17.2. Producer flow control
HornetQ also can limit the amount of data sent from a client to a server to prevent the server being overwhelmed.
17.2.1. Window based flow control
In a similar way to consumer window based flow control, HornetQ producers, by default, can only send messages to an address as long as they have sufficient credits to do so. The amount of credits required to send a message is given by the size of the message.
As producers run low on credits they request more from the server. When the server sends them more credits they can send more messages.
The amount of credits a producer requests in one go is known as the window size.
The window size therefore determines the amount of bytes that can be in-flight at any one time before more need to be requested; this prevents the remoting connection from getting overloaded.
17.2.1.1. Using Core API
If the HornetQ core API is being used, window size can be set via the
ClientSessionFactory.setProducerWindowSize(int producerWindowSize) method.
17.2.1.2. Using JMS
If JNDI is used to look up the connection factory, the producer window size can be configured in
JBOSS_DIST/jboss-as/server/<PROFILE>/deploy/hornetq/hornetq-jms.xml:
<connection-factory name="NettyConnectionFactory">
<connectors>
<connector-ref connector-name="netty-connector"/>
</connectors>
<entries>
<entry name="/ConnectionFactory"/>
</entries>
<producer-window-size>10</producer-window-size>
</connection-factory>
If the connection factory is directly instantiated, the producer window size can be set via the
HornetQConnectionFactory.setProducerWindowSize(int producerWindowSize) method.
17.2.1.3. Blocking producer window based flow control
Normally the server will always give the same number of credits as has been requested. However, it is also possible to set a maximum size on any address. This then blocks how many credits can be sent to the address so that its memory cannot be exceeded.
For example, if there is a JMS queue called "myqueue", the maximum memory size could be set to 10 MB, and the server will control the number of credits sent to any producers which are sending any messages to myqueue. This means that the total messages in the queue never exceeds 10 MB.
When the address gets full, producers will block on the client side until more space frees up on the address; that is, until messages are consumed from the queue thus freeing up space for more messages to be sent. This is called blocking producer flow control.
To configure an address with a maximum size and tell the server that you want to block producers for this address if it becomes full, you need to define an AddressSettings (Section 23.3, “Configuring Queues Through Address Settings”) block for the address and specify
max-size-bytes and address-full-policy
The address block applies to all queues registered to that address. That is, the total memory for all queues bound to that address will not exceed
max-size-bytes. In the case of JMS topics this means the total memory of all subscriptions in the topic will not exceed max-size-bytes.
Here is an example:
<address-settings>
<address-setting match="jms.queue.exampleQueue">
<max-size-bytes>100000</max-size-bytes>
<address-full-policy>PAGE</address-full-policy>
</address-setting>
</address-settings>
The above example would set the max size of the JMS queue "exampleQueue" to be 100,000 bytes and would block any producers sending to that address to prevent that max size being exceeded.
Note the policy must be set to
BLOCK to enable blocking producer flow control.
