Single Active Consumer

What is the Single Active Consumer?

A Single Active Consumer permits only one consumer to process messages from a specific queue. If the active consumer becomes unavailable, the queue automatically fails to the next available consumer.

What benefits does the Single Active Consumer have?

Guarantees that messages are processed in the exact order they arrive in the queue.
Ensures a single consumer executes a task at a time.
Eliminates conflicts from multiple consumers processing the same message.

When to use a Single Active Consumer?

Maintaining event order is critical (e.g., when processing ticket requests).
Preventing conflicts from parallel processing is necessary.
Tasks require exclusive, sequential execution.

Single Active Consumer can be enabled when declaring a queue, with the x-single-active-consumer argument set to True.

Example: Declaring a queue with a single active consumer

import pika

connection = pika.BlockingConnection(pika.URLParameters('host-url'))
channel = connection.channel()

channel.queue_declare(
    queue="sac",
    durable=True,
    arguments={"x-single-active-consumer":True}
)

connection.close()

For a queue with single active consumer enabled, registering consumers results in the following:

The first consumer registered with the queue becomes the single active consumer.
If the active consumer becomes unavailable, the queue selects the next consumer from the pool in connection order.

Consumer Exclusivity vs Single Active Consumer

A Single Active Consumer is not the only way to ensure that a single consumer processes messages from a queue. Consumer Exclusivity offers an alternative. It assigns one consumer as the sole processor for a specific queue, ensuring only one consumer handles messages. However, if the exclusive consumer disconnects or stops, the application must register a new consumer to continue processing—unlike the Single Active Consumer, which automatically fails over to the next available consumer.

Example: Consumer exclusivity

import pika

connection = pika.BlockingConnection(pika.URLParameters('host-url'))
channel = connection.channel()

def callback(ch, method, properties, msg):
		print(f"[✅]  { msg }")
		ch.basic_ack(delivery_tag=method.delivery_tag)

channel.basic_consume(
    "ex_queue",
    callback,
    auto_ack=False,
    exclusive**=**True

)
connection.close()

Introduction

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AMQP 0-9-1 Overview

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Queue Deep-dive

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LavinMQ CLI

Single Active Consumer

What is the Single Active Consumer?

What benefits does the Single Active Consumer have?

When to use a Single Active Consumer?

Consumer Exclusivity vs Single Active Consumer

Managed LavinMQ instance on CloudAMQP

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