Amazon SQS - Simple Queuing Service - FIFO vs Standard Queue - AWS Certification Cheat Sheet

Let’s get a quick overview of Amazon SQS from an AWS certification perspective. We will look at important certification questions regarding Amazon SQS.

You will learn

  • What is Amazon SQS?
  • Why do we need Amazon SQS?
  • When do we use Amazon SQS?
  • What is difference between FIFO vs Standard Queue?
  • What are the steps involved in processing a message from Amazon SQS queue?
  • How do you implement auto scaling with CloudWatch Alarms for SQS Queue?

Simple Queuing Service

Simple Queuing Service is a highly reliable, scalable, fully-managed message queuing service.

Here are some of the important features:

  • High availability
  • Unlimited scaling
    • Auto scale to process billions of messages per day
  • Low cost (Pay for use)

Two Types of Simple Queuing Service Queues

There are Two Types of Simple Queuing Service Queues:

  • Standard Queue
    • Unlimited throughput
    • BUT NO guarantee of ordering (Best-Effort Ordering)
    • and NO guarantee of exactly-once processing
      • Guarantees at-least-once delivery (some messages can be processed twice)
  • FIFO (first-in-first-out) Queue
    • First-In-First-out Delivery
    • Exactly-Once Processing
    • BUT throughput is lower
      • Upto 300 messages per second (300 send, receive, or delete operations per second)
      • If you batch 10 messages per operation (maximum), up to 3,000 messages per second

Choose between Two Types of Simple Queuing Service Queues

  • Standard SQS queue if throughput is important
  • FIFO Queue if order of events is important

Sending and receiving a SQS Message - Best case scenario

Let’s see the steps involved in sending and receiving a message on the SQS queue:

  • Step I: Producer places message on queue
    • Receives globally unique message ID ABCDEFGHIJ (used to track the message)
  • Step II: Consumer polls for messages
    • Receives the message ABCDEFGHIJ along with a receipt handle XYZ
  • Step III: Message remains in the queue while the consumer processes the message
    • Other consumers will not receive ABCDEFGHIJ even if they poll for messages
  • Step IV: Consumer processes the message successfully
    • Calls delete message (using receipt handle XYZ)
    • Message is removed from the queue

Simple Queuing Service Lifecycle of a message

The picture below shows the service lifecycle of a message

When a message is sent to queue, it is redundantly distributed among SQS servers

SQS - Auto Scaling

You can also implement auto scaling for your SQS queue using CloudWatch based on number of messages in the queue.


  • Use target tracking scaling policy
  • Use a SQS metric like ApproximateNumberOfMessages

SQS Queue - Important configuration

The table below shows some of the important SQS queue configuration:

Configuration Description
Visibility timeout Other consumers will not receive a message being processed for the configured time period (default - 30 seconds, min - 0, max - 12 hours)
Consumer processing a message can call ChangeMessageVisibility to increase visibility timeout of a message (before visibility timeout)
DelaySeconds Time period before a new message is visible on the queue
Delay Queue = Create Queue + Delay Seconds
default - 0, max - 15 minutes
Can be set at Queue creation or updated using SetQueueAttributes
Use message timers to configure a message specific DelaySeconds value
Message retention period Maximum period a message can be on the queue
Default - 4 days, Min - 60 seconds, Max - 14 days
MaxReceiveCount Maximum number of failures in processing a message

Simple Queuing Service Security

Here are the important things to consider about security of the SQS queue:

  • You can provide access to other AWS resources to access SQS using IAM roles (EC2 -> SQS)
  • By default only the queue owner is allowed to use the queue
    • Configure SQS Queue Access Policy to provide access to other AWS accounts

SQS - Certification and Interview Questions

Let’s look a few scenarios regarding SQS.

Scenario Result
Consumer takes more than visibility timeout to process the message Message is visible on queue after visibility timeout and another consumer might receive the message
Consumer calls ChangeMessageVisibility before visibility timeout Visibility timeout is extended to requested time
DelaySeconds is configured on the queue Message is delayed for DelaySeconds before it is available
Receiver wants to decide how to handle the message without looking at message body Configure Message Attributes
How to reduce number of API calls to SQS? Use Long Polling - When looking for messages, you can specify a WaitTimeSeconds upto 20 seconds
Your receive messages and start processing them after a week. You see that some messages are not processed at all! Exceeded message retention period. Default message retention period is 4 days. Max 14 days.
Give high priority to premium customers Create separate queues for free and premium customers

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GCP IAM - GCP Certification Cheat Sheet

GCP App Engine - GCP Certification Cheat Sheet


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