Module: ZK::Election

Defined in:

lib/zk/election.rb

Overview

NOTE: this module should be considered experimental.

==== Overview

This module implements the "leader election" protocols described here[http://hadoop.apache.org/zookeeper/docs/current/recipes.html#sc_leaderElection].

There are Candidates and Observers. Candidates take part in elections and all have equal ability and chance to become the leader. When a leader is decided, they hold onto the leadership role until they die. When the leader dies, an election is held and the winner has its +on_winning_election+ callbacks fired, and the losers have their +on_losing_election+ callbacks fired. When all of the +on_winning_election+ callbacks have completed (completing whatever steps are necessary to assume the leadership role), the leader will "acknowledge" that it has taken over by creating an ephemeral node at a known location (with optional data that the Observers can then read and take action upon). Note that when this node is created, it means the leader has finished taking over, but it does not mean that all the slaves have completed their tasks.

Observers are interested parties in the election, the "constituents" of the process. They can register callbacks to be fired when a new leader has been elected and when a leader has died. The new leader callbacks will only fire once the leader has acknowledged its role, so they can be sure that the leader is ready to perform its duties.

==== Use Case / Example

One problem this pattern can be used to solve is failover between two database nodes. Candidates set up callbacks to both take over as master and to follow the master if they lose the election. On the client side, Obesrvers are set up to follow the "leader ack" node. The leader writes its connection info to the "leader ack" node, and the clients can reconnect to the currently active leader.

def server candidate = @zk.election_candidate("database_election", "dbhost2.fqdn.tld:4567", :follow => :leader) candidate.on_winning_election { become_master_node! } candidate.on_losing_election { become_slave_of_master! }

@zk.on_connected do
  candidate.vote!
end

end

Note that as soon as vote! is called, either the on_winning_election or on_losing_election callbacks will be called.

Defined Under Namespace

Classes: Base, Candidate, Observer

Constant Summary

VOTE_PREFIX =

'ballot'.freeze

ROOT_NODE =

'/_zkelection'.freeze

VALID_FOLLOW_OPTIONS =

[:next_node, :leader].freeze

DEFAULT_OPTS =

{
  :root_election_node => ROOT_NODE,
}.freeze