com.neovisionaries.zookeeper

Enum LeaderElection.State

java.lang.Object

java.lang.Enum<LeaderElection.State>

com.neovisionaries.zookeeper.LeaderElection.State

All Implemented Interfaces:

Serializable, Comparable<LeaderElection.State>

Enclosing class:

LeaderElection

public static enum LeaderElection.State
extends Enum<LeaderElection.State>

Leader election state.

           State Transition

                     +----------------+
                     |  +----------+  |
                     |  |  LEADER  |  |
                     |  +----------+  |
                     |        A       |
                     |        |       |
                     |        V       |
 +---------+         |  +----------+  |
 | CREATED |----------->| ELECTING |  |
 +---------+         |  +----------+  |
      |              |        A       |
      |              |        |       |
      V              |        V       |
 +---------+         |  +----------+  |
 |  DONE   |<--------|  | FOLLOWER |  |
 +---------+         |  +----------+  |
                     +----------------+
 

The initial state of LeaderElection is CREATED.

LeaderElection.start() method can be called only when the state is CREATED. When start() method is called when the state is not CREATED, an IllegalStateException is thrown.

In the implementation of start() method, the state is changed to ELECTING before the method returns. This means that Listener.onStateChanged() is called before start() method is returned. In some unusual cases, however, the state is changed to DONE. This happens when the given ZooKeeper instance reports AUTH_FAILED or CLOSED, or when you have called LeaderElection.finish() before calling start() method.

As a result of leader election, the state is changed to either LEADER or FOLLOWER. The state LEADER means that the LeaderElection instance has won the leader election and now is the leader. The state FOLLOWER means that the LeaderElection instance has lost the leader election and now is a follower.

The state may be changed back to ELECTING from LEADER or FOLLOWER. This happens when it is detected that the znode for leader election has been deleted. In this case, another new leader election will be executed without delay, and as a result, the state will be changed to either LEADER or FOLLOWER again.

The implementation of LeaderElection triggers a ZooKeeper callback as necessary. At the timing, the state of the given ZooKeeper instance is checked by calling ZooKeeper.getState() method. If the ZooKeeper's state is either AUTH_FAILED or CLOSED, a ZooKeeper callback is not triggered, and instead, the state of LeaderElection is changed to DONE.

Since:

1.1

Enum Constant Summary

Enum Constants

Enum Constant and Description
CREATED The initial state of a LeaderElection instance.
DONE The LeaderElection instance has stopped working and will not join leader election any further.
ELECTING Leader election is now being conducted.
FOLLOWER The LeaderElection instance is now a follower.
LEADER The LeaderElection instance is now the leader.

Method Summary

Modifier and Type	Method and Description
static LeaderElection.State	valueOf(String name) Returns the enum constant of this type with the specified name.
static LeaderElection.State[]	values() Returns an array containing the constants of this enum type, in the order they are declared.

Methods inherited from class java.lang.Enum

clone, compareTo, equals, finalize, getDeclaringClass, hashCode, name, ordinal, toString, valueOf

Methods inherited from class java.lang.Object

getClass, notify, notifyAll, wait, wait, wait

Enum Constant Detail

CREATED

public static final LeaderElection.State CREATED

The initial state of a LeaderElection instance.

LEADER

public static final LeaderElection.State LEADER

The LeaderElection instance is now the leader.

FOLLOWER

public static final LeaderElection.State FOLLOWER

The LeaderElection instance is now a follower.

ELECTING

public static final LeaderElection.State ELECTING

Leader election is now being conducted.

DONE

public static final LeaderElection.State DONE

The LeaderElection instance has stopped working and will not join leader election any further.

Method Detail

values

public static LeaderElection.State[] values()

Returns an array containing the constants of this enum type, in the order they are declared. This method may be used to iterate over the constants as follows:

for (LeaderElection.State c : LeaderElection.State.values())
    System.out.println(c);

Returns:

an array containing the constants of this enum type, in the order they are declared

valueOf

public static LeaderElection.State valueOf(String name)

Returns the enum constant of this type with the specified name. The string must match exactly an identifier used to declare an enum constant in this type. (Extraneous whitespace characters are not permitted.)

Parameters:

name - the name of the enum constant to be returned.

Returns:

the enum constant with the specified name

Throws:

IllegalArgumentException - if this enum type has no constant with the specified name

NullPointerException - if the argument is null