Chapter 30. High Availability

JBoss EAP 7 messaging allows servers to be linked together as live - backup pairs where each live server has a backup. Live servers receive messages from clients, while a backup server is not operational until failover occurs. A backup server can be owned by only one live server, and it will remain in passive mode, waiting to take over the live server’s work.

When a live server crashes or is brought down in the correct mode, the backup server currently in passive mode will become the new live server. If the new live server is configured to allow automatic failback, it will detect the old live server coming back up and automatically stop, allowing the old live server to start receiving messages again.

JBoss EAP messaging supports two different strategies for backing up a server: replication and shared store. Use the ha-policy attribute of the server configuration element to assign the policy of your choice to the given server. There are four valid values for ha-policy:

As you can see, the value specifies whether the server uses a data replication or a shared store ha policy, and whether it takes the role of master or slave.

Use the management CLI console to add an ha-policy to the server of your choice.

/subsystem=messaging-activemq/server=SERVER/ha-policy=POLICY:add

For example, use the following command to add the replication-master policy to the default server.

/subsystem=messaging-activemq/server=default/ha-policy=replication-master:add

The replication-master policy is configured with the default values. Values to override the default configuration can be included when you add the policy. The management CLI command to read the current configuration uses the following basic syntax.

/subsystem=messaging-activemq/server=SERVER/ha-policy=POLICY:read-resource

For example, use the following command to read the current configuration for the replication-master policy that was added above to the default server. The output is also is also included to highlight the default configuration.

/subsystem=messaging-activemq/server=default/ha-policy=replication-master:read-resource

{
    "outcome" => "success",
    "result" => {
        "check-for-live-server" => true,
        "cluster-name" => undefined,
        "group-name" => undefined,
        "initial-replication-sync-timeout" => 30000L
    }
}

See Data Replication and Shared Store for details on the configuration options available for each policy.

When using replication, the live and the backup server pairs do not share the same data directories, all data synchronization is done over the network. Therefore all (persistent) data received by the live server will be duplicated to the backup.

If the live server is cleanly shut down, the backup server will activate and clients will failover to backup. This behavior is pre-determined and is therefore not configurable when using data replication.

The backup server will first need to synchronize all existing data from the live server before replacing it. Unlike shared storage, therefore, a replicating backup will not be fully operational immediately after startup. The time it will take for the synchronization to happen depends on the amount of data to be synchronized and the network speed. Also note that clients are blocked for the duration of initial-replication-sync-timeout when the backup is started. After this timeout elapses, clients will be unblocked, even if synchronization is not completed.

After a successful failover, the backup’s journal will start holding newer data than the data on the live server. You can configure the original live server to perform a failback and become the live server once restarted. A failback will synchronize data between the backup and the live server before the live server comes back online.

In cases were both servers are shut down, the administrator will have to determine which server’s journal has the latest data. If the backup journal has the latest data, copy that journal to the live server. Otherwise, whenever it activates again, the backup will replicate the stale journal data from the live server and will delete its own journal data. If the live server’s data is the latest, no action is needed and the servers can be started normally.

The replicating live and backup pair must be part of a cluster, and it is the cluster-connection configuration element that defines how a backup server will find its live pair. See Configuring Cluster Connections for details on how to configure a cluster connection. When configuring a cluster-connection, keep in mind the following:

Specify a pair of live / backup servers by configuring the group-name attribute in either the master or the slave element. A backup server will only connect to a live server that shares the same group-name.

As a simple example of using a group-name, suppose you have 2 live servers and 2 backup servers. Because each live server needs to pair with its own backup, you would assign names like so:

In the above example, server backup1 with will search for the live server with the same group-name, pair1, which in this case is the server live1.

Much like in the shared store case, when the live server stops or crashes, its replicating, paired backup will become active and take over its duties. Specifically, the paired backup will become active when it loses connection to its live server. This can be problematic because this can also happen because of a temporary network problem. In order to address this issue, the paired backup will try to determine whether it still can connect to the other servers in the cluster. If it can connect to more than half the servers, it will become active. If it loses communication to its live server plus more than half the other servers in the cluster, the paired backup will wait and try reconnecting with the live server. This reduces the risk of a "split brain" situation where both the backup and live servers are processing messages without the other knowing it.

/subsystem=messaging-activemq/server=default/ha-policy=POLICY:write-attribute(name=ATTRIBUTE,value=VALUE)

/subsystem=messaging-activemq/server=default/ha-policy=replication-slave:write-attribute(name=restart-backup,value=true)

AMQ222207: The backup server is not responding promptly introducing latency beyond the limit. Replication server being disconnected now.

This style of high availability differs from data replication in that it requires a shared file system which is accessible by both the live and backup node. This means that the server pairs use the same location for their paging, message journal, bindings journal, and large messages in their configuration.

Also, each participating server in the pair, live and backup, will need to have a cluster-connection defined, even if not part of a cluster, because the cluster-connection defines how the backup server announces its presence to its live server and any other nodes. See Configuring Cluster Connections for details on how this is done.

When failover occurs and a backup server takes over, it will need to load the persistent storage from the shared file system before clients can connect to it. This style of high availability differs from data replication in that it requires a shared file system which is accessible by both the live and backup pair. Typically this will be some kind of high performance Storage Area Network, or SAN. Red Hat does not recommend using Network Attached Storage, known as a NAS, for your storage solution.

The advantage of shared store high availability is that no replication occurs between the live and backup nodes, this means it does not suffer any performance penalties due to the overhead of replication during normal operation.

The disadvantage of shared store replication is that when the backup server activates it needs to load the journal from the shared store which can take some time depending on the amount of data in the store. Also, it requires a shared storage solution supported by JBoss EAP.

If you require the highest performance during normal operation, Red Hat recommends having access to a highly performant SAN and accept the slightly slower failover costs. Exact costs will depend on the amount of data.

/subsystem=messaging-activemq/server=default/ha-policy=POLICY:write-attribute(name=ATTRIBUTE,value=VALUE)

/subsystem=messaging-activemq/server=default/ha-policy=shared-store-slave:write-attribute(name=restart-backup,value=true)

After a live server has failed and a backup taken has taken over its duties, you may want to restart the live server and have clients fail back to it.

In case of a shared store, simply restart the original live server and kill the new live server by killing the process itself. Alternatively, you can set allow-fail-back to true on the slave which will force it to automatically stop once the master is back online. The management CLI command to set allow-fail-back looks like the following:

/subsystem=messaging-activemq/server=default/ha-policy=shared-store-slave:write-attribute(name=allow-fail-back,value=true)

In replication HA mode you need make sure the check-for-live-server attribute is set to true in the master configuration. This is now the default value in JBoss EAP 7.1.

/subsystem=messaging-activemq/server=default/ha-policy=replication-master:write-attribute(name=check-for-live-server,value=true)

If set to true, a live server will search the cluster during startup for another server using its nodeID. If it finds one, it will contact this server and try to "fail-back". Since this is a remote replication scenario, the original live server will have to synchronize its data with the backup running with its ID. Once they are in sync, it will request the backup server to shut down so it can take over active processing. This behavior allows the original live server to determine whether there was a fail-over, and if so whether the server that took its duties is still running or not.

For shared stores, it is also possible to cause failover to occur on normal server shut down, to enable this set failover-on-server-shutdown to true in the HA configuration on either the master or slave like so:

/subsystem=messaging-activemq/server=default/ha-policy=shared-store-slave:write-attribute(name=failover-on-server-shutdown,value=true)

You can also force the running backup server to shut down when the original live server comes back up, allowing the original live server to take over automatically, by setting allow-failback to true.

/subsystem=messaging-activemq/server=default/ha-policy=shared-store-slave:write-attribute(name=allow-failback,value=true)

JBoss EAP also makes it possible to colocate backup messaging servers in the same JVM as another live server. Take for example a simple two node cluster of standalone servers where each live server colocates the backup for the other. You can use either a shared store or a replicated HA policy when colocating servers in this way. There are two important things to remember when configuring messaging servers for colocation.

First, each server element in the configuration will need its own remote-connector and remote-acceptor or http-connector and http-acceptor. For example, a live server with a remote-acceptor can be configured to listen on port 5445, while a remote-acceptor from a colocated backup uses port 5446. The ports are defined in socket-binding elements that must be added to the default socket-binding-group. In the case of http-acceptors, the live and colocated backup can share the same http-listener. Cluster-related configuration elements in each server configuration will use the remote-connector or http-connector used by the server. The relevant configuration is included in each of the examples that follow.

Second, remember to properly configure paths for journal related directories. For example, in a shared store colocated topology, both the live server and its backup, colocated on another live server, must be configured to share directory locations for the binding and message journals, for large messages, and for paging.

JBoss EAP messaging defines two types of client failover:

JBoss EAP messaging also provides 100% transparent automatic reattachment of connections to the same server (e.g. in case of transient network problems). This is similar to failover, except it is reconnecting to the same server and is discussed in Client Reconnection and Session Reattachment.

During failover, if the client has consumers on any non persistent or temporary queues, those queues will be automatically recreated during failover on the backup node, since the backup node will not have any knowledge of non persistent queues.

/subsystem=messaging-activemq/server=default/connection-factory=RemoteConnectionFactory:read-resource

/subsystem=messaging-activemq/server=default/cluster-connection=my-cluster:read-resource

/subsystem=messaging-activemq/server=default/connection-factory=RemoteConnectionFactory:write-attribute(name=reconnect-attempts,value=<NEW_VALUE>)

/subsystem=messaging-activemq/server=default/connection-factory=RemoteConnectionFactory:write-attribute(name=initial-connect-attempts,value=<NEW_VALUE>)

This section discusses connection time to live (TTL) and explains how JBoss EAP messaging handles crashed clients and clients that have exited without cleanly closing their resources.

Cleaning up Dead Connection Resources on the Server

Before a JBoss EAP client application exits, it should close its resources in a controlled manner, using a finally block.

Below is an example of a core client appropriately closing its session and session factory in a finally block:

ServerLocator locator = null;
ClientSessionFactory sf = null;
ClientSession session = null;

try {
   locator = ActiveMQClient.createServerLocatorWithoutHA(..);

   sf = locator.createClientSessionFactory();;

   session = sf.createSession(...);

   ... do some stuff with the session...
}
finally {
   if (session != null) {
      session.close();
   }

   if (sf != null) {
      sf.close();
   }

   if(locator != null) {
      locator.close();
   }
}

And here is an example of a well behaved JMS client application:

Connection jmsConnection = null;

try {
   ConnectionFactory jmsConnectionFactory = ActiveMQJMSClient.createConnectionFactoryWithoutHA(...);

   jmsConnection = jmsConnectionFactory.createConnection();

   ... do some stuff with the connection...
}
finally {
   if (connection != null) {
      connection.close();
   }
}

Unfortunately sometimes clients crash and do not have a chance to clean up their resources. If this occurs, it can leave server side resources hanging on the server. If these resources are not removed they would cause a resource leak on the server, and over time this likely would result in the server running out of memory or other resources.

When looking to clean up dead client resources, it is important to be aware of the fact that sometimes the network between the client and the server can fail and then come back, allowing the client to reconnect. Because JBoss EAP supports client reconnection, it is important that it not clean up "dead" server side resources too soon, or clients will be prevented any client from reconnecting and regaining their old sessions on the server.

JBoss EAP makes all of this configurable. For each ClientSessionFactory configured, a Time-To-Live, or TTL, property can be used to set how long the server will keep a connection alive in milliseconds in the absence of any data from the client. The client will automatically send "ping" packets periodically to prevent the server from closing its connection. If the server does not receive any packets on a connection for the length of the TTL time, it will automatically close all the sessions on the server that relate to that connection.

If you are using JMS, the connection TTL is defined by the ConnectionTTL attribute on a ActiveMQConnectionFactory instance, or if you are deploying JMS connection factory instances direct into JNDI on the server side, you can specify it in the xml config, using the parameter connectionTtl.

The default value for ConnectionTTL on an network-based connection, such as an http-connector, is 60000, i.e. 1 minute. The default value for connection TTL on a internal connection, e.g. an in-vm connection, is -1. A value of -1 for ConnectionTTL means the server will never time out the connection on the server side.

If you do not want clients to specify their own connection TTL, you can set a global value on the server side. This can be done by specifying the connection-ttl-override attribute in the server configuration. The default value for connection-ttl-override is -1 which means "do not override", i.e. let clients use their own values.

Closing Core Sessions or JMS Connections

It is important that all core client sessions and JMS connections are always closed explicitly in a finally block when you are finished using them.

If you fail to do so, JBoss EAP will detect this at garbage collection time. It will then close the connection and log a warning similar to the following:

[Finalizer] 20:14:43,244 WARNING [org.apache.activemq.artemis.core.client.impl.DelegatingSession]  I'm closing a ClientSession you left open. Please make sure you close all ClientSessions explicitly before let
ting them go out of scope!
[Finalizer] 20:14:43,244 WARNING [org.apache.activemq.artemis.core.client.impl.DelegatingSession]  The session you didn't close was created here:
java.lang.Exception
   at org.apache.activemq.artemis.core.client.impl.DelegatingSession.<init>(DelegatingSession.java:83)
   at org.acme.yourproject.YourClass (YourClass.java:666)

Note that if you are using JMS the warning will involve a JMS connection, not a client session. Also, the log will tell you the exact line of code where the unclosed JMS connection or core client session was instantiated. This will enable you to pinpoint the error in your code and correct it appropriately.

Detecting Failure from the Client Side

As long as the client is receiving data from the server it will consider the connection to be alive. If the client does not receive any packets for client-failure-check-period milliseconds, it will consider the connection failed and will either initiate failover, or call any FailureListener instances, or ExceptionListener instances if you are using JMS, depending on how the client has been configured.

If you are using JMS the behavior is defined by the ClientFailureCheckPeriod attribute on a ActiveMQConnectionFactory instance.

The default value for client failure check period on a network connection, for example an HTTP connection, is 30000, or 30 seconds. The default value for client failure check period on an in-vm connection, is -1. A value of -1 means the client will never fail the connection on the client side if no data is received from the server. Whatever the type of connection, the check period is typically much lower than the value for connection TTL on the server so that clients can reconnect in case of transitory failure.

Configuring Asynchronous Connection Execution

Most packets received on the server side are executed on the remoting thread. These packets represent short-running operations and are always executed on the remoting thread for performance reasons.

However, by default some kinds of packets are executed using a thread from a thread pool so that the remoting thread is not tied up for too long. Please note that processing operations asynchronously on another thread adds a little more latency. These packets are:

org.apache.activemq.artemis.core.protocol.core.impl.wireformat.RollbackMessage

org.apache.activemq.artemis.core.protocol.core.impl.wireformat.SessionCloseMessage

org.apache.activemq.artemis.core.protocol.core.impl.wireformat.SessionCommitMessage

org.apache.activemq.artemis.core.protocol.core.impl.wireformat.SessionXACommitMessage

org.apache.activemq.artemis.core.protocol.core.impl.wireformat.SessionXAPrepareMessage

org.apache.activemq.artemis.core.protocol.core.impl.wireformat.SessionXARollbackMessage

To disable asynchronous connection execution, set the parameter async-connection-execution-enabled to false. The default value is true.

JBoss EAP messaging clients can be configured to automatically reconnect or reattach to the server in the event that a failure is detected in the connection between the client and the server.

Transparent Session Reattachment

If the failure was due to some transient cause such as a temporary network outage, and the target server was not restarted, the sessions will still exist on the server, assuming the client has not been disconnected for more than the value of connection-ttl. See Detecting Dead Connections.

In this scenario, JBoss EAP will automatically reattach the client sessions to the server sessions when the re-connection is made. This is done 100% transparently and the client can continue exactly as if nothing had happened.

As JBoss EAP messaging clients send commands to their servers they store each sent command in an in-memory buffer. When a connection fails and the client subsequently attempts to reattach to the same server, as part of the reattachment protocol, the server gives the client the id of the last command it successfully received.

If the client has sent more commands than were received before failover it can replay any sent commands from its buffer so that the client and server can reconcile their states.

The size in bytes of this buffer is set by the confirmationWindowSize property. When the server has received confirmationWindowSize bytes of commands and processed them it will send back a command confirmation to the client, and the client can then free up space in the buffer.

If you are using the JMS service on the server to load your JMS connection factory instances into JNDI, then this property can be configured in the server configuration, by setting the confirmation-window-size attribute of the chosen connection-factory. If you are using JMS but not using JNDI then you can set these values directly on the ActiveMQConnectionFactory instance using the appropriate setter method, setConfirmationWindowSize. If you are using the core API, the ServerLocator instance has a setConfirmationWindowSize method exposed as well.

Setting confirmationWindowSize to -1, which is also the default, disables any buffering and prevents any reattachment from occurring, forcing a reconnect instead.

Session Reconnection

Alternatively, the server might have actually been restarted after crashing or it might have been stopped. In such a case any sessions will no longer exist on the server and it will not be possible to 100% transparently reattach to them.

In this case, JBoss EAP will automatically reconnect the connection and recreate any sessions and consumers on the server corresponding to the sessions and consumers on the client. This process is exactly the same as what happens when failing over to a backup server.

Client reconnection is also used internally by components such as core bridges to allow them to reconnect to their target servers.

See the section on Automatic Client Failover to get a full understanding of how transacted and non-transacted sessions are reconnected during a reconnect and what you need to do to maintain once and only once delivery guarantees.

Configuring Reconnection Attributes

Client reconnection is configured by setting the following properties:

If you are using JMS and JNDI on the client to look up your JMS connection factory instances then you can specify these parameters in the JNDI context environment. For example, your jndi.properties file might look like the following.

java.naming.factory.initial = ActiveMQInitialContextFactory
connection.ConnectionFactory=tcp://localhost:8080?retryInterval=1000&retryIntervalMultiplier=1.5&maxRetryInterval=60000&reconnectAttempts=1000

If you are using JMS, but instantiating your JMS connection factory directly, you can specify the parameters using the appropriate setter methods on the ActiveMQConnectionFactory immediately after creating it.

If you are using the core API and instantiating the ServerLocator instance directly you can also specify the parameters using the appropriate setter methods on the ServerLocator immediately after creating it.

If your client does manage to reconnect but the session is no longer available on the server, for instance if the server has been restarted or it has timed out, then the client will not be able to reattach, and any ExceptionListener or FailureListener instances registered on the connection or session will be called.

ExceptionListeners and SessionFailureListeners

Note that when a client reconnects or reattaches, any registered JMS ExceptionListener or core API SessionFailureListener will be called.