SQS vs. SNS

Loose coupling with Amazon Simple Queue Service

Amazon Simple Queue Service (SQS) is a fully managed message queuing service that use use to you to decouple and scale microservices, distributed systems, and serverless applications The service works on a massive scale, processing billions of messages per day. It stores all message queues and messages within a single, highly available AWS Region with multiple redundant Availability Zones. This ensures that no single computer, network, or Availability Zone failure can make messages inaccessible. Messages can be sent and read simultaneously.

A loosely coupled workload involves processing a large number of smaller jobs. The loss of one node or job in a loosely coupled workload usually doesn’t delay the entire calculation. The lost work can be picked up later or omitted altogether.

With Amazon SQS, you can decouple pre-processing steps from compute steps and post-processing steps. Building applications from individual components that perform discrete functions improves scalability and reliability. Decoupling components is a best practice for designing modern applications. Amazon SQS frequently lies at the heart of cloud-native loosely coupled solutions.

SQS queue types

Amazon SQS offers two types of message queues

STANDARD QUEUES

Standard queues support at-least-once message delivery and provide best-effort ordering. Messages are generally delivered in the same order in which they are sent. However, because of the highly distributed architecture, more than one copy of a message might be delivered out of order. Standard queues can handle a nearly unlimited number of API calls per second. You can use standard message queues if your application can process messages that arrive repetitively and out of order.

FIFO QUEUES

FIFO (First-In-First-Out) queues are designed to enhance messaging between applications when the order of operations and events is critical or where duplicates can’t be tolerated. FIFO queues also provide exactly-once processing but have a limited number of API calls per second. FIFO queues are designed to enhance messaging between applications when the order of operations and events is critical.

Optimizing your Amazon SQS queue configurations

When creating an Amazon SQS queue, you need to consider how your application interacts with the queue. This information will help you optimize the configuration of your queue to control costs and increase performance.

TUNE YOUR VISIBILITY TIMEOUT

When a consumer receives an SQS message, that message remains in the queue until the consumer deletes it. You can configure the SQS queue’s visibility timeout setting to make that message invisible to other consumers for a period of time. This helps to prevent another consumer from processing the same message. The default visibility timeout is 30 seconds. The consumer deletes the message once it completes processing the message. If the consumer fails to delete the message before the visibility timeout expires, it becomes visible to other consumers and can be processed again. 

Typically, you should set the visibility timeout to the maximum time that it takes your application to process and delete a message from the queue. Setting too short of a timeout increases the possibility of your application processing a message twice. Too long of a visibility timeout delays subsequent attempts at processing a message.

CHOOSE THE RIGHT POLLING TYPE

You can configure an Amazon SQS queue to use either short polling or long polling. Queues with short polling:

• Send a response to the consumer immediately after receiving a request providing a faster response
• Increases the number of responses and therefore costs. 

SQS queues with long polling:

• Do not return a response until at least one message arrives or the poll times out.
• Less frequent responses but decreases costs.

Depending on the frequency of messages arriving in your queue, many of the responses from a queue using short polling could just be reporting an empty queue. Unless your application requires an immediate response to its poll requests, long polling is the preferable option.

Amazon SNS

Amazon SNS is a web service that makes it easy to set up, operate, and send notifications from the cloud. The service follows the publish-subscribe (pub-sub) messaging paradigm, with notifications being delivered to clients using a push mechanism.

Amazon SNS publisher to multiple SQS queues

Using highly available services, such as Amazon SNS, to perform basic message routing is an effective way of distributing messages to microservices. The two main forms of communications between microservices are request-response, and observer. In the example, an observer type is used to fan out orders to two different SQS queues based on the order type. 

To deliver Amazon SNS notifications to an SQS queue, you subscribe to a topic specifying Amazon SQS as the transport and a valid SQS queue as the endpoint. To permit the SQS queue to receive notifications from Amazon SNS, the SQS queue owner must subscribe the SQS queue to the topic for Amazon SNS. If the user owns the Amazon SNS topic being subscribed to and the SQS queue receiving the notifications, nothing else is required. Any message published to the topic will automatically be delivered to the specified SQS queue. If the owner of the SQS queue is not the owner of the topic, Amazon SNS requires an explicit confirmation to the subscription request.

Amazon SNS and Amazon SQS

Features	Amazon SNS	Amazon SQS
Message persistence	No	Yes
Delivery mechanism	Push (passive)	Poll (active)
Producer and consumer	Publisher and subscriber	Send or receive
Distribution model	One to many	One to one

Regards

Osama