SQS queue concept in AWS - Step-by-Step Execution

Process Flow - SQS queue concept

Message sent by Producer

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Message stored in SQS Queue

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Message waits in queue until consumed

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Consumer polls queue

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Message received and processed

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Message deleted from queue

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End

Messages are sent by producers to the queue, stored until consumers poll and process them, then deleted after processing.

Execution Sample

AWS

SendMessage -> Queue stores message -> ReceiveMessage -> Process -> DeleteMessage

Shows the basic flow of sending, receiving, processing, and deleting a message in SQS.

Process Table

Step	Action	Queue State	Message Visibility	Result
1	Producer sends message	1 message added	Visible	Message stored in queue
2	Consumer polls queue	1 message in queue	Invisible	Message received by consumer
3	Consumer processes message	1 message invisible	Invisible	Processing in progress
4	Consumer deletes message	0 messages	N/A	Message removed from queue
5	Consumer polls again	0 messages	N/A	No messages to receive

💡 Queue empty, no messages left to process

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
Queue Message Count	0	1	1	1	0	0
Message Visibility	N/A	Visible	Invisible	Invisible	N/A	N/A

Key Moments - 3 Insights

Why does the message become invisible after the consumer polls it?

When the consumer polls the message (Step 2), it becomes invisible to other consumers to prevent duplicate processing, as shown in the execution_table row 2.

What happens if the consumer does not delete the message after processing?

If the message is not deleted, it becomes visible again after the visibility timeout, allowing other consumers to receive it, which can cause duplicate processing. This is implied between Steps 3 and 5.

Why does the queue message count remain 1 during processing?

The message stays in the queue until deleted, so during processing (Step 3), the count is still 1 but the message is invisible, as shown in the variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table at Step 2, what is the message visibility state?

A Deleted

B Visible

C Invisible

D Not in queue

💡 Hint

Check the 'Message Visibility' column at Step 2 in the execution_table.

At which step does the queue become empty?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look at the 'Queue State' column in the execution_table to find when message count is zero.

If the consumer never deletes the message, what would happen at Step 5?

A Message would become visible again

B Queue would be empty

C Message would still be invisible

D Message would be deleted automatically

💡 Hint

Refer to the key_moments explanation about message visibility timeout and deletion.

Concept Snapshot

SQS queue stores messages sent by producers.
Consumers poll messages, which become invisible during processing.
After processing, consumers delete messages to remove them.
If not deleted, messages reappear after visibility timeout.
This ensures reliable, at-least-once delivery.

Full Transcript

Amazon SQS is a message queue service where producers send messages that are stored until consumers poll them. When a consumer receives a message, it becomes invisible to others to avoid duplicate processing. The consumer processes the message and then deletes it from the queue. If the message is not deleted, it becomes visible again after a timeout, allowing reprocessing. This flow ensures messages are reliably delivered and processed.