The Apache Qpid Broker for Java supports a flow control mechanism to which can be used to prevent either a single queue or a virtualhost exceeding configured limits. These two mechanisms are described next.

Flow control is enabled on a producer when it sends a message to a Queue which is "overfull". The producer flow control will be rescinded when all Queues on which a producer is blocking become "underfull". A Queue is defined as overfull when the size (in bytes) of the messages on the queue exceeds the capacity of the Queue. A Queue becomes "underfull" when its size becomes less than the resume capacity. The capacity and resume capacity can be specified when the queue is created. This can be done using the Flow Control Settings within the Queue creation dialogue.

There are four Broker log messages that may occur if flow control through queue capacity limits is enabled. Firstly, when a capacity limited queue becomes overfull, a log message similar to the following is produced MESSAGE [vh(/test)/qu(MyQueue)] [vh(/test)/qu(MyQueue)] QUE-1003 : Overfull : Size : 1,200 bytes, Capacity : 1,000 Then for each channel which becomes blocked upon the overful queue a log message similar to the following is produced: MESSAGE [con:2(guest@anonymous(713889609)/test)/ch:1] [con:2(guest@anonymous(713889609)/test)/ch:1] CHN-1005 : Flow Control Enforced (Queue MyQueue) When enough messages have been consumed from the queue that it becomes underfull, then the following log is generated: MESSAGE [vh(/test)/qu(MyQueue)] [vh(/test)/qu(MyQueue)] QUE-1004 : Underfull : Size : 600 bytes, Resume Capacity : 800 And for every channel which becomes unblocked you will see a message similar to: MESSAGE [con:2(guest@anonymous(713889609)/test)/ch:1] [con:2(guest@anonymous(713889609)/test)/ch:1] CHN-1006 : Flow Control Removed Obviously the details of connection, virtual host, queue, size, capacity, etc would depend on the configuration in use.

Flow control can also be triggered when a configured disk quota is exceeded. This is supported by the BDB and Derby virtualhosts. This functionality blocks all producers on reaching the disk overflow limit. When consumers consume the messages, causing disk space usage to falls below the underflow limit, the producers are unblocked and continue working as normal. Two limits can be configured: overfull limit - the maximum space on disk (in bytes). underfull limit - when the space on disk drops below this limit, producers are allowed to resume publishing. The overfull and underful limit can be specified when a new virtualhost is created or an exiting virtualhost is edited. This can be done using the Store Overflow and Store Underfull settings within the virtual host creation and edit dialogue. If editing an existing virtualhost, the virtualhost must be restarted for the new values to take effect. The disk quota functionality is based on "best effort" principle. This means the broker cannot guarantee that the disk space limit will not be exceeded. If several concurrent transactions are started before the limit is reached, which collectively cause the limit to be exceeded, the broker may allow all of them to be committed. The Broker will also impose flow control if the filesystem hosting a virtualhost exceeds a configured percentage..

There are two broker log messages that may occur if flow control through disk quota limits is enabled. When the virtual host is blocked due to exceeding of the disk quota limit the following message appears in the broker log [vh(/test)/ms(BDBMessageStore)] MST-1008 : Store overfull, flow control will be enforced When virtual host is unblocked after cleaning the disk space the following message appears in the broker log [vh(/test)/ms(BDBMessageStore)] MST-1009 : Store overfull condition cleared

If a producer sends to a queue which is overfull, the broker will respond by instructing the client not to send any more messages. The impact of this is that any future attempts to send will block until the broker rescinds the flow control order. While blocking the client will periodically log the fact that it is blocked waiting on flow control. WARN Message send delayed by 5s due to broker enforced flow control WARN Message send delayed by 10s due to broker enforced flow control After a set period the send will timeout and throw a JMSException to the calling code. If such a JMSException is thrown, the message will not be sent to the broker, however the underlying Session may still be active - in particular if the Session is transactional then the current transaction will not be automatically rolled back. Users may choose to either attempt to resend the message, or to roll back any transactional work and close the Session. Both the timeout delay and the periodicity of the warning messages can be set using Java system properties. The amount of time (in milliseconds) to wait before timing out is controlled by the property qpid.flow_control_wait_failure. The frequency at which the log message informing that the producer is flow controlled is sent is controlled by the system property qpid.flow_control_wait_notify_period. Adding the following to the command line to start the client would result in a timeout of one minute, with warning messages every ten seconds: -Dqpid.flow_control_wait_failure=60000 -Dqpid.flow_control_wait_notify_period=10000