Raft

Implementation of replicated state machine (raft) protocol.

Implemented:

Leader election
Log replication
Cluster changes
Log compaction (snapshotting)

Usage

Before starting a new raft server, you need to implement raft_server behavior.

init/0: return with the new staring state
handle_command/2: execute commands that can modify state
handle_query/2: execute queries which cannot change state

Start new raft server

{ok, Pid} = raft:start(callback_module).

Commands

raft:command(Pid, {do_stuff, Stuff}).

Query

raft:query(Pid, {get_stuff, Stuff}).

Cluster changes

Join new member to the cluster

raft:join(PidA, PidB).

Leave member

raft:leave(PidA, PidB).

Get status info

#{role => leader | follower | candidate,
  term => 1,
  leader => <0.261.0>,
  cluster_members => [<0.261.0>, <0.253.0>, <0.243.0>],
  log_last_index => 1923,
  committed_index => 1922,
  last_applied => 1922,
  log => Last10LogEntry
} = raft:status(Pid)

Config parameters

Timeouts

Normal heartbeat timeout

How many times the followers wait for the heartbeat.

max_heartbeat_timeout - rand(heartbeat_grace_time) + heartbeat_grace_time

Consensus timeout

When electing a new leader candidates wait until this amount of time, and starts a new election.

max_heartbeat_timeout - rand(heartbeat_grace_time)

Leader timeout

Leader sending heartbeats (empty append_entries_req) if nothing sent to the followers for a while.

min_heartbeat_timeout for one heartbeat, than change to idle timeout until the next command

Idle timeout

After one period of heartbeat leader change it's heartbeat time to this. (max_heartbeat_timeout-rand(heartbeat_grace_time))

Append entries chunk size

max_append_entries_chunk_sizeIf a follower wants to catch up, leader can send more than one command in one message. This parameters configures the max amount of commands send in one message. default: 100

Automatic snapshot chunks size

snapshot_chunk_size After a certain amount of commands every raft server does log compactions (create a snapshot). default: 100000

Build

WARNING: Not yet prod ready

$ rebar3 compile

systream/raft