Consumer Flow Control

BlazingMQ delivers messages to consumer applications using a push model i.e. BlazingMQ message broker pushes new messages as quickly as possible to the consumers. In order to ensure that consumers do not get overwhelmed and can implement flow control, BlazingMQ provide consumer applications APIs to specify maximum number and/or bytes of messages (or “flow control parameters”) that BlazingMQ broker can send them. Consumers can specify flow control parameters when they are attaching to a queue via the openQueue API , and also update them anytime later on via the configureQueue API. Flow control parameters are captured in maxUnconfirmedMessages and maxUnconfirmedBytes parameters of bmqt::QueueOptions.

Once BlazingMQ has sent the specified number of messages or bytes to the consumer instance, it will not send any new messages to it, and such consumer is considered to be “at capacity”. Any new messages arriving in the BlazingMQ queue will stay in the queue and will be delivered to the consumer only when consumer processes (i.e., confirms) pending messages that were delivered previously. BlazingMQ keeps track of outstanding messages sent to every consumer to determine if a consumer instance is at capacity or not.

As an example, if a consumer instance wants no more than 1,000 messages or 100MB worth of messages, it can specify that in the flow control API. Specifying the value of zero of number or bytes of messages leads to effectively pausing the consumer – BlazingMQ sees that consumer has no capacity and thus does not route any messages to it. This can be used by consumer to pause itself temporarily in case it is not able to process messages for some reason. Similarly, specifying a value of 1 of number of messages is equivalent to pulling one message at a time from the queue.

Lastly, flow control parameters can be specified for every subscription separately if desired.