Queues

 Q what is the difference between SNS & SQS & Event Bridge?

• Abbreviation
• SNS stands for simple notification service.
• SQS stands for simple Queue service.
• Usage
• SNS uses a Publisher Subscriber system, we own a topic and we publish to that topic and subscribers get notified of events that are delivered to that topic.
• One to many fan out.
• High throughput.
• Many subscribers.
• SQS is a queuing service for message processing. SQS can be a subscriber to SNS. When somebody publishes a message to SNS our SQS will get a message that may be processed at a later time.
• Allows application owner to publish messages to a queue and be decouple application from one another.
• One of the oldest service from AWS.
• SQS has a synchronous communication.
• Temporary message holding pool.
• Ordered message processing.
• Event Bridge is an improved version of SNS. Event Bridge provides third-party integration, such as Shopify, Pager duty, data docs, etc.
• One too many.
• AWS, SQS, third party application integration.
• Components
• SQS has following components.
• Queues
• Queues can be created from console, CLI or infrastructure code.
• We publish messages to queue.
• They are temporary holding pools.
• A queue can be a FIFO queue or regular queue.
• By default SQS does not respect order of messages. We need to create FIFO queue for it.
• Messages
• Messages are a raw data, Json blob.
• We can subscribe messages one by one or in groups.
• As a message is subscribed by consumer. It is hidden from other consumers.
• Polling
• Polling is a methodology via which application which wants to receive the message interacts with queue and processes them.
• SNS has following components.
• Topics
• Topics are not holding pools for messages.
• As soon as the message is published, in a topic, the topic delivers an identical copy of messages to all the subscribers.
• There may be a failure scenario if one of the service is down this can be handled by adding a SQS between SNS, and receiving service.
• once a message is delivered, it disappears from the topic.
• Messages
• Messages are same as in SQS I.e json blobs.
• Publish/Subscribe(PubSub)
• owners publishes data into topic and receiver who has subscribed to topic receive data.
• Event bridge has following components.
• Message bus
• Message bus is same as topic, we publish the event bus.
• Message filtering allows us to filter messages. For example, we may have a target that receives delivered messages.
• Events 
• Events can be pending, shipped , or delivered events.
• Can be constructed by applications, or emitted by AWS services like EC2.
• We can integrate events with third-party providers like SAP, etc.
• Rules
• Rules, match the coming events and match them to the corresponding targets.
• For a particular rule we can have maximum of five targets.
• Targets
• Targets are destination, endpoints what will be invoked on an event, etc.
• Framework
• Publishing a message to a topic can deliver to many subscribers. It uses fanout framework. The topic may be delivered to different types of services like SQS, lambda, email.
• In SQS The system must poll the Queue to discover new events. So whenever an event is delivered to Queue there is no notification. There should be a mechanism of pulling the queue for new events, process the events and deleting the events. This can be done by a separate thread.
• Consumers
• Messages in an SQS queue are typically processed by a single consumer or a single service with a narrow responsibility and different services if they care about same events they can have their own separate queues.
• Messages in SNS are broadcasted to all those consumers Who have subscribed to topic.
• When to use
• If other systems care about event we should use SNS.
• If our system gives about event we should use SQS.
• Use case
• When we buy something online our transaction processing service may send event to SNS.
• The SNS invokes lambda which sends an email back to the customer.
• The SNS may store order success info in Dynamo DB.
• We may publish same event in analytics queue which is SQS.
• Then our SQS will be polled by a service.
• So if a message is sent from SNS to SQS it will be there guaranteed and we will not loose it.
• Example
• SQS
• Auto service creates/updates, orders, and publishes them to Queue.
• Analytics service may poll the queue at regular intervals to subscribe messages and check the status of the order.
• SNS
• Order service publishes message to the topic in queue.
• All the services who have subscribed to topic in queue, get the message, such as accounting service, analytics service, order dashboard service, etc.