103 lines
4.6 KiB
Markdown
103 lines
4.6 KiB
Markdown
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# Chat Example
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This application shows how to use use the
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[websocket](https://github.com/gorilla/websocket) package to implement a simple
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web chat application.
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## Running the example
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The example requires a working Go development environment. The [Getting
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Started](http://golang.org/doc/install) page describes how to install the
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development environment.
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Once you have Go up and running, you can download, build and run the example
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using the following commands.
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$ go get github.com/gorilla/websocket
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$ cd `go list -f '{{.Dir}}' github.com/gorilla/websocket/examples/chat`
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$ go run *.go
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To use the chat example, open http://localhost:8080/ in your browser.
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## Server
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The server application defines two types, `Client` and `Hub`. The server
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creates an instance of the `Client` type for each websocket connection. A
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`Client` acts as an intermediary between the websocket connection and a single
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instance of the `Hub` type. The `Hub` maintains a set of registered clients and
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broadcasts messages to the clients.
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The application runs one goroutine for the `Hub` and two goroutines for each
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`Client`. The goroutines communicate with each other using channels. The `Hub`
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has channels for registering clients, unregistering clients and broadcasting
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messages. A `Client` has a buffered channel of outbound messages. One of the
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client's goroutines reads messages from this channel and writes the messages to
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the websocket. The other client goroutine reads messages from the websocket and
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sends them to the hub.
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### Hub
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The code for the `Hub` type is in
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[hub.go](https://github.com/gorilla/websocket/blob/master/examples/chat/hub.go).
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The application's `main` function starts the hub's `run` method as a goroutine.
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Clients send requests to the hub using the `register`, `unregister` and
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`broadcast` channels.
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The hub registers clients by adding the client pointer as a key in the
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`clients` map. The map value is always true.
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The unregister code is a little more complicated. In addition to deleting the
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client pointer from the `clients` map, the hub closes the clients's `send`
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channel to signal the client that no more messages will be sent to the client.
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The hub handles messages by looping over the registered clients and sending the
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message to the client's `send` channel. If the client's `send` buffer is full,
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then the hub assumes that the client is dead or stuck. In this case, the hub
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unregisters the client and closes the websocket.
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### Client
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The code for the `Client` type is in [client.go](https://github.com/gorilla/websocket/blob/master/examples/chat/client.go).
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The `serveWs` function is registered by the application's `main` function as
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an HTTP handler. The handler upgrades the HTTP connection to the WebSocket
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protocol, creates a client, registers the client with the hub and schedules the
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client to be unregistered using a defer statement.
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Next, the HTTP handler starts the client's `writePump` method as a goroutine.
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This method transfers messages from the client's send channel to the websocket
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connection. The writer method exits when the channel is closed by the hub or
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there's an error writing to the websocket connection.
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Finally, the HTTP handler calls the client's `readPump` method. This method
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transfers inbound messages from the websocket to the hub.
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WebSocket connections [support one concurrent reader and one concurrent
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writer](https://godoc.org/github.com/gorilla/websocket#hdr-Concurrency). The
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application ensures that these concurrency requirements are met by executing
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all reads from the `readPump` goroutine and all writes from the `writePump`
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goroutine.
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To improve efficiency under high load, the `writePump` function coalesces
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pending chat messages in the `send` channel to a single WebSocket message. This
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reduces the number of system calls and the amount of data sent over the
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network.
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## Frontend
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The frontend code is in [home.html](https://github.com/gorilla/websocket/blob/master/examples/chat/home.html).
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On document load, the script checks for websocket functionality in the browser.
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If websocket functionality is available, then the script opens a connection to
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the server and registers a callback to handle messages from the server. The
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callback appends the message to the chat log using the appendLog function.
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To allow the user to manually scroll through the chat log without interruption
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from new messages, the `appendLog` function checks the scroll position before
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adding new content. If the chat log is scrolled to the bottom, then the
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function scrolls new content into view after adding the content. Otherwise, the
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scroll position is not changed.
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The form handler writes the user input to the websocket and clears the input
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field.
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