Sending and receiving messages in AWS - Commands & Configuration

Introduction

Sometimes apps need to talk to each other without being connected all the time. Sending and receiving messages lets apps pass information safely and reliably, even if one app is busy or offline.

When you want to send a task to another app to do later without waiting.

When you need to keep messages safe until the receiver is ready.

When you want to connect different parts of your system without them crashing each other.

When you want to handle many messages quickly and in order.

When you want to retry sending messages if something goes wrong.

Config File - sqs-queue.json

sqs-queue.json

{
  "QueueName": "example-queue",
  "Attributes": {
    "DelaySeconds": "0",
    "MessageRetentionPeriod": "86400"
  }
}

This JSON file creates a simple AWS SQS queue named example-queue. DelaySeconds sets no delay before messages are visible. MessageRetentionPeriod keeps messages for 1 day (86400 seconds) before deleting.

Commands

This command creates a new message queue named example-queue with no delay and messages kept for 1 day.

Terminal

aws sqs create-queue --queue-name example-queue --attributes DelaySeconds=0,MessageRetentionPeriod=86400

Expected Output

{"QueueUrl":"https://sqs.us-east-1.amazonaws.com/123456789012/example-queue"}

→

--queue-name - Sets the name of the queue to create.

→

--attributes - Sets queue properties like delay and retention.

This command sends a message with the text 'Hello from my app!' to the example-queue.

Terminal

aws sqs send-message --queue-url https://sqs.us-east-1.amazonaws.com/123456789012/example-queue --message-body "Hello from my app!"

Expected Output

{"MD5OfMessageBody":"5d41402abc4b2a76b9719d911017c592","MessageId":"f1a2b3c4-5678-90ab-cdef-EXAMPLE11111"}

→

--queue-url - Specifies the exact queue to send the message to.

→

--message-body - The content of the message being sent.

This command retrieves one message from the example-queue so your app can read it.

Terminal

aws sqs receive-message --queue-url https://sqs.us-east-1.amazonaws.com/123456789012/example-queue --max-number-of-messages 1

Expected Output

{"Messages":[{"MessageId":"f1a2b3c4-5678-90ab-cdef-EXAMPLE11111","ReceiptHandle":"AQEBwJnKyrHigUMZj6rYigCgxlaS3SLy0a...","MD5OfBody":"5d41402abc4b2a76b9719d911017c592","Body":"Hello from my app!"}]}

→

--queue-url - Specifies which queue to get messages from.

→

--max-number-of-messages - Limits how many messages to receive at once.

This command deletes the message from the queue after it has been processed, so it won't be received again.

Terminal

aws sqs delete-message --queue-url https://sqs.us-east-1.amazonaws.com/123456789012/example-queue --receipt-handle AQEBwJnKyrHigUMZj6rYigCgxlaS3SLy0a...

Expected Output

No output (command runs silently)

→

--queue-url - Specifies the queue where the message lives.

→

--receipt-handle - Unique identifier for the message to delete.

Key Concept

If you remember nothing else from this pattern, remember: messages are sent to a queue to wait safely until the receiver is ready to get and delete them.

Common Mistakes

Not deleting messages after receiving them

Messages stay in the queue and get delivered again, causing duplicate processing.

Always delete messages using their receipt handle after processing.

Using the wrong queue URL when sending or receiving messages

Commands fail or messages go to the wrong place.

Copy the exact queue URL from the create-queue output and reuse it.

Trying to receive messages immediately after sending without delay

Sometimes messages take a moment to appear, causing empty receives.

Wait a few seconds or retry receive-message if no messages appear.

Summary

Create an SQS queue to hold messages safely.

Send messages to the queue with send-message command.

Receive messages from the queue with receive-message command.

Delete messages after processing to avoid duplicates.

Practice

1. What is the main purpose of sending messages in a cloud message queue service like AWS SQS?

easy

A. To put data into a queue so other parts can use it later

B. To delete data permanently from the system

C. To directly update a database record

D. To create a new virtual machine

Solution

1. Step 1: Understand message sending in queues

   Sending messages means placing data into a queue for later use by other parts of the system.

2. Step 2: Identify the purpose of sending messages

   The purpose is to store data temporarily so it can be processed asynchronously, not to delete or update directly.

3. Final Answer:

   To put data into a queue so other parts can use it later -> Option A

4. Quick Check:

   Sending messages = storing data in queue [OK]

Hint: Sending messages means putting data in a queue for later use [OK]

Common Mistakes:

• Confusing sending with deleting data
• Thinking sending updates databases directly
• Assuming sending creates compute resources

2. Which AWS service is primarily used for sending and receiving messages in a queue?

easy

A. Amazon EC2

B. Amazon SQS

C. Amazon RDS

D. Amazon S3

Solution

1. Step 1: Identify AWS services for messaging

   Amazon SQS (Simple Queue Service) is designed for sending and receiving messages in queues.

2. Step 2: Differentiate from other services

   EC2 is for servers, RDS for databases, and S3 for storage, not messaging queues.

3. Final Answer:

   Amazon SQS -> Option B

4. Quick Check:

   Message queue service = Amazon SQS [OK]

Hint: SQS is AWS's queue service for messages [OK]

Common Mistakes:

• Choosing EC2 which is for servers
• Confusing S3 storage with messaging
• Selecting RDS which is a database

3. Consider this AWS CLI command to receive a message from an SQS queue:
aws sqs receive-message --queue-url https://sqs.us-east-1.amazonaws.com/123456789012/MyQueue --max-number-of-messages 1
What will this command do?

medium

A. Create a new queue named MyQueue

B. Send one message to the queue

C. Delete one message from the queue

D. Receive up to one message from the queue

Solution

1. Step 1: Analyze the AWS CLI command

   The command uses receive-message with --max-number-of-messages 1, which means it tries to get up to one message from the queue.

2. Step 2: Understand the effect of the command

   It does not send, delete, or create queues. It only retrieves messages for processing.

3. Final Answer:

   Receive up to one message from the queue -> Option D

4. Quick Check:

   receive-message = get messages [OK]

Hint: receive-message gets messages, send-message puts them [OK]

Common Mistakes:

• Thinking receive-message sends messages
• Confusing receive with delete
• Assuming it creates queues

4. You wrote this code snippet to receive messages from an AWS SQS queue but it returns no messages:

response = sqs.receive_message(QueueUrl=queue_url, MaxNumberOfMessages=5)
messages = response['Messages']
for msg in messages:
    print(msg['Body'])

What is the likely cause of the error?

medium

A. The response has no 'Messages' key if the queue is empty

B. MaxNumberOfMessages must be 1, not 5

C. The queue URL is incorrect or empty

D. You must delete messages before receiving

Solution

1. Step 1: Understand SQS receive_message response

   If the queue is empty, the response does not include the 'Messages' key, causing a KeyError when accessing it.

2. Step 2: Identify the error cause

   Not checking if 'Messages' exists before looping causes failure when no messages are returned.

3. Final Answer:

   The response has no 'Messages' key if the queue is empty -> Option A

4. Quick Check:

   Empty queue means no 'Messages' key [OK]

Hint: Check if 'Messages' key exists before using it [OK]

Common Mistakes:

• Assuming 'Messages' always exists
• Setting MaxNumberOfMessages incorrectly
• Thinking messages must be deleted before receiving

5. You want to build a system that sends messages to an AWS SQS queue and ensures each message is processed exactly once by a receiver. Which combination of features should you use?

hard

A. Standard queue with short polling and no message deletion

B. Standard queue with long polling and message deletion after processing

C. FIFO queue with deduplication and message deletion after processing

D. FIFO queue without deduplication and no message deletion

Solution

1. Step 1: Understand message processing guarantees

   Standard queues can deliver messages more than once; FIFO queues preserve order and support exactly-once processing with deduplication.

2. Step 2: Ensure messages are deleted after processing

   Deleting messages after processing prevents reprocessing the same message.

3. Step 3: Combine features for exactly-once processing

   Use FIFO queue with deduplication and delete messages after processing to guarantee exactly-once delivery.

4. Final Answer:

   FIFO queue with deduplication and message deletion after processing -> Option C

5. Quick Check:

   Exactly-once = FIFO + deduplication + delete [OK]

Hint: Use FIFO queue with deduplication and delete messages after processing [OK]

Common Mistakes:

• Using standard queue for exactly-once needs
• Not deleting messages after processing
• Ignoring deduplication feature