queue: add pubsub abstraction (#792)

* add pubsub abstraction

ROADMAP.md

@@ -45,9 +45,9 @@ This document defines the roadmap for Kratos development.
         - [ ] Streaming Handler
 - [ ] Cache
     - [ ] go-redis
-- [ ] Queue
+- [x] Pubsub
-    - [ ] Broker API
+    - [x] Absctraction
-    - [ ] Kafka
+    - [x] Kafka
     - [ ] Nats
 - [ ] Database
     - [ ] Ent

pubsub/README.md

@@ -0,0 +1,3 @@
+# Pubsub
+
+* [Kafka](https://github.com/go-kratos/kafka)

pubsub/pubsub.go

@@ -0,0 +1,39 @@
+package pubsub
+
+import (
+	"context"
+)
+
+// Message is an absctraction for all messages that
+// are sent to quque or received from queue.
+type Message struct {
+	Key    string
+	Value  []byte
+	Header map[string]string
+}
+
+// Event given to a subscription handler for processing.
+type Event interface {
+	Message() *Message
+	Ack() error
+	Nack() error
+}
+
+// Handler is a callback function that processes messages delivered
+// to asynchronous subscribers.
+type Handler func(context.Context, Event) error
+
+// Publisher is absctraction for sending messages
+// to queue.
+type Publisher interface {
+	Publish(ctx context.Context, msg *Message) error
+	PublishAsync(ctx context.Context, msg *Message, callback func(err error)) error
+	Close() error
+}
+
+// Subscriber is an absctraction for receiving messages
+// from queue.
+type Subscriber interface {
+	Subscribe(ctx context.Context, h Handler) error
+	Close() error
+}