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
|
# SWI-Prolog package paxos
This package provides the library `paxos.pl` that implements a
replicating key-value store of Prolog terms on top of SWI-Prolog
`broadcast` libraries and its TIPC or UDP based extension that
allow broadcasting outside the process using networking.
## Features and usage
This library allows a cluster of Prolog processes that can communicate
using TIPC or UDP to maintain a replicated key-value store. Both keys
and values are arbitrary ground Prolog terms. Nodes may be dynamically
added and removed from the cluster while the cluster maintains a shared
view of the available nodes and their status. The library uses the
_paxos_ algorithm to guarantee consistency and progress in the context
of appearing and failing nodes as well as networking that may drop
packages, have packages arrive out-of-order or be duplicated.
Changes to key values are available as reliable _broadcast_ events in
the participating nodes.
## Application areas
This package has been primarily developed to support clustered HTTP
servers that need to share status information such as _session cookies_.
## Dependencies
This package requires either the TIPC or `clib` package to provide
network enabled broadcasting.
## Status
The current implementation is incomplete and has barely been tested.
## Plans
- Complete and test the implementation
- Make the storage _hookable_ and provide scalable backends based
on e.g. RocksDB.
- The current version only support the _quorum_. There is only
partial support for _learners_ that follow the _quorum_, but
do not participate in voting.
## History
This library was originally conceived by Jeffrey Rosenwald as
`tipc_paxos.pl` as part of the [TIPC
package](https://github.com/SWI-Prolog/contrib-tipc). Jeffrey later
implemented `udp_broadcast.pl` as an alternative network enabled
broadcast transport that could be used for `tipc_paxos.pl`. Jan
Wielemaker renamed the library to `paxos.pl` and added hooks to work
with both the TIPC and UDP (and potentially other) transport mechanisms.
As the project grows more complicated and both requires support
libraries and a testing framework it has been moved to be an extension
package of SWI-Prolog.
|
