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Competing consumers boost throughput but break strict ordering: messages spread across workers can finish out of order.

Some workflows need both a hot standby and guaranteed sequence.

Single Active Consumer (SAC) ensures only one consumer receives messages from a queue at a time, even if many consumers subscribe.

Others wait in line as standbys.

Set the x-single-active-consumer argument to true when declaring the queue.

If the active consumer disconnects or its channel closes, RabbitMQ promotes the next registered consumer to active automatically.

This gives high availability without losing ordering.

Consumers become active in the order they registered. The first to subscribe becomes active; the rest queue up as standbys.

• Exclusive: only one consumer is even allowed; others get an error
• SAC: many can subscribe, but only one is active; others stand by for failover

Standby consumers subscribe normally. They simply receive no messages until promoted.

Because only one consumer processes at a time, messages are handled in FIFO order as long as the consumer acks sequentially and prefetch keeps the pipeline tidy.

SAC sacrifices parallelism: throughput is bound to a single consumer. Use it only when ordering matters more than raw speed.

The management UI and CLI mark which consumer is active. This helps verify failover behaved as expected during incidents.

• Sequential state machines
• Per-entity event streams that must stay ordered
• Workflows needing a warm standby worker

Check your understanding of SAC.

Single Active Consumer routes all messages to one consumer at a time, preserving FIFO ordering while keeping standbys ready for instant failover.

Enable it with x-single-active-consumer, and accept the throughput trade-off when order matters most.