Amazon SQS: Simple Queue Service

Amazon Simple Queue Service (SQS) is a fully managed message queuing service that enables you to decouple and scale microservices, distributed systems, and serverless applications. SQS was the very first service AWS ever launched (in 2004) and remains one of the most heavily used services in the entire platform.

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What Is a Message Queue?

A message queue is a form of asynchronous communication. A producer places a message on the queue, and a consumer retrieves it later. The queue acts as a buffer between the two, decoupling their execution.

Producer  ——>  [SQS Queue]  ——>  Consumer

The producer does not wait for the consumer to process the message. The consumer pulls messages from the queue at its own pace.

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Why Use SQS?

Problem Without a Queue	How SQS Solves It
Consumer is temporarily offline	Messages are stored in the queue until the consumer is available
Producer is faster than consumer	Messages buffer in the queue, preventing the consumer from being overwhelmed
Tight coupling between services	Producer and consumer only need to know about the queue, not each other
Single point of failure	SQS is highly available and durable across multiple AZs
Difficulty scaling consumers	Multiple consumers can read from the same queue in parallel

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SQS Queue Types

SQS offers two types of queues:

Standard Queue

• Nearly unlimited throughput — supports thousands of messages per second
• At-least-once delivery — a message may be delivered more than once
• Best-effort ordering — messages are generally delivered in order, but ordering is not guaranteed
• Use case: high-throughput workloads where occasional duplicate or out-of-order messages are acceptable

FIFO Queue

• Exactly-once processing — duplicates are eliminated
• Strict ordering — messages are delivered in the exact order they were sent
• Throughput: up to 300 messages per second (or 3,000 with batching and high-throughput mode)
• Use case: workloads where order and exactly-once delivery are critical (e.g. financial transactions)

Feature	Standard	FIFO
Throughput	Nearly unlimited	Up to 300 msg/s (3,000 with batching)
Ordering	Best-effort	Guaranteed
Delivery	At-least-once	Exactly-once
Queue name suffix	None	Must end in .fifo

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How SQS Works

1. Create a Queue

You create a queue in the AWS Management Console, CLI, or SDK. You choose Standard or FIFO and configure settings like message retention and visibility timeout.

2. Send Messages

A producer sends a message to the queue. The message body can be up to 256 KB of text (JSON, XML, plain text, or any format).

import boto3

sqs = boto3.client('sqs')

sqs.send_message(
    QueueUrl='https://sqs.eu-west-2.amazonaws.com/123456789012/my-queue',
    MessageBody='{"orderId": "ORD-12345", "action": "process"}'
)

3. Receive Messages

A consumer polls the queue for messages. SQS delivers messages to the consumer, and the consumer processes them.