Queue prioritization ensures that critical tasks are handled before lower-priority ones. Rqueue supports three modes of priority handling:

1. Weighted: Polling frequency is based on numeric weights assigned to queues.
2. Strict: Higher-priority queues are polled first, but starvation is considered so lower-priority queues still get polling opportunities.
3. Hard Strict: Messages are always fetched from the highest-priority queue first, moving top-down only when that queue has no available message.

Configuring Priority

To enable priority handling:

• Set priorityMode in the container factory to WEIGHTED, STRICT, or HARD_STRICT.
• Use the priority field in the @RqueueListener annotation to assign weights or levels.
• Use the priorityGroup field to group multiple related queues. By default, queues with specified priorities are added to a default group.

Examples:

• Sub-queues: A single listener can handle multiple priority levels for the same logical task: priority="critical=10,high=6,medium=4,low=1".
• Simple Numeric: Assign a weight directly: priority="60", priorityGroup="critical-group".

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Weighted Priority

Weighted priority uses a round-robin approach where the polling frequency is proportional to the assigned weights.

Consider three queues: | Queue | Weight | |——-|——–| | Q1 | 6 | | Q2 | 4 | | Q3 | 2 |

In one cycle, Q1 is polled 6 times, Q2 4 times, and Q3 2 times. If a queue is empty during a poll, its effective weight is dynamically reduced to prevent wasting resources on empty queues. Weights are reset once all queues in the group have become inactive or finished their cycle.

For implementation details, see WeightedPriorityPoller.

Strict Priority

In strict mode, the poller prefers the highest-priority queue first, but it also considers starvation so every queue continues to get polling chances over time.

• The poller starts with Q1.
• If Q1 is empty, it moves to Q2, then Q3.
• If a queue is empty, it becomes inactive for the duration of the polling interval.
• If messages are not fetched from a lower-priority queue for a certain interval, that queue becomes eligible again so it can be polled even while higher-priority queues continue to receive traffic.
• This gives lower-priority queues a chance to make progress and prevents permanent starvation.

For implementation details, see StrictPriorityPoller.

Hard Strict Priority

In hard strict mode, the poller always follows a top-down priority order.

• The poller starts with the highest-priority queue.
• It continues to fetch from that queue as long as messages are available.
• It moves to the next queue only when the current higher-priority queue has no message available.
• This approach keeps the queue order fully top-down and may starve lower-priority queues while higher-priority queues continue to receive traffic.

Additional Configuration

• rqueue.add.default.queue.with.queue.level.priority: Determines if a “default” priority level should be automatically added to listeners using the priority="key=value" syntax. Default: true.

• rqueue.default.queue.with.queue.level.priority: Specifies the weight/priority of the automatically added default queue. By default, it is placed in the middle of the defined priority levels.

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