120 lines
3.4 KiB
Markdown
120 lines
3.4 KiB
Markdown
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## Application
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A Koa application is not a 1-to-1 representation of an HTTP server,
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as one or more Koa applications may be mounted together to form larger
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applications, with a single HTTP server.
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The following is a useless Koa application bound to port `3000`:
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```js
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var koa = require('koa');
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var app = koa();
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app.listen(3000);
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```
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The `app.listen(...)` method is simply sugar for the following:
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```js
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var http = require('http');
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var koa = require('koa');
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var app = koa();
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http.createServer(app.callback()).listen(3000);
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```
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This means you can spin up the same application as both HTTP and HTTPS,
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or on multiple addresses:
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```js
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var http = require('http');
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var koa = require('koa');
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var app = koa();
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http.createServer(app.callback()).listen(3000);
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http.createServer(app.callback()).listen(3001);
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```
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### Settings
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Application settings are properties on the `app` instance, currently
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the following are supported:
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- `app.name` optionally give your application a name
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- `app.env` defaulting to the __NODE_ENV__ or "development"
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- `app.proxy` when true proxy header fields will be trusted
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- `app.subdomainOffset` offset of `.subdomains` to ignore [2]
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- `app.jsonSpaces` default JSON response spaces [2]
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- `app.outputErrors` output err.stack to stderr [false in "test" environment]
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### app.listen(...)
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Create and return an HTTP server, passing the given arguments to
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`Server#listen()`. These arguments are documented on [nodejs.org](http://nodejs.org/api/http.html#http_server_listen_port_hostname_backlog_callback).
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### app.callback()
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Return a callback function suitable for the `http.createServer()`
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method to handle a request.
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### app.use(function)
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Add the given middleware function to this application. See [Middleware](#middleware) for
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more information.
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## Handling Requests
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Koa requests are manipulated using a `Context` object containing both a Koa `Request` and `Response` object. For more information on these view:
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- [Context](context.md)
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- [Request](request.md)
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- [Response](response.md)
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## Error Handling
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By default outputs all errors to stderr unless __NODE_ENV__ is "test". To perform custom error-handling logic such as centralized logging you
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can add an "error" event listener:
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```js
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app.on('error', function(err){
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log.error('server error', err);
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});
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```
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If an error in the req/res cycle and it is _not_ possible to respond to the client, the `Context` instance is also passed:
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```js
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app.on('error', function(err){
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log.error('server error', err);
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});
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```
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When an error occurs _and_ it is still possible to respond to the client, aka no data has been written to the socket, Koa will respond
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appropriately with a 500 "Internal Server Error". In either case
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an app-level "error" is emitted for logging purposes.
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## Notes
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### HEAD Support
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Koa's upstream response middleware supports __HEAD__ for you,
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however expensive requests would benefit from custom handling. For
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example instead of reading a file into memory and piping it to the
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client, you may wish to `stat()` and set the `Content-*` header fields
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appropriately to bypass the read.
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### Socket Errors
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Node http servers emit a "clientError" event when a socket error occurs. You'll probably want to delegate this to your
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Koa handler by doing the following, in order to centralize
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logging:
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```js
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var app = koa();
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var srv = app.listen(3000);
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srv.on('clientError', function(err){
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app.emit('error', err);
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});
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```
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# License
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MIT
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