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<div class="textblock"><p>WiredTiger applications will generally use the following classes to access and manage data:</p>
<ul>
<li>a <a class="el" href="struct_w_t___c_o_n_n_e_c_t_i_o_n.html" title="A connection to a WiredTiger database. ">WT_CONNECTION</a> represents a connection to a database. Most applications will only open one connection to a database for each process. All methods in <a class="el" href="struct_w_t___c_o_n_n_e_c_t_i_o_n.html" title="A connection to a WiredTiger database. ">WT_CONNECTION</a> are thread safe.</li>
<li>a <a class="el" href="struct_w_t___s_e_s_s_i_o_n.html" title="All data operations are performed in the context of a WT_SESSION. ">WT_SESSION</a> represents a context in which database operations are performed. Sessions are opened on a specified connection, and applications must open a single session for each thread accessing the database.</li>
<li>a <a class="el" href="struct_w_t___c_u_r_s_o_r.html" title="A WT_CURSOR handle is the interface to a cursor. ">WT_CURSOR</a> represents a cursor over a collection of data. Cursors are opened in the context of a session (which may have an associated transaction), and can query and update records. In the common case, a cursor is used to access records in a table. However, cursors can be used on subsets of tables (such as a single column or a projection of multiple columns), as an interface to statistics, configuration data or application-specific data sources.</li>
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<p>Handles and operations are <a class="el" href="config_strings.html">configured using strings</a>, which keeps the set of methods in the API relatively small and makes the interface very similar regardless of the programming language used in the application. WiredTiger supports the C, C++, Java and Python programming languages (among others).</p>
<p>By default, WiredTiger works as a traditional key/value store, where the keys and values are raw byte arrays accessed using a <a class="el" href="group__wt.html#struct_w_t___i_t_e_m" title="A raw item of data to be managed, including a pointer to the data and a length. ">WT_ITEM</a> structure. Keys and values may be up to (4GB - 512B) bytes in size, but depending on how <a class="el" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb" title="Create a table, column group, index or file. ">WT_SESSION::create</a> "maximum item sizes" are configured, large key and value items will be stored on overflow pages.</p>
<p>WiredTiger also supports a <a class="el" href="schema.html">schema layer</a> so that keys and values types can be chosen from a list, or composite keys or values made up of columns with any combination of types. The size (4GB - 512B) byte limit on keys and values still applies.</p>
<p>All applications that use WiredTiger will be structured roughly as follows. The code below is taken from the complete example program <a class="el" href="ex_access_8c-example.html">ex_access.c</a>.</p>
<h1><a class="anchor" id="basic_connection"></a>
Connecting to a database</h1>
<p>To access a database, first open a connection and create a session handle for the single thread accessing the database:</p>
<div class="fragment"><div class="line"> <a class="code" href="struct_w_t___c_o_n_n_e_c_t_i_o_n.html">WT_CONNECTION</a> *conn;</div><div class="line"> <a class="code" href="struct_w_t___c_u_r_s_o_r.html">WT_CURSOR</a> *cursor;</div><div class="line"> <a class="code" href="struct_w_t___s_e_s_s_i_o_n.html">WT_SESSION</a> *session;</div><div class="line"> <span class="keyword">const</span> <span class="keywordtype">char</span> *key, *value;</div><div class="line"> <span class="keywordtype">int</span> ret;</div><div class="line"></div><div class="line"> <span class="comment">/* Open a connection to the database, creating it if necessary. */</span></div><div class="line"> error_check(<a class="code" href="group__wt.html#gacbe8d118f978f5bfc8ccb4c77c9e8813">wiredtiger_open</a>(home, NULL, <span class="stringliteral">"create"</span>, &conn));</div><div class="line"></div><div class="line"> <span class="comment">/* Open a session handle for the database. */</span></div><div class="line"> error_check(conn-><a class="code" href="struct_w_t___c_o_n_n_e_c_t_i_o_n.html#adad5965cd4a60f65b5ac01f7ca6d1fc0">open_session</a>(conn, NULL, NULL, &session));</div></div><!-- fragment --><p> The configuration string <code>"create"</code> is passed to <a class="el" href="group__wt.html#gacbe8d118f978f5bfc8ccb4c77c9e8813" title="Open a connection to a database. ">wiredtiger_open</a> to indicate the database should be created if it does not already exist.</p>
<p>The code block above also shows simple error handling with <a class="el" href="group__wt.html#gae8bf720ddb4a7a7390b70424594c40fd" title="Return information about a WiredTiger error as a string (see WT_SESSION::strerror for a thread-safe A...">wiredtiger_strerror</a> (a function that returns a string describing an error code passed as its argument). More complex error handling can be configured by passing an implementation of <a class="el" href="struct_w_t___e_v_e_n_t___h_a_n_d_l_e_r.html" title="The interface implemented by applications to handle error, informational and progress messages...">WT_EVENT_HANDLER</a> to <a class="el" href="group__wt.html#gacbe8d118f978f5bfc8ccb4c77c9e8813" title="Open a connection to a database. ">wiredtiger_open</a> or <a class="el" href="struct_w_t___c_o_n_n_e_c_t_i_o_n.html#adad5965cd4a60f65b5ac01f7ca6d1fc0" title="Open a session. ">WT_CONNECTION::open_session</a>.</p>
<h1><a class="anchor" id="basic_create_table"></a>
Creating a table</h1>
<p>Create a table we can use to store data:</p>
<div class="fragment"><div class="line"> error_check(session-><a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb">create</a>(session, <span class="stringliteral">"table:access"</span>, <span class="stringliteral">"key_format=S,value_format=S"</span>));</div></div><!-- fragment --><p> This call creates a table called <code>"access"</code>, configured to use strings for its key and value columns. (See <a class="el" href="schema.html">Schema, Columns, Column Groups, Indices and Projections</a> for more information on tables with other types of key and value columns.)</p>
<h1><a class="anchor" id="basic_cursors"></a>
Accessing data with cursors</h1>
<p>Now that we have a table, we open a cursor to perform some operations on it:</p>
<div class="fragment"><div class="line"> error_check(session-><a class="code" href="struct_w_t___s_e_s_s_i_o_n.html#afb5b4a69c2c5cafe411b2b04fdc1c75d">open_cursor</a>(session, <span class="stringliteral">"table:access"</span>, NULL, NULL, &cursor));</div></div><!-- fragment --><p> Here, the string <code>"table:access"</code> specifies that we are opening the cursor on the table named <code>"access"</code>.</p>
<p>Then we insert a new row into the table. The <a class="el" href="struct_w_t___c_u_r_s_o_r.html#ad1088d719df40babc1f57d086691ebdc" title="Set the key for the next operation. ">WT_CURSOR::set_key</a> and <a class="el" href="struct_w_t___c_u_r_s_o_r.html#a27f7cbd0cd3e561f6a145704813ad64c" title="Set the value for the next operation. ">WT_CURSOR::set_value</a> calls put the application's key and value into the cursor, respectively. The <a class="el" href="struct_w_t___c_u_r_s_o_r.html#aac90d9fbcc031570f924db55f8a1cee3" title="Insert a record and optionally update an existing record. ">WT_CURSOR::insert</a> call creates a record containing that value and inserts it into the table.</p>
<div class="fragment"><div class="line"> cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#ad1088d719df40babc1f57d086691ebdc">set_key</a>(cursor, <span class="stringliteral">"key1"</span>); <span class="comment">/* Insert a record. */</span></div><div class="line"> cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#a27f7cbd0cd3e561f6a145704813ad64c">set_value</a>(cursor, <span class="stringliteral">"value1"</span>);</div><div class="line"> error_check(cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#aac90d9fbcc031570f924db55f8a1cee3">insert</a>(cursor));</div></div><!-- fragment --><p> Now we iterate through all of the records in the table, printing them out as we go:</p>
<div class="fragment"><div class="line"> error_check(cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#afc1b42c22c9c85e1ba08ce3b34437565">reset</a>(cursor)); <span class="comment">/* Restart the scan. */</span></div><div class="line"> <span class="keywordflow">while</span> ((ret = cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#a0503f16bd8f3d05aa3552f229b3a8e1b">next</a>(cursor)) == 0) {</div><div class="line"> error_check(cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#af19f6f9d9c7fc248ab38879032620b2f">get_key</a>(cursor, &key));</div><div class="line"> error_check(cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#af85364a5af50b95bbc46c82e72f75c01">get_value</a>(cursor, &value));</div><div class="line"></div><div class="line"> printf(<span class="stringliteral">"Got record: %s : %s\n"</span>, key, value);</div><div class="line"> }</div><div class="line"> scan_end_check(ret == <a class="code" href="group__wt.html#ga3c9e1b494d95cf34404ab7a974af6bf8">WT_NOTFOUND</a>); <span class="comment">/* Check for end-of-table. */</span></div></div><!-- fragment --><p> Note that the key and value parts of the records are returned as C strings because the table was created that way (even if it was created by a previous run of the example). No data extraction or conversion is required in the application.</p>
<p>Because the cursor was positioned in the table after the <a class="el" href="struct_w_t___c_u_r_s_o_r.html#aac90d9fbcc031570f924db55f8a1cee3" title="Insert a record and optionally update an existing record. ">WT_CURSOR::insert</a> call, we had to re-position it using the <a class="el" href="struct_w_t___c_u_r_s_o_r.html#afc1b42c22c9c85e1ba08ce3b34437565" title="Reset the cursor. ">WT_CURSOR::reset</a> call; if we weren't using the cursor for the call to <a class="el" href="struct_w_t___c_u_r_s_o_r.html#aac90d9fbcc031570f924db55f8a1cee3" title="Insert a record and optionally update an existing record. ">WT_CURSOR::insert</a> above, this loop would simplify to:</p>
<div class="fragment"><div class="line"><span class="keywordflow">while</span> ((ret = cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#a0503f16bd8f3d05aa3552f229b3a8e1b">next</a>(cursor)) == 0) {</div><div class="line"> ...</div><div class="line">}</div></div><!-- fragment --><h1><a class="anchor" id="basic_close"></a>
Closing handles</h1>
<p>Lastly, we close the connection, which implicitly closes the cursor and session handles:</p>
<div class="fragment"><div class="line"> error_check(conn-><a class="code" href="struct_w_t___c_o_n_n_e_c_t_i_o_n.html#af535c517df851eeac8ebf3594d40b545">close</a>(conn, NULL)); <span class="comment">/* Close all handles. */</span></div></div><!-- fragment --></div></div><!-- contents -->
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