activemq-artemis/docs/user-manual/en/client-reconnection.md

Client Reconnection and Session Reattachment

Apache ActiveMQ Artemis clients can be configured to automatically reconnect or re-attach to the server in the event that a failure is detected in the connection between the client and the server.

100% Transparent session re-attachment

If the failure was due to some transient failure such as a temporary network failure, and the target server was not restarted, then the sessions will still be existent on the server, assuming the client hasn't been disconnected for more than connection-ttl

In this scenario, Apache ActiveMQ Artemis will automatically re-attach the client sessions to the server sessions when the connection reconnects. This is done 100% transparently and the client can continue exactly as if nothing had happened.

The way this works is as follows:

As Apache ActiveMQ Artemis clients send commands to their servers they store each sent command in an in-memory buffer. In the case that connection failure occurs and the client subsequently reattaches to the same server, as part of the reattachment protocol the server informs the client during reattachment with the id of the last command it successfully received from that client.

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.Ac

The size of this buffer is configured with the confirmationWindowSize parameter on the connection URL. 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.

The window is specified in bytes.

Setting this parameter to -1 disables any buffering and prevents any re-attachment from occurring, forcing reconnect instead. The default value for this parameter is -1. (Which means by default no auto re-attachment will occur)

Session reconnection

Alternatively, the server might have actually been restarted after crashing or being stopped. In this case any sessions will no longer be existent on the server and it won't be possible to 100% transparently re-attach to them.

In this case, Apache ActiveMQ Artemis 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 during failover onto a backup server.

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

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

Configuring reconnection/reattachment attributes

Client reconnection is configured using the following parameters:

• retryInterval. This optional parameter determines the period in milliseconds between subsequent reconnection attempts, if the connection to the target server has failed. The default value is 2000 milliseconds.

• retryIntervalMultiplier. This optional parameter determines determines a multiplier to apply to the time since the last retry to compute the time to the next retry.

  This allows you to implement an exponential backoff between retry attempts.

  Let's take an example:

  If we set retryInterval to 1000 ms and we set retryIntervalMultiplier to 2.0, then, if the first reconnect attempt fails, we will wait 1000 ms then 2000 ms then 4000 ms between subsequent reconnection attempts.

  The default value is 1.0 meaning each reconnect attempt is spaced at equal intervals.

• maxRetryInterval. This optional parameter determines the maximum retry interval that will be used. When setting retryIntervalMultiplier it would otherwise be possible that subsequent retries exponentially increase to ridiculously large values. By setting this parameter you can set an upper limit on that value. The default value is 2000 milliseconds.

• reconnectAttempts. This optional parameter determines the total number of reconnect attempts to make before giving up and shutting down. A value of -1 signifies an unlimited number of attempts. The default value is 0.

All of these parameters are set on the URL used to connect to the broker.

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 won't be able to re-attach, and any ExceptionListener or FailureListener instances registered on the connection or session will be called.

ExceptionListeners and SessionFailureListeners

Please note, that when a client reconnects or re-attaches, any registered JMS ExceptionListener or core API SessionFailureListener will be called.