gen_paxos/paxos_fsm

2012.3.17 Author: greg@burd.me

Motivation

gen_paxos is an implementation of the Paxos protocol for Erlang/OTP. Paxos implementation is to be a large finite state machine. My motivation is to reduce the cost of re-invention of wheels. PAXOS is a consensus method in distributed and masterless environment, which is mathmatically guaranteed to come to a conclusion in a finite amount of time.

Current status

UNDER DEVELOPMENT

Prototype of paxos consensus protocol (paxos_fsm) has successfully worked, but there is major work still to do before this is viable for production use. See "future work" below for the list.

Usage

With setting (1) Name of Subject, (2) Identity Number in PAXOS group, (3) Value that PAXOS agent will propose, and (4) Other PAXOS agents into paxos_fsm:start/4, PAXOS consensus IPC (part-time parliament) starts. With calling paxos_fsm:get_result/1 you'll get result of the parliament. With calling paxos_fsm:stop/1 you'll garbage-collect the part-time parliament.

Example

in node A:

 nodeA> node().
 'nodeA'.
 nodeA> nodes().
 ['nodeB'].
 nodeA> gen_paxos:start_link( 0, nodes() ).
 nodeA> gen_paxos:ask( somekey, somevalue ).
 ...(paxos consensus protocol started; messages will be printed)...
 paxos mediation done. decided: hogehoge - (N=2)
 nodeA> gen_paxos:stop().
 paxos_fsm terminated (normal).
 ok
 StateName: decided
 StateData: ....
 nodeA>

in node B:

 nodeB> node().
 'nodeB'.
 nodeB> nodes().
 ['nodeA'].
 nodeB> gen_paxos:start_link( 1, nodes() ).
 ...(paxos consensus protocol started; messages will be printed)...
 paxos mediation done. decided: hogehoge - (N=2)
 nodeB> gen_paxos:ask( somekey, void ).
 somevalue
 nodeB>

Future Work

• replace use of disterl with UDP sockets
• 2-phase paxos consensus algorithm works, but not so fast as other replication/consensus protocols.
• to refine the code so as to work as a generic erlang module - deprecated.
• automation of basic tests.
• to expand coverege of tested combinations of states.
  • because paxos is complex FSM as a whole
• to create a gen_leader[2] compatible interface?
• gen_serverize
• master election module with lease time, with replication consistency
• performance test
• quick check
• dialyzer
• documentation :P

Related works

• libpaxos ( http://sourceforge.net/projects/libpaxos ) libpaxos is almost same implementation of PAXOS consensus protocol. It has Simple version and Fast version of paxos and seems well working. Its implementation has paxos coordinator process, SPOF in a really distributed environment, which is solved in gen_paxos.

• gen_leader ( http://www.cs.chalmers.se/~hanssv/leader_election/ ) gen_leader is a master election module whose behaviour looks like gen_server. It's siginificant in churn environment (maybe) and small rate of mis-election, controllability with proirity. Valid information and details are in [2].

License

The first author of this package UENISHI Kota has given permission to re-license his works under the Apache Software License version 2 (ASLv2) in an email to me (Greg Burd) he wrote: "... so if your code comes successfully working you can change the license to APL and remove my signature and copyright notion." Copyright notices need not change, names need not be removed, but starting with this version of this software (released in 2012) the code is available for use under the Apache Software License (see the LICENSE file for complete terms and conditions of use).

Authors

• UENISHI Kota kuenishi+paxos@gmail.com
• Greg Burd greg@burd.me @gregburd

Appendix

[1] Chandra, Tushar; Robert Griesemer, Joshua Redstone (2007). "Paxos Made Live – An Engineering Perspective". PODC '07: 26th ACM Symposium on Principles of Distributed Computing. http://labs.google.com/papers/paxos_made_live.html.

[2] Svensson, H. and Arts, T. 2005. A new leader election implementation. In Proceedings of the 2005 ACM SIGPLAN Workshop on Erlang (Tallinn, Estonia, September 26 - 28, 2005). ERLANG '05. ACM, New York, NY, 35-39. DOI= http://doi.acm.org/10.1145/1088361.1088368