Sending and receiving messages in AWS - Step-by-Step Execution

Process Flow - Sending and receiving messages

Create Queue

↓

Send Message to Queue

↓

Message Stored in Queue

↓

Receive Message from Queue

↓

Process Message

↓

Delete Message from Queue

This flow shows how a message is sent to a queue, stored, received, processed, and then deleted.

Execution Sample

AWS

aws sqs create-queue --queue-name MyQueue
aws sqs send-message --queue-url <url> --message-body "Hello"
aws sqs receive-message --queue-url <url>
aws sqs delete-message --queue-url <url> --receipt-handle <handle>

This sequence creates a queue, sends a message, receives it, and deletes it from the queue.

Process Table

Step	Action	Queue State	Message Content	Result
1	Create Queue 'MyQueue'	Empty	-	Queue created successfully
2	Send Message 'Hello'	Contains 1 message	Hello	Message sent and stored
3	Receive Message	Contains 1 message (invisible)	Hello	Message received for processing
4	Process Message	Contains 1 message (invisible)	Hello	Message processed
5	Delete Message	Empty	-	Message deleted from queue

💡 Message deleted, queue is empty, process complete

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 5
Queue Messages	0	1 (Hello)	1 (invisible Hello)	0
Message Visibility	N/A	Visible	Invisible (received)	Deleted

Key Moments - 2 Insights

Why does the message remain in the queue after receiving it?

When a message is received, it becomes invisible temporarily but is not deleted until explicitly removed (see Step 3 and Step 5 in execution_table).

What happens if the message is not deleted after processing?

If not deleted, the message becomes visible again after a timeout and can be received again, causing duplicate processing.

Visual Quiz - 3 Questions

Look at the execution_table, what is the queue state immediately after sending the message?

A Empty

B Contains 1 message

C Contains 1 invisible message

D Deleted

💡 Hint

Check Step 2 in the execution_table under 'Queue State'

At which step does the message become invisible to other receivers?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look at Step 3 in execution_table where message visibility changes

If the delete message step is skipped, what happens to the message?

A It is permanently removed

B It stays invisible forever

C It becomes visible again after timeout

D It duplicates automatically

💡 Hint

Refer to key_moments about message deletion and visibility timeout

Concept Snapshot

AWS SQS message flow:
1. Create a queue
2. Send message to queue (stored and visible)
3. Receive message (becomes invisible)
4. Process message
5. Delete message (removes from queue)
Message must be deleted to avoid reprocessing.

Full Transcript

This visual execution shows how AWS SQS handles sending and receiving messages. First, a queue is created. Then a message is sent and stored in the queue. When a receiver gets the message, it becomes invisible to others but remains in the queue until deleted. Processing happens while the message is invisible. Finally, deleting the message removes it permanently. If deletion is skipped, the message reappears after a timeout, risking duplicate processing.

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