Breaking the pub/sub docs out

This deletes the use cases README
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Aaron Schlesinger 2020-05-05 14:26:43 -07:00
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# Publish/Subscribe Messaging
Dapr enables developers to design their application with a pub/sub pattern using a message broker, where event consumers and producers are decoupled from one another, and communicate by sending and receiving messages that are associated with a namespace, usually in the form of topics.
The [publish/subscribe pattern](https://en.wikipedia.org/wiki/Publish%E2%80%93subscribe_pattern) allows your microservices to communicate with each other purely by sending messages. In this system, the **producer** of a message sends it to a **topic**, with no knowledge of what service will receive the message. A messages can even be sent if there's no consumer for it.
This allows event producers to send messages to consumers that aren't running, and consumers to receive messages based on subscriptions to topics.
Similarly, a **consumer** will receive messages from a topic without knowledge of what producer sent it. This pattern is especially useful when you need to decouple microservices from one another.
Dapr provides At-Least-Once messaging guarantees, and integrates with various message brokers implementations.
These implementations are pluggable, and developed outside of the Dapr runtime in [components-contrib](https://github.com/dapr/components-contrib/tree/master/pubsub).
Dapr provides a publish/subscribe API that provides at-least-once guarantees and integrates with various message brokers implementations. These implementations are pluggable, and developed outside of the Dapr runtime in [components-contrib](https://github.com/dapr/components-contrib/tree/master/pubsub).
## Publish/Subscribe API
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## Behavior and guarantees
Dapr guarantees At-Least-Once semantics for message delivery.
That is, when an application publishes a message to a topic using the Publish/Subscribe API, it can assume the message is delivered at least once to any subscriber when the response status code from that endpoint is `200`, or returns no error if using the gRPC client.
That means that when an application publishes a message to a topic using the Publish/Subscribe API, it can assume the message is delivered at least once to any subscriber when the response status code from that endpoint is `200`, or returns no error if using the gRPC client.
The burden of dealing with concepts like consumer groups and multiple instances inside consumer groups is all catered for by Dapr.

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# Dapr use cases
This document briefly describes scenarios that Dapr enables for your microservices. Each use case is accompanied with a link to learn more details.
## Pub/Sub
Dapr provides an API that enables event-driven architectures in your system. Each microservice can utilize the same API to leverage one of [many supported event buses](https://github.com/dapr/docs/tree/master/howto/setup-pub-sub-message-broker#reference). Microservices can communicate each other by sending or subscribing to _events_ on specific _topics_. By leveraging this communication style, your microservices remain flexible and loosely coupled.
Visit the following documents to learn more:
- [More details on pub/sub](https://github.com/dapr/docs/blob/master/concepts/publish-subscribe-messaging/README.md)
- [How to get started with pub/sub](https://github.com/dapr/docs/tree/master/howto/setup-pub-sub-message-broker)