By setting a connection factory’s imqReconnectEnabled attribute to true, you can enable a client to reconnect automatically to a broker if a connection fails. The imqReconnectAttempts attribute controls the number of reconnection attempts to a given broker address; imqReconnectInterval specifies the interval, in milliseconds, to wait between attempts.
In a broker cluster, where the broker address list (imqAddressList ) specifies multiple addresses, a failed connection can be restored not only on the original broker, but also on a different one in the cluster. If reconnection to the original broker fails, the client runtime will try the other addresses in the list. The imqAddressListBehavior and imqAddressListIterations attributes control the order in which addresses are tried and the number of iterations through the list, as described in the preceding section. Each address is tried repeatedly at intervals of imqReconnectInterval milliseconds, up to the maximum number of attempts specified by imqReconnectAttempts.
Automatic reconnection supports all client acknowledgment modes for message consumption. Once a connection has been reestablished, the broker will redeliver all unacknowledged messages it had previously delivered, marking them with a Redeliver flag. Application code can use this flag to determine whether any message has already been consumed but not yet acknowledged. (In the case of nondurable subscribers, however, the broker does not hold messages once their connections have been closed. Thus any messages produced for such subscribers while the connection is down cannot be delivered after reconnection and will be lost.) Message production is blocked while automatic reconnection is in progress; message producers cannot send messages to the broker until after the connection has been reestablished.
Automatic reconnection provides connection failover, but not data failover: persistent messages and other state information held by a failed or disconnected broker can be lost when the client is reconnected to a different broker instance. While attempting to reestablish a connection, Message Queue does maintain objects (such as sessions, message consumers, and message producers) provided by the client runtime. Temporary destinations are also maintained for a time when a connection fails, because clients might reconnect and access them again; after giving clients time to reconnect and use these destinations, the broker will delete them. In circumstances where the client-side state cannot be fully restored on the broker on reconnection (for example, when using transacted sessions, which exist only for the duration of a connection), automatic reconnection will not take place and the connection’s exception handler will be called instead. It is then up to the application code to catch the exception, reconnect, and restore state.