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.
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.
This package has been primarily developed to support clustered HTTP servers that need to share status information such as session cookies.
This package requires either the TIPC or clib package to provide
network enabled broadcasting.
The current implementation is incomplete and has barely been tested.
- 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.
This library was originally conceived by Jeffrey Rosenwald as
tipc_paxos.pl as part of the TIPC
package. 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.