SQS queue concept in AWS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an Amazon SQS queue?

An Amazon SQS queue is a service that stores messages temporarily so that different parts of an application can communicate by sending and receiving messages asynchronously.

beginner

What are the two types of SQS queues?

The two types of SQS queues are Standard queues, which offer high throughput and at-least-once delivery, and FIFO queues, which guarantee exactly-once processing and preserve message order.

beginner

How does SQS help in decoupling application components?

SQS acts like a mailbox where one part of the app puts messages and another part picks them up later, so they don't need to work at the same time or know about each other directly.

intermediate

What is the visibility timeout in SQS?

Visibility timeout is the time period after a message is received during which it is hidden from other consumers, preventing multiple processing of the same message.

intermediate

Why would you use a FIFO queue instead of a Standard queue?

Use a FIFO queue when the order of messages matters and you want to make sure each message is processed exactly once without duplicates.

What does SQS stand for?

A Simple Query Service

B Secure Query System

C Standard Queue Setup

D Simple Queue Service

SQS stands for Simple Queue Service, which is AWS's message queuing service.

Which SQS queue type guarantees message order?

A Standard queue

B Both Standard and FIFO

C FIFO queue

D Neither

FIFO queues guarantee that messages are processed in the exact order they are sent.

What happens to a message during the visibility timeout?

A It is hidden from other consumers

B It is visible to all consumers

C It is deleted immediately

D It is moved to a dead-letter queue

During visibility timeout, the message is hidden from other consumers to avoid duplicate processing.

Which feature of SQS helps different parts of an app work independently?

A Decoupling

B Encryption

C Auto-scaling

D Load balancing

Decoupling allows parts of an app to communicate without depending on each other’s timing or availability.

What is a common use case for SQS?

A Storing large files

B Passing messages between microservices

C Hosting websites

D Sending emails directly

SQS is commonly used to pass messages between microservices or components asynchronously.

Explain how SQS helps improve application reliability and scalability.

Think about how messages wait safely and how components don’t need to be active at the same time.

Describe the differences between Standard and FIFO SQS queues and when to use each.

Consider message order and duplicate handling.

Practice

1. What is the main purpose of an AWS SQS queue?

easy

A. To store messages safely between different parts of a system

B. To run code automatically when triggered

C. To host websites and web applications

D. To store large files like videos and images

Solution

1. Step 1: Understand what SQS does

   AWS SQS is designed to hold messages so different parts of a system can communicate safely without losing data.

2. Step 2: Compare options with SQS purpose

   Running code automatically is Lambda's job, hosting websites is for services like S3 or EC2, and storing large files is for S3. Only storing messages fits SQS.

3. Final Answer:

   To store messages safely between different parts of a system -> Option A

4. Quick Check:

   SQS = message storage between system parts [OK]

Hint: SQS holds messages, not code or files [OK]

Common Mistakes:

• Confusing SQS with Lambda or S3
• Thinking SQS runs code
• Assuming SQS stores large files

2. Which of the following is the correct way to create a standard SQS queue using AWS CLI?

easy

A. aws sqs create-queue --queue-name MyQueue --attributes DelaySeconds=0

B. aws sqs create-queue MyQueue --delay 0

C. aws sqs new-queue --name MyQueue

D. aws sqs make-queue --queue MyQueue --delay 0

Solution

1. Step 1: Recall AWS CLI syntax for creating SQS queue

   The correct command uses 'aws sqs create-queue' with '--queue-name' and optional '--attributes'.

2. Step 2: Check each option's syntax

   aws sqs create-queue --queue-name MyQueue --attributes DelaySeconds=0 matches the correct syntax. Options B, C, and D use incorrect commands or flags.

3. Final Answer:

   aws sqs create-queue --queue-name MyQueue --attributes DelaySeconds=0 -> Option A

4. Quick Check:

   Correct CLI syntax = aws sqs create-queue --queue-name MyQueue --attributes DelaySeconds=0 [OK]

Hint: Use 'aws sqs create-queue --queue-name' to create queues [OK]

Common Mistakes:

• Using wrong command verbs like 'new-queue' or 'make-queue'
• Missing '--queue-name' flag
• Incorrect attribute syntax

3. Given this code snippet using AWS SDK to receive messages from an SQS queue, what will be the output if the queue is empty?

const params = { QueueUrl: 'https://sqs.us-east-1.amazonaws.com/123456789012/MyQueue', MaxNumberOfMessages: 1 };
const data = await sqs.receiveMessage(params).promise();
console.log(data.Messages);

medium

A. []

B. null

C. undefined

D. An error is thrown

Solution

1. Step 1: Understand SQS receiveMessage behavior when empty

   If no messages are available, the 'Messages' property is not included in the response, so data.Messages is undefined.

2. Step 2: Check what console.log prints

   Logging undefined prints 'undefined' in JavaScript, not null or empty array. No error is thrown.

3. Final Answer:

   undefined -> Option C

4. Quick Check:

   Empty queue means data.Messages = undefined [OK]

Hint: Empty queue returns undefined Messages, not empty array [OK]

Common Mistakes:

• Assuming empty array is returned
• Expecting null instead of undefined
• Thinking an error occurs on empty queue

4. You set the VisibilityTimeout of an SQS queue to 30 seconds. A message is received and not deleted. What happens after 30 seconds?

medium

A. The message is deleted automatically

B. The message becomes visible again and can be received again

C. The message is moved to a dead-letter queue

D. The queue stops delivering messages

Solution

1. Step 1: Understand VisibilityTimeout effect

   VisibilityTimeout hides a message from other consumers for the set time after being received.

2. Step 2: What happens if message is not deleted after timeout?

   If not deleted, after 30 seconds the message becomes visible again for processing by others.

3. Final Answer:

   The message becomes visible again and can be received again -> Option B

4. Quick Check:

   VisibilityTimeout expiry = message visible again [OK]

Hint: VisibilityTimeout hides message temporarily, then shows it again [OK]

Common Mistakes:

• Thinking message deletes automatically after timeout
• Confusing dead-letter queue behavior
• Assuming queue stops working

5. You want to delay processing of messages by 10 minutes after they are sent to an SQS queue. Which configuration should you use?

hard

A. Set the VisibilityTimeout to 600 seconds

B. Set the ReceiveMessageWaitTimeSeconds to 600 seconds

C. Set the MessageRetentionPeriod to 600 seconds

D. Set the DelaySeconds attribute to 600 on the queue or per message

Solution

1. Step 1: Identify delay setting for message delivery

   DelaySeconds controls how long a message waits before becoming visible after sending.

2. Step 2: Compare other attributes

   VisibilityTimeout controls message invisibility after receipt, MessageRetentionPeriod controls how long messages stay in queue, ReceiveMessageWaitTimeSeconds controls long polling wait time.

3. Final Answer:

   Set the DelaySeconds attribute to 600 on the queue or per message -> Option D

4. Quick Check:

   DelaySeconds = message delivery delay [OK]

Hint: Use DelaySeconds to delay message visibility after sending [OK]

Common Mistakes:

• Confusing VisibilityTimeout with DelaySeconds
• Using MessageRetentionPeriod for delay
• Mistaking ReceiveMessageWaitTimeSeconds for delay