/**
* Identifies the master of the replication group, resulting from the last successful election.
* This method relies on the helper nodes supplied to the monitor and queries them for the master.
*
* <p>This method is useful when a Monitor first starts up and the Master needs to be determined.
* Once a Monitor is registered and the Listener has been started, it's kept up to date via events
* that are delivered to the Listener.
*
* @return the id associated with the master replication node.
* @throws UnknownMasterException if the master could not be determined from the set of helpers
* made available to the Monitor.
* @throws EnvironmentFailureException if an unexpected, internal or environment-wide failure
* occurs.
* @throws IllegalStateException if the monitor has been shutdown.
*/
public String getMasterNodeName() throws UnknownMasterException {
if (shutdown.get()) {
throw new IllegalStateException("The monitor has been shutdown");
}
return repGroupAdmin.getMasterNodeName();
}
/**
* Registers the monitor with the group so that it can be kept informed about the outcome of
* elections and group membership changes. The monitor, just like a replication node, is
* identified by its nodeName. The Monitor uses the helper nodes to locate a master with which it
* can register itself. If the helper nodes are not available the registration will fail.
*
* <p>A monitor must be registered at least once, if it needs to be kept up to date about ongoing
* election results and group changes. Attempts to re-register the same monitor are ignored.
* Registration, once it has been completed successfully persists beyond the lifetime of the
* Monitor instance and does not need to be repeated. Repeated registrations are benign and merely
* confirm that the current monitor configuration is consistent with earlier registrations of this
* monitor.
*
* @return the node that is the current master
* @throws EnvironmentFailureException if an unexpected, internal or environment-wide failure
* occurs.
* @throws IllegalStateException if the monitor has been shutdown, or no helper sockets were
* specified at Monitor initialization.
*/
public ReplicationNode register() throws EnvironmentFailureException {
if (shutdown.get()) {
throw new IllegalStateException("The monitor has been shutdown");
}
if (repGroupAdmin.getHelperSockets().size() == 0) {
throw new IllegalStateException("No helper sockets were specified at Monitor initialization");
}
RepNodeImpl monitorNode =
new RepNodeImpl(
nameIdPair,
NodeType.MONITOR,
monitorConfig.getNodeHostname(),
monitorConfig.getNodePort());
/* Ensure that the monitor is part of the group. */
return repGroupAdmin.ensureMonitor(monitorNode);
}
/**
* Returns the current composition of the group. It does so by first querying the helpers to
* determine the master and then obtaining the group information from the master.
*
* @return an instance of RepGroup denoting the current composition of the group
* @throws UnknownMasterException if the master could not be determined from the set of helpers
* made available to the Monitor.
* @throws EnvironmentFailureException if an unexpected, internal or environment-wide failure
* occurs.
* @throws IllegalStateException if the monitor has been shutdown.
*/
public ReplicationGroup getGroup() throws UnknownMasterException, DatabaseException {
if (shutdown.get()) {
throw new IllegalStateException("The monitor has been shutdown");
}
/*
* TODO: Should we use this information to update the helper set as an
* optimization?
*/
return repGroupAdmin.getGroup();
}
/**
* Starts the Listener so it's actively listening for election results and broadcasts of
* replication group changes.
*
* <p>The Monitor must have been previously registered with the replication group via the <code>
* register()</code> method so that other nodes in the group are aware of it and can keep it
* current. If the monitor has not been registered, it will not be updated, that is, its listener
* will not be invoked. The registration needs to be done exactly once.
*
* <p>Once the registration has been completed, the Monitor can start listening even when none of
* the other nodes in the group are available. It will be contacted automatically by the other
* nodes when they come up and hold a successful election.
*
* <p>Invoking <code>startListener</code> results in a synchronous callback to the application via
* the {@link MonitorChangeListener#notify(NewMasterEvent)} method, if there is a current Master.
* If there is no Master at this time then the callback takes place asynchronously, after the
* method returns, when a Master is eventually elected.
*
* @param newListener the listener used to monitor events of interest.
* @throws EnvironmentFailureException if an unexpected, internal or environment-wide failure
* occurs.
* @throws IOException if the monitor socket could not be set up
* @throws IllegalArgumentException if an invalid parameter is specified.
* @throws IllegalStateException if the monitor has been shutdown, or a listener has already been
* established.
*/
public void startListener(MonitorChangeListener newListener)
throws DatabaseException, IOException {
if (shutdown.get()) {
throw new IllegalStateException("The monitor has been shutdown");
}
if (newListener == null) {
throw new IllegalArgumentException(
"A MonitorChangeListener must be associated with "
+ " this Monitor when invoking this method");
}
if (this.monitorChangeListener != null) {
throw new IllegalStateException("A Listener has already been established");
}
this.monitorChangeListener = newListener;
serviceDispatcher = new ServiceDispatcher(monitorConfig.getNodeSocketAddress());
serviceDispatcher.start();
Protocol electionProtocol =
new Protocol(
TimebasedProposalGenerator.getParser(),
MasterValue.getParser(),
monitorConfig.getGroupName(),
nameIdPair,
null);
learner = new Learner(electionProtocol, serviceDispatcher, nameIdPair);
serviceDispatcher.register(new MonitorService(this, serviceDispatcher));
masterChangeListener = new MasterChangeListener();
learner.addListener(masterChangeListener);
learner.start();
try {
/* Notify the listener about the current master. */
final ReplicationGroup repGroup = repGroupAdmin.getGroup();
final RepGroupImpl group = RepInternal.getRepGroupImpl(repGroup);
/*
* In the absence of a network failure, the query should result in
* a call to the notify method of MonitorChanegListener.
*/
learner.queryForMaster(group.getLearnerSockets());
/* Notify JoinGroupEvents for those current active nodes. */
notifyJoinGroupEventsForActiveNodes(repGroup);
} catch (UnknownMasterException ume) {
/* The Listener will be informed when a Master is elected. */
LoggerUtils.logMsg(logger, formatter, Level.INFO, "No current master.");
}
}
