A library to help elect leaders using cassandra. Cassandra is already leveraging paxos, and with cassandra's expiring columns we can build a simple leadership election module.
Installation
To install
<dependency>
<groupId>io.paradoxical</groupId>
<artifactId>cassandra-leadership</artifactId>
<version>1.0</version>
</dependency>
Usage
First you need some sort of leadership election tracking table. You can custom the schema names, but you must have something like:
CREATE TABLE leadership_election (
group text PRIMARY KEY,
leader_id text
);
In order for this to work. The default schema uses the one above.
LeadershipSchema.Default
Election
Once you have your leadership tables created you can create a leadership election factory. The factory takes a LeadershipGroup
as a key. This lets you create leadership per group. For example, if you want to elect the leader for a group of workers,
pin all the workers to the same leadership group. Each worker should have its own unique ID and it will be elected leader.
While doing work, if you want to maintain leadership status you will need to heartbeat. It is recommended to heartbeat half the ttl.
For example, if you set the lease time TTL to be 30 seconds, you should attempt to heartbeat no later than every 15 seconds.
Leadership here is re-entrant. If a leader is already active and that same leader asks to elect a leader it will return itself until either its lease time is up, or it explicitly renounces leadership.
As an example
final LeadershipElection leadership = new CassandraLeadershipElectionFactory(session).create(LeadershipGroup.random());
final LeaderIdentity user1 = LeaderIdentity.valueOf("user1");
final LeaderIdentity user2 = LeaderIdentity.valueOf("user2");
assertThat(leadership.tryClaimLeader(user1, Duration.ofSeconds(2))).isPresent();
Thread.sleep(Duration.ofSeconds(3).toMillis());
assertThat(leadership.tryClaimLeader(user2, Duration.ofSeconds(3))).isPresent();
Listing current leader groups
Listing current leader groups is provided for via the LeadershipStatus contract which will list you the current group and its leader id. It also gives
you the amount of seconds that the leader will be active for. When the leader is no longer active the entry is removed.
Wiring into Guice
If you use guice, you can easily create your factories
public class LeadershipModule extends AbstractModule {
@Override
protected void configure() {
bind(LeadershipSchema.class).toInstance(LeadershipSchema.Default);
bind(LeadershipStatus.class).to(LeadershipStatusImpl.class);
bind(LeadershipElectionFactory.class).to(CassandraLeadershipElectionFactory.class);
}
}
