Amazon Simple Queue Service
Message queue
A message queue: senders put work in, receivers poll it out to process; keeps both sides independent of each other's speed.
- Cost$ Low
- Difficulty●●●●●
- Costs money when idle?No
- Serverless
- In examsCLF · SAA · DVA · SOA
Think of it asLike an order drop box: customers keep dropping orders in, and staff handle them whenever they're free.
Where does SQS fit?
A real example, step by step
- A website takes orders very quickly, but the warehouse system needs a few seconds per order.
- Instead of calling the warehouse directly, the website puts each order on an SQS queue and answers the customer immediately.
- Workers (Lambda or EC2) pull orders from the queue at their own pace; unprocessed orders wait safely.
- An order that fails 5 times in a row moves to a dead-letter queue for someone to check, without blocking the rest.
- On a sale day, orders arrive 20 times faster: the queue grows, workers scale on the number of waiting messages, and no order is lost.
▶ Watch the “Event-driven image processing” scenario on the map
When to use it
✓ Use it when
Decoupling systems, background processing, absorbing traffic spikes.
✗ Avoid it when
One message needs to reach many receivers at once → SNS / EventBridge.
Common uses
- Decouple web servers from background workers
- Buffer sudden traffic spikes
- FIFO queues that preserve order (orders, payments)
- Dead-letter queues hold messages that failed processing
SQS, SNS or EventBridge?
| SQS |  SNS |  EventBridge | |
|---|---|---|---|
| Type | Queue: messages wait until processed | Pub/sub: pushes immediately to many | Rule-based event routing |
| Receivers per message | Usually one | Many | Many targets by rule |
| Keeps messages | Up to 14 days | No (retries delivery a few times) | Archive for replay |
| Best for | Decoupling, absorbing spikes | Notifications, fan-out | Reacting to AWS/SaaS events |
What does it cost?
- Pricing model: ~$0.40/1 million requests (Standard), ~$0.50 (FIFO); each 64 KB chunk of a message counts as one request.
- Free tier: Always Free: 1 million requests/month.
Short polling calls constantly even when the queue is empty — use long polling; large messages count as multiple requests.
Decouple components to absorb sudden spikes → SQS. Strict ordering and exactly-once processing → a FIFO queue. Messages that keep failing → a dead-letter queue. Messages processed twice → increase the visibility timeout. Fewer empty receives → long polling.
Try real exam questions
1A development team is collaborating with another company to create an integrated product. The other company needs to access an Amazon Simple Queue Service (Amazon SQS) queue that is contained in the development team's account. The other company wants to poll the queue without giving up its own account permissions to do so. How should a solutions architect provide access to the SQS queue?
- ACreate an instance profile that provides the other company access to the SQS queue.
- BCreate an IAM policy that provides the other company access to the SQS queue.
- CCreate an SQS access policy that provides the other company access to the SQS queue.
- DCreate an Amazon Simple Notification Service (Amazon SNS) access policy that provides the other company access to the SQS queue.
Show answer
Correct answer: C
Use an SQS access policy (a policy attached to the queue itself) that allows the other company's account to use the queue. An IAM policy in your account can't grant permissions to another account.
2A company is designing an application. The application uses an AWS Lambda function to receive information through Amazon API Gateway and to store the information in an Amazon Aurora PostgreSQL database. During the proof-of-concept stage, the company has to increase the Lambda quotas significantly to handle the high volumes of data that the company needs to load into the database. A solutions architect must recommend a new design to improve scalability and minimize the configuration effort. Which solution will meet these requirements?
- ARefactor the Lambda function code to Apache Tomcat code that runs on Amazon EC2 instances. Connect the database by using native Java Database Connectivity (JDBC) drivers.
- BChange the platform from Aurora to Amazon DynamoDB. Provision a DynamoDB Accelerator (DAX) cluster. Use the DAX client SDK to point the existing DynamoDB API calls at the DAX cluster.
- CSet up two Lambda functions. Configure one function to receive the information. Configure the other function to load the information into the database. Integrate the Lambda functions by using Amazon Simple Notification Service (Amazon SNS).
- DSet up two Lambda functions. Configure one function to receive the information. Configure the other function to load the information into the database. Integrate the Lambda functions by using an Amazon Simple Queue Service (Amazon SQS) queue.
Show answer
Correct answer: D
Split it into two functions with an SQS queue in between: SQS holds messages safely and the database-writing function pulls them in batches at a controlled rate, so the Lambda quotas don't need raising. SNS pushes messages immediately, so it doesn't reduce load; moving to EC2 or DynamoDB is a large, costly change.
Frequently asked questions
What is the difference between standard and FIFO queues?
Standard queues have nearly unlimited throughput, but a message may be delivered more than once and out of order. FIFO queues keep strict order within a message group and process each message exactly once, with limited throughput.
How is SQS priced?
By the number of requests (send, receive, delete), with a free monthly allowance; FIFO costs slightly more than standard. Use long polling to cut empty receives and reduce cost.
What do AWS exams ask about SQS?
Decoupling components to absorb traffic spikes, visibility timeout, dead-letter queues for failed messages, long polling, choosing standard or FIFO, and Auto Scaling based on the number of waiting messages.