JBoss.orgCommunity Documentation
HornetQ 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 Chapter 17, Detecting Dead Connections.
In this scenario, HornetQ 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 HornetQ 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 ConfirmationWindowSize
parameter, 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 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 hornetq-jms.xml
using the element confirmation-window-size
a. If you're using JMS but not using JNDI then
you can set these values directly on the HornetQConnectionFactory
instance using the appropriate setter method.
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, HornetQ 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 Section 39.2.1, “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.
Client reconnection is configured using the following parameters:
retry-interval
. 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.
retry-interval-multiplier
. 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 retry-interval
to 1000
ms and
we set retry-interval-multiplier
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.
max-retry-interval
. This optional parameter determines the
maximum retry interval that will be used. When setting retry-interval-multiplier
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.
reconnect-attempts
. 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
.
If you're using JMS, and you're using the JMS Service on the server to load your JMS
connection factory instances directly into JNDI, then you can specify these parameters
in the xml configuration in hornetq-jms.xml
, for example:
<connection-factory name="ConnectionFactory"> <connectors> <connector-ref connector-name="netty"/> </connectors> <entries> <entry name="ConnectionFactory"/> <entry name="XAConnectionFactory"/> </entries> <retry-interval>1000</retry-interval> <retry-interval-multiplier>1.5</retry-interval-multiplier> <max-retry-interval>60000</max-retry-interval> <reconnect-attempts>1000</reconnect-attempts> </connection-factory>
If you're using JMS, but instantiating your JMS connection factory directly, you can
specify the parameters using the appropriate setter methods on the HornetQConnectionFactory
immediately after creating it.
If you're 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 won't be able to re-attach, and any ExceptionListener
or
FailureListener
instances registered on the connection or session
will be called.