Two forms of constructor are supported:

    $db = new BerkeleyDB::Recno
                [ -Filename      => "filename", ]
                [ -Subname       => "sub-database name", ]
                [ -Flags         => flags,]
                [ -Property      => flags,]
                [ -Mode          => number,]
                [ -Cachesize     => number,]
                [ -Lorder        => number,]
                [ -Pagesize      => number,]
                [ -Env           => $env,]
                [ -Txn           => $txn,]
                [ -Encrypt       => { Password => "string",
	                              Flags    => number }, ],
                # BerkeleyDB::Recno specific
                [ -Delim           => byte,]
                [ -Len             => number,]
                [ -Pad             => byte,]
                [ -Source          => filename,]

and this

    [$db =] tie @arry, 'BerkeleyDB::Recno', 
                [ -Filename      => "filename", ]
                [ -Subname       => "sub-database name", ]
                [ -Flags         => flags,]
                [ -Property      => flags,]
                [ -Mode          => number,]
                [ -Cachesize     => number,]
                [ -Lorder        => number,]
                [ -Pagesize      => number,]
                [ -Env           => $env,]
                [ -Txn           => $txn,]
                [ -Encrypt       => { Password => "string",
	                              Flags    => number }, ],
                # BerkeleyDB::Recno specific
                [ -Delim           => byte,]
                [ -Len             => number,]
                [ -Pad             => byte,]
                [ -Source          => filename,]

=head2 A Recno Example

Here is a simple example that uses RECNO (if you are using a version 
of Perl earlier than 5.004_57 this example won't work -- see 
L<Extra RECNO Methods> for a workaround).

## simpleRecno

Here is the output from the script:

    The array contains 5 entries
    popped black
    shifted white
    Element 1 Exists with value blue
    The last element is green
    The 2nd last element is yellow

=head1 BerkeleyDB::Queue

Equivalent to calling B<db_create> followed by B<DB-E<gt>open> with
type B<DB_QUEUE> in Berkeley DB 3.x or greater. This database format
isn't available if you use Berkeley DB 2.x.

Two forms of constructor are supported:

    $db = new BerkeleyDB::Queue
                [ -Filename      => "filename", ]
                [ -Subname       => "sub-database name", ]
                [ -Flags         => flags,]
                [ -Property      => flags,]
                [ -Mode          => number,]
                [ -Cachesize     => number,]
                [ -Lorder        => number,]
                [ -Pagesize      => number,]
                [ -Env           => $env,]
                [ -Txn           => $txn,]
                [ -Encrypt       => { Password => "string",
	                              Flags    => number }, ],
                # BerkeleyDB::Queue specific
                [ -Len             => number,]
                [ -Pad             => byte,]
                [ -ExtentSize    => number, ]

and this

    [$db =] tie @arry, 'BerkeleyDB::Queue', 
                [ -Filename      => "filename", ]
                [ -Subname       => "sub-database name", ]
                [ -Flags         => flags,]
                [ -Property      => flags,]
                [ -Mode          => number,]
                [ -Cachesize     => number,]
                [ -Lorder        => number,]
                [ -Pagesize      => number,]
                [ -Env           => $env,]
                [ -Txn           => $txn,]
                [ -Encrypt       => { Password => "string",
	                              Flags    => number }, ],
                # BerkeleyDB::Queue specific
                [ -Len             => number,]
                [ -Pad             => byte,]


=head1 BerkeleyDB::Heap

Equivalent to calling B<db_create> followed by B<DB-E<gt>open> with
type B<DB_HEAP> in Berkeley DB 5.2.x or greater. This database format
isn't available if you use an older version of Berkeley DB.

One form of constructor is supported:

    $db = new BerkeleyDB::Heap
                [ -Filename      => "filename", ]
                [ -Subname       => "sub-database name", ]
                [ -Flags         => flags,]
                [ -Property      => flags,]
                [ -Mode          => number,]
                [ -Cachesize     => number,]
                [ -Lorder        => number,]
                [ -Pagesize      => number,]
                [ -Env           => $env,]
                [ -Txn           => $txn,]
                [ -Encrypt       => { Password => "string",
	                              Flags    => number }, ],
                [ -BlobThreshold=> $number, ]
                [ -BlobDir      => directory, ]
                # BerkeleyDB::Heap specific
                [ -HeapSize      => number, ]
                [ -HeapSizeGb    => number, ]

=head1 BerkeleyDB::Unknown

This class is used to open an existing database. 

Equivalent to calling B<db_open> with type B<DB_UNKNOWN> in Berkeley DB 2.x and
calling B<db_create> followed by B<DB-E<gt>open> with type B<DB_UNKNOWN> in
Berkeley DB 3.x or greater. 

The constructor looks like this:

    $db = new BerkeleyDB::Unknown
                [ -Filename      => "filename", ]
                [ -Subname       => "sub-database name", ]
                [ -Flags         => flags,]
                [ -Property      => flags,]
                [ -Mode          => number,]
                [ -Cachesize     => number,]
                [ -Lorder        => number,]
                [ -Pagesize      => number,]
                [ -Env           => $env,]
                [ -Txn           => $txn,]
                [ -Encrypt       => { Password => "string",
	                              Flags    => number }, ],


=head2 An example 

=head1 COMMON OPTIONS

All database access class constructors support the common set of
options defined below. All are optional.

=over 5

=item -Filename

The database filename. If no filename is specified, a temporary file will
be created and removed once the program terminates.

=item -Subname

Specifies the name of the sub-database to open.
This option is only valid if you are using Berkeley DB 3.x or greater.

=item -Flags

Specify how the database will be opened/created. The valid flags are:

B<DB_CREATE>

Create any underlying files, as necessary. If the files do not already
exist and the B<DB_CREATE> flag is not specified, the call will fail.

B<DB_NOMMAP>

Not supported by BerkeleyDB.

B<DB_RDONLY>

Opens the database in read-only mode.

B<DB_THREAD>

Not supported by BerkeleyDB.

B<DB_TRUNCATE>

If the database file already exists, remove all the data before
opening it.

=item -Mode

Determines the file protection when the database is created. Defaults
to 0666.

=item -Cachesize

=item -Lorder

=item -Pagesize

=item -Env

When working under a Berkeley DB environment, this parameter

Defaults to no environment.

=item -Encrypt

If present, this parameter will enable encryption of  all data before
it is written to the database. This parameters must be given a hash
reference. The format is shown below.

    -Encrypt => { -Password => "abc", Flags => DB_ENCRYPT_AES }

Valid values for the Flags are 0 or C<DB_ENCRYPT_AES>.

This option requires Berkeley DB 4.1 or better.

=item -Txn

TODO.

=back

=head1 COMMON DATABASE METHODS

All the database interfaces support the common set of methods defined
below.

All the methods below return 0 to indicate success.

=head2 $env = $db->Env();

Returns the environment object the database is associated with or C<undef>
when no environment was used when opening the database.

=head2 $status = $db->db_get($key, $value [, $flags])

Given a key (C<$key>) this method reads the value associated with it
from the database. If it exists, the value read from the database is
returned in the C<$value> parameter.

The B<$flags> parameter is optional. If present, it must be set to B<one>
of the following values:

=over 5

=item B<DB_GET_BOTH>

When the B<DB_GET_BOTH> flag is specified, B<db_get> checks for the
existence of B<both> the C<$key> B<and> C<$value> in the database.

=item B<DB_SET_RECNO>

TODO.

=back

In addition, the following value may be set by bitwise OR'ing it into
the B<$flags> parameter:

=over 5

=item B<DB_RMW>

TODO

=back

The variant C<db_pget> allows you to query a secondary database:

	$status = $sdb->db_pget($skey, $pkey, $value);

using the key C<$skey> in the secondary db to lookup C<$pkey> and C<$value>
from the primary db.

=head2 $status = $db->db_exists($key [, $flags])

This method checks for the existence of the given key (C<$key>), but
does not read the value. If the key is not found, B<db_exists> will
return B<DB_NOTFOUND>. Requires BDB 4.6 or better.

=head2 $status = $db->db_put($key, $value [, $flags])

Stores a key/value pair in the database.

The B<$flags> parameter is optional. If present it must be set to B<one>
of the following values:

=over 5

=item B<DB_APPEND>

This flag is only applicable when accessing a B<BerkeleyDB::Recno>
database.

TODO.


=item B<DB_NOOVERWRITE>

If this flag is specified and C<$key> already exists in the database,
the call to B<db_put> will return B<DB_KEYEXIST>.

=back

=head2 $status = $db->db_del($key [, $flags])

Deletes a key/value pair in the database associated with C<$key>.
If duplicate keys are enabled in the database, B<db_del> will delete
B<all> key/value pairs with key C<$key>.

The B<$flags> parameter is optional and is currently unused.

=head2 $status = $env->stat_print([$flags])

Prints statistical information. 

If the C<MsgFile> option is specified the output will be sent to the
file. Otherwise output is sent to standard output.

This option requires Berkeley DB 4.3 or better.

=head2 $status = $db->db_sync()

If any parts of the database are in memory, write them to the database.

=head2 $cursor = $db->db_cursor([$flags])

Creates a cursor object. This is used to access the contents of the
database sequentially. See L<CURSORS> for details of the methods
available when working with cursors.

The B<$flags> parameter is optional. If present it must be set to B<one>
of the following values:

=over 5

=item B<DB_RMW>

TODO.

=back

=head2 ($flag, $old_offset, $old_length) = $db->partial_set($offset, $length) ;

TODO

=head2 ($flag, $old_offset, $old_length) = $db->partial_clear() ;

TODO

=head2 $db->byteswapped()

TODO

=head2 $status = $db->get_blob_threshold($t1) ;

Sets the parameter $t1 to the threshold value (in bytes) that is used to
determine when a data item is stored as a Blob.

=head2 $status = $db->get_blob_dir($dir) ;

Sets the $dir parameter to the directory where blob files are stored.

=head2 $db->type()

Returns the type of the database. The possible return code are B<DB_HASH>
for a B<BerkeleyDB::Hash> database, B<DB_BTREE> for a B<BerkeleyDB::Btree>
database and B<DB_RECNO> for a B<BerkeleyDB::Recno> database. This method
is typically used when a database has been opened with
B<BerkeleyDB::Unknown>.

=head2   $bool = $env->cds_enabled();

Returns true if the Berkeley DB environment C<$env> has been opened on
CDS mode.

=head2   $bool = $db->cds_enabled();

Returns true if the database C<$db> has been opened on CDS mode.

=head2 $lock = $db->cds_lock();

Creates a CDS write lock object C<$lock>.

It is a fatal error to attempt to create a cds_lock if the Berkeley DB
environment has not been opened in CDS mode.

=head2 $lock->cds_unlock();

Removes a CDS lock. The destruction of the CDS lock object automatically
calls this method.

Note that if multiple CDS lock objects are created, the underlying write
lock will not be released until all CDS lock objects are either explicitly
unlocked with this method, or the CDS lock objects have been destroyed.

=head2 $ref = $db->db_stat()

Returns a reference to an associative array containing information about
the database. The keys of the associative array correspond directly to the
names of the fields defined in the Berkeley DB documentation. For example,
in the DB documentation, the field B<bt_version> stores the version of the
Btree database. Assuming you called B<db_stat> on a Btree database the
equivalent field would be accessed as follows:

    $version = $ref->{'bt_version'} ;

If you are using Berkeley DB 3.x or better, this method will work will
all database formats. When DB 2.x is used, it only works with
B<BerkeleyDB::Btree>.

=head2 $status = $db->status()

Returns the status of the last C<$db> method called.

=head2 $status = $db->truncate($count)

Truncates the database and returns the number or records deleted
in C<$count>.

=head2  $status = $db->compact($start, $stop, $c_data, $flags, $end);

Compacts the database C<$db>. 

All the parameters are optional - if only want to make use of some of them,
use C<undef> for those you don't want.  Trailing unused parameters can be
omitted. For example, if you only want to use the C<$c_data> parameter to
set the C<compact_fillpercent>, write you code like this

    my %hash;
    $hash{compact_fillpercent} = 50;
    $db->compact(undef, undef, \%hash);

The parameters operate identically to the C equivalent of this method.
The C<$c_data> needs a bit of explanation - it must be a hash reference.
The values of the following keys can be set before calling C<compact> and
will affect the operation of the compaction.

=over 5

=item * compact_fillpercent

=item * compact_timeout
    
=back

The following keys, along with associated values, will be created in the
hash reference if the C<compact> operation was successful.

=over 5

=item * compact_deadlock

=item * compact_levels

=item * compact_pages_free

=item * compact_pages_examine

=item * compact_pages_truncated

=back

You need to be running Berkeley DB 4.4 or better if you want to make use of
C<compact>.

=head2  $status = $db->associate($secondary, \&key_callback)

Associate C<$db> with the secondary DB C<$secondary>

New key/value pairs inserted to the database will be passed to the callback
which must set its third argument to the secondary key to allow lookup. If
an array reference is set multiple keys secondary keys will be associated
with the primary database entry.

Data may be retrieved fro the secondary database using C<db_pget> to also
obtain the primary key.

Secondary databased are maintained automatically.

=head2  $status = $db->associate_foreign($secondary, callback, $flags)

Associate a foreign key database C<$db> with the secondary DB
C<$secondary>.

The second parameter must be a reference to a sub or C<undef>. 

The C<$flags> parameter must be either C<DB_FOREIGN_CASCADE>, 
C<DB_FOREIGN_ABORT> or C<DB_FOREIGN_NULLIFY>. 

When the flags parameter is C<DB_FOREIGN_NULLIFY> the second parameter is a
reference to a sub of the form

    sub foreign_cb
    {
        my $key = \$_[0];
        my $value = \$_[1];
        my $foreignkey = \$_[2];
        my $changed = \$_[3] ;

        # for ... set $$value and set $$changed to 1

        return 0;
    }

    $foreign_db->associate_foreign($secondary, \&foreign_cb, DB_FOREIGN_NULLIFY);

=head1 CURSORS

A cursor is used whenever you want to access the contents of a database
in sequential order.
A cursor object is created with the C<db_cursor>

A cursor object has the following methods available:

=head2 $newcursor = $cursor->c_dup($flags)

Creates a duplicate of C<$cursor>. This method needs Berkeley DB 3.0.x or better.

The C<$flags> parameter is optional and can take the following value:

=over 5

=item DB_POSITION

When present this flag will position the new cursor at the same place as the
existing cursor.

=back

=head2 $status = $cursor->c_get($key, $value, $flags)

Reads a key/value pair from the database, returning the data in C<$key>
and C<$value>. The key/value pair actually read is controlled by the
C<$flags> parameter, which can take B<one> of the following values:

=over 5

=item B<DB_FIRST>

Set the cursor to point to the first key/value pair in the
database. Return the key/value pair in C<$key> and C<$value>.

=item B<DB_LAST>

Set the cursor to point to the last key/value pair in the database. Return
the key/value pair in C<$key> and C<$value>.

=item B<DB_NEXT>

If the cursor is already pointing to a key/value pair, it will be
incremented to point to the next key/value pair and return its contents.

If the cursor isn't initialised, B<DB_NEXT> works just like B<DB_FIRST>.

If the cursor is already positioned at the last key/value pair, B<c_get>
will return B<DB_NOTFOUND>.

=item B<DB_NEXT_DUP>

This flag is only valid when duplicate keys have been enabled in
a database.
If the cursor is already pointing to a key/value pair and the key of
the next key/value pair is identical, the cursor will be incremented to
point to it and their contents returned.

=item B<DB_PREV>

If the cursor is already pointing to a key/value pair, it will be
decremented to point to the previous key/value pair and return its
contents.

If the cursor isn't initialised, B<DB_PREV> works just like B<DB_LAST>.

If the cursor is already positioned at the first key/value pair, B<c_get>
will return B<DB_NOTFOUND>.

=item B<DB_CURRENT>

If the cursor has been set to point to a key/value pair, return their
contents.
If the key/value pair referenced by the cursor has been deleted, B<c_get>
will return B<DB_KEYEMPTY>.

=item B<DB_SET>

Set the cursor to point to the key/value pair referenced by B<$key>
and return the value in B<$value>.

=item B<DB_SET_RANGE>

This flag is a variation on the B<DB_SET> flag. As well as returning
the value, it also returns the key, via B<$key>.
When used with a B<BerkeleyDB::Btree> database the key matched by B<c_get>
will be the shortest key (in length) which is greater than or equal to
the key supplied, via B<$key>. This allows partial key searches.
See ??? for an example of how to use this flag.

=item B<DB_GET_BOTH>

Another variation on B<DB_SET>. This one returns both the key and
the value.

=item B<DB_SET_RECNO>

TODO.

=item B<DB_GET_RECNO>

TODO.

=back

In addition, the following value may be set by bitwise OR'ing it into
the B<$flags> parameter:

=over 5

=item B<DB_RMW>

TODO.

=back

=head2  $status = $cursor->c_put($key, $value, $flags)

Stores the key/value pair in the database. The position that the data is
stored in the database is controlled by the C<$flags> parameter, which
must take B<one> of the following values:

=over 5

=item B<DB_AFTER>

When used with a Btree or Hash database, a duplicate of the key referenced
by the current cursor position will be created and the contents of
B<$value> will be associated with it - B<$key> is ignored.
The new key/value pair will be stored immediately after the current
cursor position.
Obviously the database has to have been opened with B<DB_DUP>.

When used with a Recno ... TODO


=item B<DB_BEFORE>

When used with a Btree or Hash database, a duplicate of the key referenced
by the current cursor position will be created and the contents of
B<$value> will be associated with it - B<$key> is ignored.
The new key/value pair will be stored immediately before the current
cursor position.
Obviously the database has to have been opened with B<DB_DUP>.

When used with a Recno ... TODO

=item B<DB_CURRENT>

If the cursor has been initialised, replace the value of the key/value
pair stored in the database with the contents of B<$value>.

=item B<DB_KEYFIRST>

Only valid with a Btree or Hash database. This flag is only really
used when duplicates are enabled in the database and sorted duplicates
haven't been specified.
In this case the key/value pair will be inserted as the first entry in
the duplicates for the particular key.

=item B<DB_KEYLAST>

Only valid with a Btree or Hash database. This flag is only really
used when duplicates are enabled in the database and sorted duplicates
haven't been specified.
In this case the key/value pair will be inserted as the last entry in
the duplicates for the particular key.

=back

=head2  $status = $cursor->c_del([$flags])

This method deletes the key/value pair associated with the current cursor
position. The cursor position will not be changed by this operation, so
any subsequent cursor operation must first initialise the cursor to
point to a valid key/value pair.

If the key/value pair associated with the cursor have already been
deleted, B<c_del> will return B<DB_KEYEMPTY>.

The B<$flags> parameter is not used at present.

=head2 $status = $cursor->c_count($cnt [, $flags])

Stores the number of duplicates at the current cursor position in B<$cnt>.

The B<$flags> parameter is not used at present. This method needs 
Berkeley DB 3.1 or better.

=head2  $status = $cursor->status()

Returns the status of the last cursor method as a dual type.

=head2  $status = $cursor->c_pget() ;

See C<db_pget>

=head2  $status = $cursor->c_close()

Closes the cursor B<$cursor>.

=head2 $stream = $cursor->db_stream($flags);

Create a BerkeleyDB::DbStream object to read the blob at the current cursor location.
See L<Blob> for details of the the BerkeleyDB::DbStream object.

$flags must be one or more of the following OR'ed together

DB_STREAM_READ
DB_STREAM_WRITE
DB_STREAM_SYNC_WRITE

For full information on the flags refer to the Berkeley DB Reference Guide.

=head2 Cursor Examples

TODO

Iterating from first to last, then in reverse.

examples of each of the flags.

=head1 JOIN

Join support for BerkeleyDB is in progress. Watch this space.

TODO

=head1 TRANSACTIONS

Transactions are created using the C<txn_begin> method on L<BerkeleyDB::Env>:

	my $txn = $env->txn_begin;

If this is a nested transaction, supply the parent transaction as an
argument:

	my $child_txn = $env->txn_begin($parent_txn);

Then in order to work with the transaction, you must set it as the current
transaction on the database handles you want to work with:

	$db->Txn($txn);

Or for multiple handles:

	$txn->Txn(@handles);

The current transaction is given by BerkeleyDB each time to the various BDB
operations. In the C api it is required explicitly as an argument to every
operation.

To commit a transaction call the C<commit> method on it:

	$txn->txn_commit;

and to roll back call abort:

	$txn->txn_abort

After committing or aborting a child transaction you need to set the active
transaction again using C<Txn>.

=head1 BerkeleyDBB::DbStream -- support for BLOB

Blob support is available in Berkeley DB starting with version 6.0. Refer
to the section "Blob Support" in the Berkeley DB Programmer Reference for
details of how Blob supports works.

A Blob is access via a BerkeleyDBB::DbStream object. This is created via a
cursor object.

    # Note - error handling not shown below.

    # Set the key we want
    my $k = "some key";

    # Don't want the value retrieved by the cursor,
    # so use partial_set to make sure no data is retrieved.
    my $v = '';
    $cursor->partial_set(0,0) ;
    $cursor->c_get($k, $v, DB_SET) ;
    $cursor->partial_clear() ;

    # Now create a stream to the blob
    my $stream = $cursor->db_stream(DB_STREAM_WRITE) ;

    # get the size of the blob
    $stream->size(my $s) ;

    # Read the first 1k of data from the blob
    my $data ;
    $stream->read($data, 0, 1024);

A BerkeleyDB::DbStream object has the following methods available:


=head2 $status = $stream->size($SIZE);

Outputs the length of the Blob in the $SIZE parameter.

=head2 $status = $stream->read($data, $offset, $size);

Read from the blob. $offset is the number of bytes from the start of the
blob to read from. $size if the number of bytes to read.

=head2 $status = $stream->write($data, $offset, $flags);

Write $data to the blob, starting at offset $offset.

Example

Below is an example of how to walk through a database when you don't know
beforehand which entries are blobs and which are not.

    while (1)
    {
        my $k = '';
        my $v = '';
        $cursor->partial_set(0,0) ;
        my $status = $cursor->c_get($k, $v, DB_NEXT) ;
        $cursor->partial_clear();

        last if $status != 0 ;

        my $stream = $cursor->db_stream(DB_STREAM_WRITE);

        if (defined $stream)
        {
            # It's a Blob
            $stream->size(my $s) ;
        }
        else
        {
            # Not a Blob
            $cursor->c_get($k, $v, DB_CURRENT) ;
        }
    }

=head1 Berkeley DB Concurrent Data Store (CDS)

The Berkeley DB I<Concurrent Data Store> (CDS) is a lightweight locking
mechanism that is useful in scenarios where transactions are overkill. 

=head2 What is CDS?

The Berkeley DB CDS interface is a simple lightweight locking mechanism
that allows safe concurrent access to Berkeley DB databases. Your
application can have multiple reader and write processes, but Berkeley DB
will arrange it so that only one process can have a write lock against the
database at a time, i.e. multiple processes can read from a database
concurrently, but all write processes will be serialised.

=head2 Should I use it?

Whilst this simple locking model is perfectly adequate for some
applications, it will be too restrictive for others. Before deciding on
using CDS mode, you need to be sure that it is suitable for the expected
behaviour of your application.

The key features of this model are 

=over 5

=item *

All writes operations are serialised.

=item *

A write operation will block until all reads have finished.

=back

There are a few of the attributes of your application that you need to be
aware of before choosing to use CDS.

Firstly, if you application needs either recoverability or transaction
support, then CDS will not be suitable.

Next what is the ratio of read operation to write operations will your
application have?

If it is carrying out mostly read operations, and very few writes, then CDS
may be appropriate.

What is the expected throughput of reads/writes in your application?

If you application does 90% writes and 10% reads, but on average you only
have a transaction every 5 seconds, then the fact that all writes are
serialised will not matter, because there will hardly ever be multiple
writes processes blocking.

In summary CDS mode may be appropriate for your application if it performs
mostly reads and very few writes or there is a low throughput.  Also, if
you do not need to be able to roll back a series of database operations if
an error occurs, then CDS is ok.

If any of these is not the case you will need to use Berkeley DB
transactions. That is outside the scope of this document.

=head2 Locking Used

Berkeley DB implements CDS mode using two kinds of lock behind the scenes -
namely read locks and write locks. A read lock allows multiple processes to
access the database for reading at the same time. A write lock will only
get access to the database when there are no read or write locks active.
The write lock will block until the process holding the lock releases it.

Multiple processes with read locks can all access the database at the same
time as long as no process has a write lock. A process with a write lock
can only access the database if there are no other active read or write
locks.

The majority of the time the Berkeley DB CDS mode will handle all locking
without your application having to do anything. There are a couple of
exceptions you need to be aware of though - these will be discussed in
L<Safely Updating Records> and L<Implicit Cursors> below.

A Berkeley DB Cursor (created with C<< $db->db_cursor >>) will by hold a
lock on the database until it is either explicitly closed or destroyed.
This means the lock has the potential to be long lived. 

By default Berkeley DB cursors create a read lock, but it is possible to
create a cursor that holds a write lock, thus

    $cursor = $db->db_cursor(DB_WRITECURSOR);


Whilst either a read or write cursor is active, it will block any other
processes that wants to write to the database. 

To avoid blocking problems, only keep cursors open as long as they are
needed. The same is true when you use the C<cursor> method or the
C<cds_lock> method.

For full information on CDS see the "Berkeley DB Concurrent Data Store
applications" section in the Berkeley DB Reference Guide.


=head2 Opening a database for CDS

Here is the typical signature that is used when opening a database in CDS
mode.

    use BerkeleyDB ;

    my $env = new BerkeleyDB::Env
                  -Home   => "./home" ,
                  -Flags  => DB_CREATE| DB_INIT_CDB | DB_INIT_MPOOL
        or die "cannot open environment: $BerkeleyDB::Error\n";

    my $db  = new BerkeleyDB::Hash
                -Filename       => 'test1.db',
                -Flags          => DB_CREATE,
                -Env            => $env
        or die "cannot open database: $BerkeleyDB::Error\n";

or this, if you use the tied interface

    tie %hash, "BerkeleyDB::Hash",
                -Filename       => 'test2.db',
                -Flags          => DB_CREATE,
                -Env            => $env
        or die "cannot open database: $BerkeleyDB::Error\n";

The first thing to note is that you B<MUST> always use a Berkeley DB
environment if you want to use locking with Berkeley DB.

Remember, that apart from the actual database files you explicitly create
yourself, Berkeley DB will create a few behind the scenes to handle locking
- they usually have names like "__db.001". It is therefore a good idea to
use the C<-Home> option, unless you are happy for all these files to be
written in the current directory.

Next, remember to include the C<DB_CREATE> flag when opening the
environment for the first time. A common mistake is to forget to add this
option and then wonder why the application doesn't work.

Finally, it is vital that all processes that are going to access the
database files use the same Berkeley DB environment.


=head2 Safely Updating a Record

One of the main gotchas when using CDS is if you want to update a record in
a database, i.e. you want to retrieve a record from a database, modify it
in some way and put it back in the database.

For example, say you are writing a web application and you want to keep a
record of the number of times your site is accessed in a Berkeley DB
database. So your code will have a line of code like this (assume, of
course, that C<%hash> has been tied to a Berkeley DB database):

    $hash{Counter} ++ ;

That may look innocent enough, but there is a race condition lurking in
there. If I rewrite the line of code using the low-level Berkeley DB API,
which is what will actually be executed, the race condition may be more
apparent:

    $db->db_get("Counter", $value);
    ++ $value ;
    $db->db_put("Counter", $value);

Consider what happens behind the scenes when you execute the commands
above.  Firstly, the existing value for the key "Counter" is fetched from
the database using C<db_get>. A read lock will be used for this part of the
update.  The value is then incremented, and the new value is written back
to the database using C<db_put>. This time a write lock will be used. 

Here's the problem - there is nothing to stop two (or more) processes
executing the read part at the same time. Remember multiple processes can
hold a read lock on the database at the same time. So both will fetch the
same value, let's say 7, from the database. Both increment the value to 8
and attempt to write it to the database. Berkeley DB will ensure that only
one of the processes gets a write lock, while the other will be blocked. So
the process that happened to get the write lock will store the value 8 to
the database and release the write lock. Now the other process will be
unblocked, and it too will write the value 8 to the database. The result,
in this example, is we have missed a hit in the counter.

To deal with this kind of scenario, you need to make the update atomic. A
convenience method, called C<cds_lock>, is supplied with the BerkeleyDB
module for this purpose. Using C<cds_lock>, the counter update code can now
be rewritten thus:

    my $lk = $dbh->cds_lock() ; 
    $hash{Counter} ++ ;
    $lk->cds_unlock;

or this, where scoping is used to limit the lifetime of the lock object

    {
        my $lk = $dbh->cds_lock() ;
        $hash{Counter} ++ ;
    }

Similarly, C<cds_lock> can be used with the native Berkeley DB API 

    my $lk = $dbh->cds_lock() ;
    $db->db_get("Counter", $value);
    ++ $value ;
    $db->db_put("Counter", $value);
    $lk->unlock;


The C<cds_lock> method will ensure that the current process has exclusive
access to the database until the lock is either explicitly released, via
the C<< $lk->cds_unlock() >> or by the lock object being destroyed.

If you are interested, all that C<cds_lock> does is open a "write" cursor.
This has the useful side-effect of holding a write-lock on the database
until the cursor is deleted. This is how you create a write-cursor

    $cursor = $db->db_cursor(DB_WRITECURSOR);

If you have instantiated multiple C<cds_lock> objects for one database
within a single process, that process will hold a write-lock on the
database until I<ALL> C<cds_lock> objects have been destroyed.

As with all write-cursors, you should try to limit the scope of the
C<cds_lock> to as short a time as possible. Remember the complete database
will be locked to other process whilst the write lock is in place.

=head2 Cannot write with a read cursor while a write cursor is active

This issue is easier to demonstrate with an example, so consider the code
below. The intention of the code is to increment the values of all the
elements in a database by one.

    # Assume $db is a database opened in a CDS environment.

    # Create a write-lock
    my $lock = $db->db_cursor(DB_WRITECURSOR);
    # or 
    # my $lock = $db->cds_lock();

    
    my $cursor = $db->db_cursor();

    # Now loop through the database, and increment
    # each value using c_put.
    while ($cursor->c_get($key, $value, DB_NEXT) == 0) 
    {
         $cursor->c_put($key, $value+1, DB_CURRENT) == 0
             or die "$BerkeleyDB::Error\n";
    }


When this code is run, it will fail on the C<c_put> line with this error

    Write attempted on read-only cursor

The read cursor has automatically disallowed a write operation to prevent a
deadlock.


So the rule is -- you B<CANNOT> carry out a write operation using a
read-only cursor (i.e. you cannot use C<c_put> or C<c_del>) whilst another
write-cursor is already active. 

The workaround for this issue is to just use C<db_put> instead of C<c_put>,
like this

    # Assume $db is a database opened in a CDS environment.

    # Create a write-lock
    my $lock = $db->db_cursor(DB_WRITECURSOR);
    # or 
    # my $lock = $db->cds_lock();

    
    my $cursor = $db->db_cursor();

    # Now loop through the database, and increment
    # each value using c_put.
    while ($cursor->c_get($key, $value, DB_NEXT) == 0) 
    {
         $db->db_put($key, $value+1) == 0
             or die "$BerkeleyDB::Error\n";
    }



=head2 Implicit Cursors

All Berkeley DB cursors will hold either a read lock or a write lock on the
database for the existence of the cursor. In order to prevent blocking of
other processes you need to make sure that they are not long lived.

There are a number of instances where the Perl interface to Berkeley DB
will create a cursor behind the scenes without you being aware of it. Most
of these are very short-lived and will not affect the running of your
script, but there are a few notable exceptions.

Consider this snippet of code

    while (my ($k, $v) = each %hash)
    {
        # do something
    }


To implement the "each" functionality, a read cursor will be created behind
the scenes to allow you to iterate through the tied hash, C<%hash>. While
that cursor is still active, a read lock will obviously be held against the
database. If your application has any other writing processes, these will
be blocked until the read cursor is closed. That won't happen until the
loop terminates.

To avoid blocking problems, only keep cursors open as long as they are
needed. The same is true when you use the C<cursor> method or the
C<cds_lock> method.


The locking behaviour of the C<values> or C<keys> functions, shown below,
is subtly different.

    foreach my $k (keys %hash)
    {
        # do something
    }

    foreach my $v (values %hash)
    {
        # do something
    }


Just as in the C<each> function, a read cursor will be created to iterate
over the database in both of these cases. Where C<keys> and C<values>
differ is the place where the cursor carries out the iteration through the
database. Whilst C<each> carried out a single iteration every time it was
invoked, the C<keys> and C<values> functions will iterate through the
entire database in one go -- the complete database will be read into memory
before the first iteration of the loop.

Apart from the fact that a read lock will be held for the amount of time
required to iterate through the database, the use of C<keys> and C<values>
is B<not> recommended because it will result in the complete database being
read into memory.


=head2 Avoiding Deadlock with multiple databases

If your CDS application uses multiple database files, and you need to write
to more than one of them, you need to be careful you don't create a
deadlock.

For example, say you have two databases, D1 and D2, and two processes, P1
and P2. Assume you want to write a record to each database. If P1 writes
the records to the databases in the order D1, D2 while process P2 writes
the records in the order D2, D1, there is the potential for a deadlock to
occur.

This scenario can be avoided by either always acquiring the write locks in
exactly the same order in your application code, or by using the
C<DB_CDB_ALLDB> flag when opening the environment. This flag will make a
write-lock apply to all the databases in the environment.

Add example here

=head1 DBM Filters

A DBM Filter is a piece of code that is be used when you I<always>
want to make the same transformation to all keys and/or values in a DBM
database. All of the database classes (BerkeleyDB::Hash,
BerkeleyDB::Btree and BerkeleyDB::Recno) support DBM Filters.

There are four methods associated with DBM Filters. All work
identically, and each is used to install (or uninstall) a single DBM
Filter. Each expects a single parameter, namely a reference to a sub.
The only difference between them is the place that the filter is
installed.

To summarise:

=over 5

=item B<filter_store_key>

If a filter has been installed with this method, it will be invoked
every time you write a key to a DBM database.

=item B<filter_store_value>

If a filter has been installed with this method, it will be invoked
every time you write a value to a DBM database.


=item B<filter_fetch_key>

If a filter has been installed with this method, it will be invoked
every time you read a key from a DBM database.

=item B<filter_fetch_value>

If a filter has been installed with this method, it will be invoked
every time you read a value from a DBM database.

=back

You can use any combination of the methods, from none, to all four.

All filter methods return the existing filter, if present, or C<undef>
in not.

To delete a filter pass C<undef> to it.

=head2 The Filter

When each filter is called by Perl, a local copy of C<$_> will contain
the key or value to be filtered. Filtering is achieved by modifying
the contents of C<$_>. The return code from the filter is ignored.

=head2 An Example -- the NULL termination problem.

Consider the following scenario. You have a DBM database that you need
to share with a third-party C application. The C application assumes
that I<all> keys and values are NULL terminated. Unfortunately when
Perl writes to DBM databases it doesn't use NULL termination, so your
Perl application will have to manage NULL termination itself. When you
write to the database you will have to use something like this:

    $hash{"$key\0"} = "$value\0" ;

Similarly the NULL needs to be taken into account when you are considering
the length of existing keys/values.

It would be much better if you could ignore the NULL terminations issue
in the main application code and have a mechanism that automatically
added the terminating NULL to all keys and values whenever you write to
the database and have them removed when you read from the database. As I'm
sure you have already guessed, this is a problem that DBM Filters can
fix very easily.

## nullFilter

Hopefully the contents of each of the filters should be
self-explanatory. Both "fetch" filters remove the terminating NULL,
and both "store" filters add a terminating NULL.


=head2 Another Example -- Key is a C int.

Here is another real-life example. By default, whenever Perl writes to
a DBM database it always writes the key and value as strings. So when
you use this:

    $hash{12345} = "something" ;

the key 12345 will get stored in the DBM database as the 5 byte string
"12345". If you actually want the key to be stored in the DBM database
as a C int, you will have to use C<pack> when writing, and C<unpack>
when reading.

Here is a DBM Filter that does it:

## intFilter

This time only two filters have been used -- we only need to manipulate
the contents of the key, so it wasn't necessary to install any value
filters.

=head1 Using BerkeleyDB with MLDBM

Both BerkeleyDB::Hash and BerkeleyDB::Btree can be used with the MLDBM
module. The code fragment below shows how to open associate MLDBM with
BerkeleyDB::Btree. To use BerkeleyDB::Hash just replace
BerkeleyDB::Btree with BerkeleyDB::Hash.

    use strict ;
    use BerkeleyDB ;
    use MLDBM qw(BerkeleyDB::Btree) ;
    use Data::Dumper;
 
    my $filename = 'testmldbm' ;
    my %o ;
     
    unlink $filename ;
    tie %o, 'MLDBM', -Filename => $filename,
                     -Flags    => DB_CREATE
                    or die "Cannot open database '$filename: $!\n";
 
See the MLDBM documentation for information on how to use the module
and for details of its limitations.

=head1 EXAMPLES

TODO.

=head1 HINTS & TIPS

=head2 Sharing Databases With C Applications

There is no technical reason why a Berkeley DB database cannot be
shared by both a Perl and a C application.

The vast majority of problems that are reported in this area boil down
to the fact that C strings are NULL terminated, whilst Perl strings
are not. See L<An Example -- the NULL termination problem.> in the DBM
FILTERS section for a generic way to work around this problem.


=head2 The untie Gotcha

TODO

=head1 COMMON QUESTIONS

This section attempts to answer some of the more common questions that
I get asked.


=head2 Relationship with DB_File

Before Berkeley DB 2.x was written there was only one Perl module that
interfaced to Berkeley DB. That module is called B<DB_File>. Although
B<DB_File> can be build with Berkeley DB 1.x, 2.x, 3.x or 4.x, it only
provides an interface to the functionality available in Berkeley DB
1.x. That means that it doesn't support transactions, locking or any of
the other new features available in DB 2.x or better.

=head2 How do I store Perl data structures with BerkeleyDB?

See L<Using BerkeleyDB with MLDBM>.

=head1 HISTORY

See the Changes file.

=head1 AVAILABILITY

The most recent version of B<BerkeleyDB> can always be found
on CPAN (see L<perlmod/CPAN> for details), in the directory
F<modules/by-module/BerkeleyDB>.

The official web site for Berkeley DB is F<http://www.oracle.com/technology/products/berkeley-db/db/index.html>.

=head1 COPYRIGHT

Copyright (c) 1997-2015 Paul Marquess. All rights reserved. This program
is free software; you can redistribute it and/or modify it under the
same terms as Perl itself.

Although B<BerkeleyDB> is covered by the Perl license, the library it
makes use of, namely Berkeley DB, is not. Berkeley DB has its own
copyright and its own license. Please take the time to read it.

Here are few words taken from the Berkeley DB FAQ (at
F<http://www.oracle.com/technology/products/berkeley-db/db/index.html>) regarding the license:

    Do I have to license DB to use it in Perl scripts?

    No. The Berkeley DB license requires that software that uses
    Berkeley DB be freely redistributable. In the case of Perl, that
    software is Perl, and not your scripts. Any Perl scripts that you
    write are your property, including scripts that make use of Berkeley
    DB. Neither the Perl license nor the Berkeley DB license
    place any restriction on what you may do with them.

If you are in any doubt about the license situation, contact either the
Berkeley DB authors or the author of BerkeleyDB.
See L<"AUTHOR"> for details.


=head1 AUTHOR

Paul Marquess E<lt>pmqs@cpan.orgE<gt>.


=head1 SEE ALSO

perl(1), DB_File, Berkeley DB.

=cut