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.
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 Detecting Dead Connections
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.
The size of this buffer is configured by the
parameter, when the server has received
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 JMS and you're using the JMS service on the server to
load your JMS connection factory instances into JNDI then this parameter
can be configured in the jms configuration using the element
confirmationWindowSize a. If you're using JMS but not using JNDI
then you can set these values directly on the
ActiveMQConnectionFactory instance using the appropriate setter
If you're using the core API you can set these values directly on the
ServerLocator instance using the appropriate setter method.
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)
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 oncedelivery guarantees.
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
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
1000 ms and we set
2.0, then, if the first reconnect
attempt fails, we will wait
1000 ms then
2000 ms then
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
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
If you're using JMS and you're using JNDI on the client to look up your
JMS connection factory instances then you can specify these parameters
in the JNDI context environment in, e.g.
java.naming.factory.initial = ActiveMQInitialContextFactory connection.ConnectionFactory=tcp://localhost:61616?retryInterval=1000&retryIntervalMultiplier=1.5&maxRetryInterval=60000&reconnectAttempts=1000
If you're 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
If you're using the core API and instantiating the
instance directly you can also specify the parameters using the
appropriate setter methods on the
ServerLocator immediately after
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
FailureListener instances registered on the
connection or session will be called.
Please note, that when a client reconnects or re-attaches, any
ExceptionListener or core API
will be called.