The publish/subscribe 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.
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 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.
The API for Publish/Subscribe can be found in the spec repo.
Dapr guarantees At-Least-Once semantics for message delivery.
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.
Dapr has the concept of an id. This is specified in Kubernetes using the dapr.io/app-id annotation and with the app-id flag using the Dapr CLI. Dapr requires an ID to be assigned to every application.
When multiple instances of the same application ID subscribe to a topic, Dapr will make sure to deliver the message to only one instance. If two different applications with different IDs subscribe to a topic, at least one instance in each application receives a copy of the same message.
Dapr follows the CloudEvents 1.0 Spec and wraps any payload sent to a topic inside a Cloud Events envelope.
The following fields from the Cloud Events spec are implemented with Dapr:
idsourcespecversiontypedatacontenttype(Optional)
Starting with Dapr v0.9, Dapr no longer wraps published content into CloudEvent if the published payload itself is already in CloudEvent format.
The following example shows an XML content in CloudEvent v1.0 serialized as JSON:
{
"specversion" : "1.0",
"type" : "xml.message",
"source" : "https://example.com/message",
"subject" : "Test XML Message",
"id" : "id-1234-5678-9101",
"time" : "2020-09-23T06:23:21Z",
"datacontenttype" : "text/xml",
"data" : "<note><to>User1</to><from>user2</from><message>hi</message></note>"
}