chat

README

Chat Example

This application shows how to use the epws package to implement a simple web chat application.

Running the example

The example requires a working Go development environment. The Getting Started page describes how to install the development environment.

Once you have Go up and running, you can download, build and run the example using the following commands.

$ go get github.com/elasticperch/epws
$ cd `go list -f '{{.Dir}}' github.com/elasticperch/epws/examples/chat`
$ go run *.go

To use the chat example, open http://localhost:8080/ in your browser.

Server

The server application defines two types, Client and Hub. The server creates an instance of the Client type for each websocket connection. A Client acts as an intermediary between the websocket connection and a single instance of the Hub type. The Hub maintains a set of registered clients and broadcasts messages to the clients.

The application runs one goroutine for the Hub and two goroutines for each Client. The goroutines communicate with each other using channels. The Hub has channels for registering clients, unregistering clients and broadcasting messages. A Client has a buffered channel of outbound messages. One of the client's goroutines reads messages from this channel and writes the messages to the websocket. The other client goroutine reads messages from the websocket and sends them to the hub.

Hub

The code for the Hub type is in hub.go. The application's main function starts the hub's run method as a goroutine. Clients send requests to the hub using the register, unregister and broadcast channels.

The hub registers clients by adding the client pointer as a key in the clients map. The map value is always true.

The unregister code is a little more complicated. In addition to deleting the client pointer from the clients map, the hub closes the clients's send channel to signal the client that no more messages will be sent to the client.

The hub handles messages by looping over the registered clients and sending the message to the client's send channel. If the client's send buffer is full, then the hub assumes that the client is dead or stuck. In this case, the hub unregisters the client and closes the websocket.

Client

The code for the Client type is in client.go.

The serveWs function is registered by the application's main function as an HTTP handler. The handler upgrades the HTTP connection to the WebSocket protocol, creates a client, registers the client with the hub and schedules the client to be unregistered using a defer statement.

Next, the HTTP handler starts the client's writePump method as a goroutine. This method transfers messages from the client's send channel to the websocket connection. The writer method exits when the channel is closed by the hub or there's an error writing to the websocket connection.

Finally, the HTTP handler calls the client's readPump method. This method transfers inbound messages from the websocket to the hub.

WebSocket connections support one concurrent reader and one concurrent writer. The application ensures that these concurrency requirements are met by executing all reads from the readPump goroutine and all writes from the writePump goroutine.

To improve efficiency under high load, the writePump function coalesces pending chat messages in the send channel to a single WebSocket message. This reduces the number of system calls and the amount of data sent over the network.

Frontend

The frontend code is in home.html.

On document load, the script checks for websocket functionality in the browser. If websocket functionality is available, then the script opens a connection to the server and registers a callback to handle messages from the server. The callback appends the message to the chat log using the appendLog function.

To allow the user to manually scroll through the chat log without interruption from new messages, the appendLog function checks the scroll position before adding new content. If the chat log is scrolled to the bottom, then the function scrolls new content into view after adding the content. Otherwise, the scroll position is not changed.

The form handler writes the user input to the websocket and clears the input field.