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zraft_lib

Erlang raft consensus protocol implementation .

Supported features:

• Runtime membership reconfiguration.
• Log truncation via snapshotting.
• Peer asynchronous RPC.
• Pluggable state machine.
• Optimistic log replication.
• Snapshot transfer via kernel sendfile command.
• Client sessions.
• Temporary data (like ephemeral nodes)
• Data change triggers.

Erlang Architecture

General Configuration

Example app configuration file.

[{zraft_lib,
     [{snapshot_listener_port,0},
      {election_timeout,500},
      {request_timeout,1000},
      {snapshot_listener_addr,"0,0,0,0"},
      {snapshot_backup,false},
      {log_dir,"./data"},
      {snapshot_dir,"./data"},
      {max_segment_size,10485760},
      {max_log_count,1000}]},
 {lager,
     [{error_logger_hwm,100},
      {error_logger_redirect,true},
      {crash_log_date,"$D0"},
      {crash_log_size,10485760},
      {crash_log_msg_size,65536},
      {handlers,
          [{lager_file_backend,
               [{file,"./log/console.log"},
                {level,info},
                {size,10485760},
                {date,"$D0"},
                {count,5}]},
           {lager_file_backend,
               [{file,"./log/error.log"},
                {level,error},
                {size,10485760},
                {date,"$D0"},
                {count,5}]}]},
      {crash_log,"./log/crash.log"},
      {crash_log_count,5}]},
 {sasl,[{sasl_error_logger,false}]}].

• "election_timeout" - Timeout(in ms) used by Follower to start new election process (default 500).
• "request_timeout" - Timeout(in ms) used by Leader to wait replication RPC reply from Follower (default 2*election_timeout).
• "snapshot_listener_port" - Default port used for snapshot transfer.(0 - any free port).
• "snapshot_listener_addr" - Bind Address for accepting snapshot transfer connections.
• "snapshot_backup" - If it turns on, all snapshot will be archived.
• "log_dir" - Directory to store RAFT logs and metadata.
• "snapshot_dir" - Directory to store snapshots.
• "max_segment_size" - Log segment Maximum size in bytes.(A New segment will be created after reach in that threshold.)
• "max_log_count" - Snapshot/LogTruncation process will be started after every "max_log_count" entries applied.

Create and Config RAFT Cluster.

zraft_client:create(Peers,BackEnd).

Parameters:

• Peers - lists of cluster peers, e.g. [{test1,'test1@host1'},{test1,'test2@host2'},{other_test,'test3@host3'}].
• BackEnd - module name used to apply user's requests.

Possible return values:

• {ok,Peers} - cluster has been created.
• {error,{PeerID,Error}} - PeerID can't be created due to "Error".
• {error,[{PeerID,Error}]} - Peers can't be created.
• {error,Reason} - cluster has been created, but the application of new configuration has failed due to "Reason".

Basic operations.

Light Session Object.

Light session object used to track current raft cluster state, e.g. leader,failed peers, etc...

Create session object by PeerID:

zraft_client:light_session(PeerID,FailTimeout,ElectionTimeout).

Parameters:

• PeerID - ID of peer from luster.
• FailTimeout - If we detect that peer has failed,then we will not send any request to this peer during this Interval.
• ElectionTimeout - If we detect that peer isn't a leader,then we will not send any request to this peer during this Interval.

Possible return values:

• LightSession - Light Session object.
• {error,Reason} - Can't read cluster configuration.

Create session by list PeerID:

zraft_client:light_session(PeersList,FailTimeout,ElectionTimeout).

This function will not try read configuration from cluster.

Write operation.

zraft_client:write(PeerID,Data,Timeout).

Parameters:

• PeerID - PeerID.
• Data - Request Data specific for BackEndModule.

Return:

• {Result,LeaderPeerID} - Result is result of applying Data to BackEndModule. LeaderPeerID is current leader ID.
• {error,Error} - Operation has failed. Typical reason is timeout,noproc.

Write using session object.

zraft_client:write(LightSessionObj,Data,Timeout).

Parameters:

• LightSessionObj - Light Sesssion Object.
• Data - Request Data specific for BackEndModule.

Return:

• {Result,LightSessionObj} - Result is result of applying Data to BackEndModule. LightSessionObj is update session object.
• {error,Error} - Operation has failed. Typical reason is timeout,all_failed. all_failed means,there are not alive peers.

WARNING: during this request Data may be applyed to backend module twice.

Read request:

zraft_client:query(PeerID,Query,Timeout).

Parameters:

• PeerID - PeerID.
• Query - Request Data specific for backend module.

Return:

• {Result,LeaderPeerID} - Result is result of query. LeaderPeerID is current leader ID.
• {error,Error} - Operation has failed. Typical reason is timeout,noproc.

Or read data using light session object:

zraft_client:query(LightSessionObj,Query,Timeout).

Return:

• {Result,LightSessionObj} - Result is result of query. LightSessionObj is update session object.
• {error,Error} - Operation has failed. Typical reason is timeout,all_failed. all_failed means,there are not alive peers.

Change Configuration:

zraft_client:set_new_conf(Peer,NewPeers,OldPeers,Timeout).

Use Session:

You can create long lived session to RAFT cluster. It can be used triggers and temporary datas.

zraft_session:start_link(PeerOrPeers,SessionTimeout)->{ok,Session}.

If first parameter is PeerID other available Peer will be readed from that Peer.

Write Data and Ephemeral data.

zraft_session:write(Session,Data, Temporary, Timeout).

If Temporary is true then data will be deleted after session wil be expired.

Read Data and Set watchers

zraft_session:query(Session,Query,Watch,Timeout).

Watch is trigger reference that will be triggered after future changes.Trigger will be triggered only once, if you need new trigger you must data again.

Example:

zraft_session:query(S1,1,my_watcher,1000).
%%Result = not_found.
zraft_session:write(S2,{1,2},1000).
receive
     {swatch_trigger,my_watcher,Reason}->
          %%Data changed. Change Reason is data_chaged or leader chaged.
          ok
end.
zraft_session:query(S1,1,my_watcher,1000). %%watch again

##Standalone Server.

You can use it for tests from erlang console.

https://github.com/dreyk/zraft

TODO:

• Write External API documentation.
• Add backend based on ets table
• Add "watcher" support (notify client about backend state changes).