Chapter 6. Network connections

A client can receive information about all master and slave brokers, so that in the event of a connection failure, it can reconnect to the slave broker. The slave broker then automatically re-creates any sessions and consumers that existed on each connection before failover. This feature saves you from having to hand-code manual reconnection logic in your applications.

When a session is recreated on the slave, it does not have any knowledge of messages already sent or acknowledged. Any in-flight sends or acknowledgements at the time of failover might also be lost. However, even without transparent failover, it is simple to guarantee once and only once delivery, even in the case of failure, by using a combination of duplicate detection and retrying of transactions.

Clients detect connection failure when they have not received packets from the broker within a configurable period of time. See Section 6.3, “Detecting dead connections” for more information.

You have a number of methods to configure clients to receive information about master and slave. One option is to configure clients to connect to a specific broker and then receive information about the other brokers in the cluster. See Section 6.7, “Configuring static discovery” for more information. The most common way, however, is to use broker discovery. For details on how to configure broker discovery, see Section 6.6, “Configuring dynamic discovery”.

Also, you can configure the client by adding parameters to the query string of the URI used to connect to the broker, as in the example below.

connectionFactory.ConnectionFactory=tcp://localhost:61616?ha=true&reconnectAttempts=3

In some cases you might not want automatic client failover, but prefer to code your own reconnection logic in a failure handler instead. This is known as application-level failover, since the failover is handled at the application level.

To implement application-level failover when using JMS, set an ExceptionListener class on the JMS connection. The ExceptionListener is called by the broker in the event that a connection failure is detected. In your ExceptionListener, you should close your old JMS connections. You might also want to look up new connection factory instances from JNDI and create new connections.

As long as the it is receiving data from the broker, the client considers a connection to be alive. Configure the client to check its connection for failure by providing a value for the client-failure-check-period property. The default check period for a network connection is 30000 milliseconds, or 30 seconds, while the default value for an In-VM connection, is -1, which means the client never fails the connection from its side if no data is received.

Typically, you set the check period to be much lower than the value used for the broker’s connection time-to-live, which ensures that clients can reconnect in case of a temporary failure.

The examples below show how to set the check period to 10000 milliseconds, or 10 seconds.

By default clients can set a time-to-live (TTL) for their own connections. The examples below show you how to set the TTL.

A client application must close its resources in a controlled manner before it exits to prevent dead connections from occurring. In Java, it is recommended to close connections inside a finally block:

Connection jmsConnection = null;
try {
   ConnectionFactory jmsConnectionFactory = ActiveMQJMSClient.createConnectionFactoryWithoutHA(...);
   jmsConnection = jmsConnectionFactory.createConnection();
   ...use the connection...
}
finally {
   if (jmsConnection != null) {
      jmsConnection.close();
   }
}

You can configure AMQ Core Protocol JMS to discover a list of brokers when attempting to establish a connection.

If you are using JNDI on the client to look up your JMS connection factory instances, you can specify these parameters in the JNDI context environment. Typically the parameters are defined in a file named jndi.properties. The host and part in the URI for the connection factory should match the group-address and group-port from the corresponding broadcast-group inside broker’s broker.xml configuration file. Below is an example of a jndi.properties file configured to connect to a broker’s discovery group.

java.naming.factory.initial = ActiveMQInitialContextFactory
connectionFactory.myConnectionFactory=udp://231.7.7.7:9876

When this connection factory is downloaded from JNDI by a client application and JMS connections are created from it, those connections will be load-balanced across the list of servers that the discovery group maintains by listening on the multicast address specified in the broker’s discovery group configuration.

As an alternative to using JNDI, you can use specify the discovery group parameters directly in your Java code when creating the JMS connection factory. The code below provides an example of how to do this.

final String groupAddress = "231.7.7.7";
final int groupPort = 9876;

DiscoveryGroupConfiguration discoveryGroupConfiguration = new DiscoveryGroupConfiguration();
UDPBroadcastEndpointFactory udpBroadcastEndpointFactory = new UDPBroadcastEndpointFactory();
udpBroadcastEndpointFactory.setGroupAddress(groupAddress).setGroupPort(groupPort);
discoveryGroupConfiguration.setBroadcastEndpointFactory(udpBroadcastEndpointFactory);

ConnectionFactory jmsConnectionFactory = ActiveMQJMSClient.createConnectionFactoryWithHA
    (discoveryGroupConfiguration, JMSFactoryType.CF);

Connection jmsConnection1 = jmsConnectionFactory.createConnection();
Connection jmsConnection2 = jmsConnectionFactory.createConnection();

The refresh timeout can be set directly on the DiscoveryGroupConfiguration by using the setter method setRefreshTimeout(). The default value is 10000 milliseconds.

On first usage, the connection factory will make sure it waits this long since creation before creating the first connection. The default wait time is 10000 milliseconds, but you can change it by passing a new value to DiscoveryGroupConfiguration.setDiscoveryInitialWaitTimeout().

Sometimes it may be impossible to use UDP on the network you are using. In this case you can configure a connection with an initial list of possible servers. The list can be just one broker that you know will always be available, or a list of brokers where at least one will be available.

This does not mean that you have to know where all your servers are going to be hosted. You can configure these servers to use the reliable servers to connect to. After they are connected, their connection details will be propagated from the server to the client.

If you are using JNDI on the client to look up your JMS connection factory instances, you can specify these parameters in the JNDI context environment. Typically the parameters are defined in a file named jndi.properties. Below is an example jndi.properties file that provides a static list of brokers instead of using dynamic discovery.

java.naming.factory.initial=org.apache.activemq.artemis.jndi.ActiveMQInitialContextFactory
connectionFactory.myConnectionFactory=(tcp://myhost:61616,tcp://myhost2:61616)

When the above connection factory is used by a client, its connections will be load-balanced across the list of brokers defined within the parentheses ().

If you are instantiating the JMS connection factory directly, you can specify the connector list explicitly when creating the JMS connection factory, as in the example below.

HashMap<String, Object> map = new HashMap<String, Object>();
map.put("host", "myhost");
map.put("port", "61616");
TransportConfiguration broker1 = new TransportConfiguration
    (NettyConnectorFactory.class.getName(), map);

HashMap<String, Object> map2 = new HashMap<String, Object>();
map2.put("host", "myhost2");
map2.put("port", "61617");
TransportConfiguration broker2 = new TransportConfiguration
    (NettyConnectorFactory.class.getName(), map2);

ActiveMQConnectionFactory cf = ActiveMQJMSClient.createConnectionFactoryWithHA
    (JMSFactoryType.CF, broker1, broker2);