Amazon SQS is a web service that gives you access to a message queue that can be used to store messages while waiting for a computer to process them.

So we have an example of a web application may be you upload an image file to this web application and then what that application will do is load the SQS that a user has uploaded it an image file and that job needs to be executed on its stored that message on the SQS system.

Amazon SQS is a distributed queue system that enables web service applications to quickly and reliably queue messages that one component in the application generates to be consumed by another component. A queue is a temporary repository for messages that are awaiting processing.

Using Amazon SQS, you can decouple the components of an application so they run independently, with Amazon SQS easing message management between components. Any component of a distributed application can store messages in a fail-safe queue. Messages can contain up to 256 KB of text in any format. Any component can later retrieve the messages pro-grammatically using the Amazon SQS API.

The queue acts as a buffer between the component producing and saving data, and the component receiving the data for processing. This means the queue resolves issues that arise if the producer is producing work faster than the consumer can process it, or if the producer or consumer are only intermittently connected to the network.

Amazon SQS ensures delivery of each message at least once, and supports multiple readers and writers interacting with the same queue. A single queue can be used simultaneously by many distributed application components, with no need for those components to coordinate with each other to share the queue.

Amazon SQS is engineered to always be available and deliver messages. One of the resulting trade offs is that SQS does not guarantee first in, first out delivery of messages. For many distributed applications, each message can stand on its own, and as long as all messages are delivered, the order is not important. If your system requires that order be preserved, you can place sequencing information in each message, so that you can reorder the messages when the queue returns them.

To illustrate, suppose you have a number of image files to encode. In an Amazon SQS message for each file specifying the command (jpeg-encode) and the location of the file in Amazon S3. A pool of Amazon EC2 instances running the needs image processing software does the following:

1. Asynchronously pulls the task messages from the queue so that go in and pull the messages from queue. SQS always pulls the messages never the message queue never pushed out you have got EC2 instances constantly polling the queue trying to polling the data down.
2. Polling the messages from the queue then Retrieves the filename of the image.
3. Processes the conversion so might apply watermark.
4. Writes the image back to Amazon S3
5. Writes a “task complete” message to another queue
6. Deletes the original task message
7. Checks for more messages in the worker queue
8. 30 Seconds visibility Time Out by Default. Maximum of 12 hours.

SQS Pricing

1. First 1 million Amazon SQS Requests per month are free.
2. $0.50 per 1 million Amazon SQS Requests per month thereafter ($0.00000050 per SQS Request).
3. A single request can have from 1 to 10 messages, up to a maximum total payload of 256KB.
4. Each 64KB ‘chunk’ of payload is billed as 1 request. For example, a single API call with a 256KB payload will be billed as four requests.