SQS

Tldr

Using Amazon SQS, you can send, store, and receive messages between software components, without losing messages or requiring other services to be available. In Amazon SQS, an application sends messages into a queue.

A user or service retrieves a message from the queue, processes it, and then deletes it from the queue (as its a pull-based system).

SQS pull-based architecture

SQS will guarantee that a message is delivered at least once, but that message may be redelivered.

SQS queues only make an “attempt” to deliver messages in order (more or less a FIFO approach) but do not guarantee FIFO. If strict FIFO is needed, that option can be selected.

SNS vs SQS

• SNS manages notifications and SQS manages messages
• SNS is a push-based system while SQS is a pull-based system

Types of SQS

Standard (Default)

Amazon SQS offers standard as the default queue type. Standard queues support a nearly unlimited number of API calls per second, per API action (SendMessage, ReceiveMessage, or DeleteMessage). Standard queues support at-least-once message delivery.

Messages delivered out of order

However, occasionally (because of the highly distributed architecture that allows nearly unlimited throughput), more than one copy of a message might be delivered out-of-order. Standard queues provide best-effort ordering which ensures that messages are generally delivered in the same order as they're sent.

FIFO

FIFO (First-In-First-Out) queues have all the capabilities of the standard queues, but are designed to enhance messaging between applications when the order of operations and events is critical, or where duplicates can't be tolerated.

SNS vs SQS

There are some key distinctions between SNS and SQS:

• SNS is a pub/sub system, while SQS is a queueing system. You'd typically use SNS to send the same message to multiple consumers via topics. In comparison, in most scenarios, each message in an SQS queue is processed by only one consumer.
• With SQS, messages are delivered through a long polling (pull) mechanism, while SNS uses a push mechanism to immediately deliver(push) messages to subscribed endpoints.
• SNS is typically used for applications that need realtime notifications, while SQS is more suited for message processing use cases.
• SNS does not persist messages - it delivers them to subscribers that are present, and then deletes them. In comparison, SQS can persist messages (from 1 minute to 14 days).

Best Practices

• It is a best practice to set the visibility timeout on your SQS queue to 6x the Lambda function timeout to allow for retries if the function is getting throttled.
• Retries – Use the maxReceiveCount on the queue's policy to limit the number of times Lambda will retry to process a failed execution.

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