Priority Queue
Priority queues deliver messages in priority order rather than FIFO order. Higher-priority messages are delivered before lower-priority ones.
Declaration
Declare a priority queue by setting the x-max-priority argument:
x-max-priority: 10
The value defines the maximum priority level (0-255). Messages with a priority higher than this value are capped to the maximum.
The x-max-priority argument is required at declaration time and cannot be changed after creation.
How It Works
Internally, a priority queue maintains separate message stores for each priority level (0 through x-max-priority). When delivering messages:
- The store with the highest priority is checked first
- If it has messages, the next message is delivered from that store
- Otherwise, the next lower priority store is checked, and so on
Messages without a priority property default to priority 0 (lowest).
Performance Considerations
Each priority level creates its own message store subdirectory on disk (prio.000, prio.001, etc.). Setting a very high x-max-priority increases resource usage proportionally. Choose the minimum number of levels needed for your use case.
Interaction with Other Features
Priority queues support all standard queue features:
- TTL (message and queue expiration)
- Dead-lettering
- Max length / max length bytes (overflow drops the oldest message from the highest-priority non-empty sub-store)
- Durability
- Consumer prefetch
- Single active consumer
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