4.11. Producer Flow Control
4.11.1. Flow Control
The broker implements producer flow control on queues that have limits set. This blocks message producers that risk overflowing a destination queue. The queue will become unblocked when enough messages are delivered and acknowledged.
Flow control relies on a reliable link between the sender and the broker. It works by holding off acknowledging sent messages, causing message producers to reach their sender replay buffer capacity and stop sending.
Queues that have been configured with a Limit Policy of type
ring do not have queue flow thresholds enabled. These queues deal with reaching capacity through the ring mechanism. All other queues with limits have two threshold values that are set by the broker when the queue is created:
- flow_stop_threshold
- the queue resource utilization level that enables flow control when exceeded. Once crossed, the queue is considered in danger of overflow, and the broker will cease acknowledging sent messages to induce producer flow control. Note that either queue size or message count capacity utilization can trigger this.
- flow_resume_threshold
- the queue resource utilization level that disables flow control when dropped below. Once crossed, the queue is no longer considered in danger of overflow, and the broker again acknowledges sent messages. Note that once trigger by either, both queue size and message count must fall below this threshold before producer flow control is deactivated.
The values for these two parameters are percentages of the capacity limits. For example, if a queue has a
qpid.max_size of 204800 (200MB), and a flow_stop_threshold of 80, then the broker will initiate producer flow control if the queue reaches 80% of 204800, or 163840 bytes of enqueued messages.
When the resource utilization of the queue falls below the
flow_resume_threshold, producer flow control is stopped. Setting the flow_resume_threshold above the flow_stop_threshold has the obvious consequence of locking producer flow control on, so don't do it.
