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A queue is a temporary repository for messages that are awaiting processing. 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. 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 any type of data in a fail-safe queue. Any other component can then later receive the data programmatically using the 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.
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 tradeoffs 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.
Be sure to read about distributed queues, which will help you understand how to design an application that works correctly with Amazon SQS. For more information, see Properties of Distributed Queues.
