Add pub/sub messaging pattern

messaging/publish_subscribe.md

@@ -0,0 +1,80 @@
+Publish & Subscribe Messaging Pattern
+============
+Publish-Subscribe is a messaging pattern used to communicate messages between 
+different components without these components knowing anything about each other's identity.
+
+It is similar to the Observer behavioral design pattern. 
+The fundamental design principals of both Observer and Publish-Subscribe is the decoupling of
+those interested in being informed about `Event Messages` from the informer (Observers or Publishers).
+Meaning that you don't have to program the messages to be sent directly to specific receivers.
+
+To accomplish this, an intermediary, called a "message broker" or "event bus", 
+receives published messages, and then routes them on to subscribers.
+
+
+There are three components **messages**, **topics**, **users**.
+
+```go
+type Message struct {
+    // Contents
+}
+
+
+type Subscription struct {
+	ch chan<- Message
+
+	Inbox chan Message
+}
+
+func (s *Subscription) Publish(msg Message) error {
+	if _, ok := ch; !ok {
+		return errors.New("Topic has been closed")
+	}
+
+	ch <- msg
+
+	return nil
+}
+```
+
+```go
+type Topic struct {
+	Subscribers    []Session
+	MessageHistory []Message
+}
+
+func (t *Topic) Subscribe(uid uint64) (Subscription, error) {
+    // Get session and create one if it's the first
+
+    // Add session to the Topic & MessageHistory
+
+    // Create a subscription
+}
+
+func (t *Topic) Unsubscribe(Subscription) error {
+	// Implementation
+}
+
+func (t *Topic) Delete() error {
+	// Implementation
+}
+```
+
+```go
+type User struct {
+    ID uint64
+    Name string
+}
+
+type Session struct {
+    User User
+    Timestamp time.Time
+}
+```
+
+Improvements
+============
+Events can be published in a parallel fashion by utilizing stackless goroutines.
+
+Performance can be improved by dealing with straggler subscribers
+by using a buffered inbox and you stop sending events once the inbox is full.