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HR-ATS-Portal/frontend/node_modules/undici/docs/docs/api/Dispatcher.md

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Dispatcher

Stability: 2 - Stable

Dispatcher is the core abstraction used to dispatch HTTP requests in undici. It extends EventEmitter and defines the low-level dispatch() contract along with the higher-level request(), stream(), pipeline(), connect(), and upgrade() methods that every concrete dispatcher provides.

Dispatcher itself is abstract: its dispatch(), close(), and destroy() methods throw Error: not implemented. Concrete dispatchers such as Client, Pool, BalancedPool, and Agent implement the contract and are what application code instantiates. Requests are not guaranteed to be dispatched in the order in which they are invoked.

import { Dispatcher, Agent } from 'undici'

const dispatcher = new Agent()
console.log(dispatcher instanceof Dispatcher) // true

Class: Dispatcher

  • Extends: {EventEmitter}

Base dispatcher class. Concrete dispatchers extend it and implement the dispatch() method on which the other request helpers are built.

dispatcher.close([callback])

  • callback {Function} (optional) Invoked once the dispatcher is closed.
    • error {Error|null}
  • Returns: {Promise} A Promise is returned only when callback is omitted. It resolves with void once the dispatcher is closed.

Closes the dispatcher and gracefully waits for enqueued requests to complete before resolving or invoking callback.

import { createServer } from 'node:http'
import { Client } from 'undici'
import { once } from 'node:events'

const server = createServer((request, response) => {
  response.end('undici')
}).listen()

await once(server, 'listening')

const client = new Client(`http://localhost:${server.address().port}`)

try {
  const { body } = await client.request({ path: '/', method: 'GET' })
  body.setEncoding('utf8')
  body.on('data', console.log)
} catch (error) {}

await client.close()

console.log('Client closed')
server.close()

dispatcher.connect(options[, callback])

  • options {Object}
    • origin {string|URL} The origin to connect to.
    • path {string} The request path.
    • headers {UndiciHeaders} Request headers. Default: null.
    • signal {AbortSignal|EventEmitter|null} A signal used to abort the request. Default: null.
    • opaque {any} An opaque value passed through to the returned ConnectData.
    • responseHeaders {string|null} Set to 'raw' to return the response headers as a raw array instead of an object. Default: null.
  • callback {Function} (optional) Invoked when the connection is established.
    • error {Error|null}
    • data {Object}
      • statusCode {number}
      • headers {Object} The response headers.
      • socket {Duplex} The established socket.
      • opaque {any} The opaque value passed in options.
  • Returns: {Promise} A Promise is returned only when callback is omitted. It resolves with the data object described above.

Starts two-way communications with the requested resource using the HTTP CONNECT method.

import { createServer } from 'node:http'
import { Client } from 'undici'
import { once } from 'node:events'

const server = createServer((request, response) => {
  throw new Error('should never get here')
}).listen()

server.on('connect', (req, socket, head) => {
  socket.write('HTTP/1.1 200 Connection established\r\n\r\n')

  let data = head.toString()
  socket.on('data', (buf) => {
    data += buf.toString()
  })

  socket.on('end', () => {
    socket.end(data)
  })
})

await once(server, 'listening')

const client = new Client(`http://localhost:${server.address().port}`)

try {
  const { socket } = await client.connect({ path: '/' })
  const wanted = 'Body'
  let data = ''
  socket.on('data', d => { data += d })
  socket.on('end', () => {
    console.log(`Data received: ${data.toString()} | Data wanted: ${wanted}`)
    client.close()
    server.close()
  })
  socket.write(wanted)
  socket.end()
} catch (error) {}

dispatcher.destroy([error[, callback]])

  • error {Error|null} (optional) The error with which pending requests are rejected. Default: null.
  • callback {Function} (optional) Invoked once the socket is closed.
  • Returns: {Promise} A Promise is returned only when callback is omitted. It resolves with void once the dispatcher is destroyed.

Destroys the dispatcher abruptly with the given error. All pending and running requests are asynchronously aborted and rejected. Because the operation is dispatched asynchronously, dispatched requests may still make some progress before being aborted.

Both arguments are optional, so the method can be called in four ways:

dispatcher.destroy() // -> Promise
dispatcher.destroy(new Error()) // -> Promise
dispatcher.destroy(() => {}) // -> void
dispatcher.destroy(new Error(), () => {}) // -> void

import { createServer } from 'node:http'
import { Client } from 'undici'
import { once } from 'node:events'

const server = createServer((request, response) => {
  response.end()
}).listen()

await once(server, 'listening')

const client = new Client(`http://localhost:${server.address().port}`)

try {
  const request = client.request({ path: '/', method: 'GET' })
  client.destroy()
    .then(() => {
      console.log('Client destroyed')
      server.close()
    })
  await request
} catch (error) {
  console.error(error)
}

dispatcher.dispatch(options, handler)

  • options {Object}
    • origin {string|URL} (optional) The origin to dispatch the request to.
    • path {string} The request path.
    • method {string} The HTTP method, e.g. 'GET' or 'POST'.
    • body {string|Buffer|Uint8Array|Readable|FormData|null} The request body. Default: null.
    • headers {UndiciHeaders} Request headers. Default: null.
    • query {Object} Query string parameters embedded into the request URL. Both keys and values are encoded with encodeURIComponent. Default: null.
    • idempotent {boolean} Whether the request can be safely retried. If false, the request is not sent until all preceding requests in the pipeline have completed. Default: true when method is 'HEAD' or 'GET'.
    • blocking {boolean} Whether the response is expected to block the pipeline. When true, further pipelining on the same connection is avoided until headers have been received. Default: method !== 'HEAD'.
    • typeOfService {number|null} The IP Type of Service (ToS) value for the request socket. Must be an integer between 0 and 255. Default: 0.
    • upgrade {boolean|string|null} Upgrades the request, e.g. to 'Websocket'. Default: null, or the request method when it is 'CONNECT'.
    • headersTimeout {number|null} The time, in milliseconds, the parser waits to receive the complete HTTP headers. Defaults to 300 seconds.
    • bodyTimeout {number|null} The time, in milliseconds, after which the request times out while receiving body data. Monitors the time between body chunks. Use 0 to disable it entirely. Defaults to 300 seconds. HTTP/1.1 headers/body parser timeouts are not guaranteed to fire with exact millisecond precision: delays up to 1000ms use native timers, while larger delays use undici's lower-overhead fast timers with a target resolution around 500ms.
    • reset {boolean} Whether the request should establish a keep-alive connection. Default: false.
    • expectContinue {boolean} For HTTP/2, appends the expect: 100-continue header and halts the request body until a 100 Continue is received from the remote server. Default: false.
  • handler {Object} A set of callbacks invoked over the request lifecycle.
    • onRequestStart {Function} Invoked before the request is dispatched on the socket. May be invoked multiple times when the request is retried.
      • controller {DispatchController}
      • context {any}
    • onRequestUpgrade {Function} (optional) Invoked when the request is upgraded. Required when options.upgrade is set or options.method is 'CONNECT'.
      • controller {DispatchController}
      • statusCode {number}
      • headers {Object}
      • socket {Duplex}
    • onResponseStart {Function} (optional) Invoked when the status code and headers have been received. May be invoked multiple times for 1xx informational responses. Not used for upgrade requests.
      • controller {DispatchController}
      • statusCode {number}
      • headers {Object}
      • statusMessage {string} (optional)
    • onResponseData {Function} (optional) Invoked when response payload data is received. Not used for upgrade requests.
      • controller {DispatchController}
      • chunk {Buffer}
    • onResponseEnd {Function} (optional) Invoked when the response payload and trailers have been received and the request has completed. Not used for upgrade requests.
      • controller {DispatchController}
      • trailers {Object}
    • onResponseError {Function} (optional) Invoked when an error occurs. Must not throw.
      • controller {DispatchController}
      • error {Error}
    • onResponseStarted {Function} (optional) Invoked when the response is received, before headers have been read.
    • onBodySent {Function} (optional) Invoked when a chunk of the request body is sent. May be invoked multiple times for chunked requests.
      • chunk {Buffer}
    • onRequestSent {Function} (optional) Invoked after the request body is fully sent.
  • Returns: {boolean} false when the dispatcher is busy and further dispatch() calls will make no progress until the 'drain' event is emitted.

This is the low-level API on which all the higher-level methods are built. This API is expected to evolve through semver-major versions and is less stable than the higher-level methods. It is primarily intended for library developers who implement higher-level APIs on top of it.

The controller passed to each handler is a DispatchController with the following shape:

  • aborted {boolean} Whether the request has been aborted.
  • paused {boolean} Whether the request is paused.
  • reason {Error|null} The reason the request was aborted, if any.
  • rawHeaders {Buffer[]|string[]|Object|null} The raw response headers, preserving duplicates and casing.
  • rawTrailers {Buffer[]|string[]|Object|null} The raw response trailers.
  • abort(reason) Aborts the request with the given reason.
  • pause() Pauses the response stream.
  • resume() Resumes the response stream.

Call controller.pause() and controller.resume() to apply backpressure rather than returning false from a handler.

import { createServer } from 'node:http'
import { Client } from 'undici'
import { once } from 'node:events'

const server = createServer((request, response) => {
  response.end('Hello, World!')
}).listen()

await once(server, 'listening')

const client = new Client(`http://localhost:${server.address().port}`)

const data = []

client.dispatch({
  path: '/',
  method: 'GET',
  headers: { 'x-foo': 'bar' }
}, {
  onRequestStart: () => {
    console.log('Connected!')
  },
  onResponseError: (_controller, error) => {
    console.error(error)
  },
  onResponseStart: (_controller, statusCode, headers) => {
    console.log(`statusCode: ${statusCode} | headers: ${JSON.stringify(headers)}`)
  },
  onResponseData: (_controller, chunk) => {
    data.push(chunk)
  },
  onResponseEnd: (_controller, trailers) => {
    console.log(`trailers: ${JSON.stringify(trailers)}`)
    console.log(`Data: ${Buffer.concat(data).toString('utf8')}`)
    client.close()
    server.close()
  }
})

dispatcher.pipeline(options, handler)

  • options {Object} Accepts every field of dispatcher.request()'s options, plus:
    • objectMode {boolean} Set to true when handler returns an object stream. Default: false.
  • handler {Function} Returns a Readable from which the result is read.
    • data {Object}
      • statusCode {number}
      • headers {Object} The response headers.
      • opaque {any} The opaque value passed in options.
      • body {Readable} The response body.
      • context {Object}
    • Returns: {Readable}
  • Returns: {Duplex} A duplex stream that writes to the request and reads from the response.

For easy use with stream.pipeline(). The handler should return a Readable from which the result is read. Usually it returns the body directly, unless a transformation is needed based on, for example, headers or statusCode. The handler should validate the response and save any required state; if there is an error, it should be thrown.

import { Readable, Writable, PassThrough, pipeline } from 'node:stream'
import { createServer } from 'node:http'
import { Client } from 'undici'
import { once } from 'node:events'

const server = createServer((request, response) => {
  request.pipe(response)
}).listen()

await once(server, 'listening')

const client = new Client(`http://localhost:${server.address().port}`)

let res = ''

pipeline(
  new Readable({
    read () {
      this.push(Buffer.from('undici'))
      this.push(null)
    }
  }),
  client.pipeline({ path: '/', method: 'GET' }, ({ statusCode, headers, body }) => {
    console.log(`response received ${statusCode}`)
    console.log('headers', headers)
    return pipeline(body, new PassThrough(), () => {})
  }),
  new Writable({
    write (chunk, _, callback) {
      res += chunk.toString()
      callback()
    },
    final (callback) {
      console.log(`Response pipelined to writable: ${res}`)
      callback()
    }
  }),
  error => {
    if (error) {
      console.error(error)
    }
    client.close()
    server.close()
  }
)

dispatcher.request(options[, callback])

  • options {Object} Accepts every field of dispatcher.dispatch()'s options, plus:
    • opaque {any} A value passed through to the returned response data. Default: null.
    • signal {AbortSignal|EventEmitter|null} A signal used to abort the request. Default: null.
    • onInfo {Function|null} Invoked for each informational (1xx) response received. Default: null.
      • info {Object}
        • statusCode {number}
        • headers {Object}
    • responseHeaders {string|null} Set to 'raw' to return the response headers as a raw array. Default: null.
    • highWaterMark {number} The high water mark of the response body stream. Default: 65536 (64 KiB).
  • callback {Function} (optional) Invoked when the response is received.
    • error {Error|null}
    • data {Object}
      • statusCode {number}
      • statusText {string} The status message, e.g. 'OK' or 'Not Found'.
      • headers {Object} The response headers. All keys are lower-cased.
      • body {Readable} The response body, which also implements the body mixin from the Fetch Standard.
      • trailers {Object} Starts out empty and is mutated to contain the trailers after body emits 'end'.
      • opaque {any} The opaque value passed in options.
      • context {Object}
  • Returns: {Promise} A Promise is returned only when callback is omitted. It resolves with the data object described above.

Performs an HTTP request. options.method must not be 'CONNECT'.

Non-idempotent requests are not pipelined, to avoid indirect failures. Idempotent requests are automatically retried if they fail due to an indirect failure from the request at the head of the pipeline; this does not apply to idempotent requests with a stream request body. All response bodies must always be fully consumed or destroyed.

The body exposes the following body mixin methods and properties: .arrayBuffer(), .blob(), .bytes(), .json(), .text(), body, and bodyUsed. A body cannot be consumed twice; for example, calling text() after json() throws a TypeError. The body also provides dump({ limit }), which discards up to limit bytes (default 131072) without destroying the socket.

The body is always a Readable, even when empty. Deserializing an empty body with json() throws. To guard against this, verify the status code is not 204 and the content-type header starts with application/json before calling json().

import { createServer } from 'node:http'
import { Client } from 'undici'
import { once } from 'node:events'

const server = createServer((request, response) => {
  response.end('Hello, World!')
}).listen()

await once(server, 'listening')

const client = new Client(`http://localhost:${server.address().port}`)

try {
  const { body, headers, statusCode, statusText, trailers } = await client.request({
    path: '/',
    method: 'GET'
  })
  console.log(`response received ${statusCode}`)
  console.log('headers', headers)
  body.setEncoding('utf8')
  body.on('data', console.log)
  body.on('error', console.error)
  body.on('end', () => {
    console.log('trailers', trailers)
  })

  client.close()
  server.close()
} catch (error) {
  console.error(error)
}

A request can be aborted with an AbortSignal, with any EventEmitter that emits an 'abort' event, or by destroying the response body:

import { Client } from 'undici'

const client = new Client('http://localhost:3000')
const abortController = new AbortController()

try {
  client.request({ path: '/', method: 'GET', signal: abortController.signal })
} catch (error) {
  console.error(error) // RequestAbortedError
}

abortController.abort()

When the response body is conditionally read, always fully consume it otherwise:

const { body, statusCode } = await client.request({ path: '/', method: 'GET' })

if (statusCode === 200) {
  return await body.arrayBuffer()
}

await body.dump()

return null

dispatcher.stream(options, factory[, callback])

  • options {Object} Accepts the same fields as dispatcher.request()'s options.
  • factory {Function} Returns the Writable to which the response is written.
    • data {Object}
      • statusCode {number}
      • headers {Object} The response headers.
      • opaque {any} The opaque value passed in options.
      • context {Object}
    • Returns: {Writable}
  • callback {Function} (optional) Invoked when the request has completed.
    • error {Error|null}
    • data {Object}
      • opaque {any} The opaque value passed in options.
      • trailers {Object}
  • Returns: {Promise} A Promise is returned only when callback is omitted. It resolves with the data object described above.

A faster version of dispatcher.request(). The factory returns a Writable to which the response is written, which avoids creating an intermediate Readable when the caller intends to pipe the response body directly to a Writable.

Use options.opaque to avoid creating a closure for the factory method. This pattern works well with Node.js web frameworks such as Fastify.

import { createServer } from 'node:http'
import { Client } from 'undici'
import { once } from 'node:events'
import { Writable } from 'node:stream'

const server = createServer((request, response) => {
  response.end('Hello, World!')
}).listen()

await once(server, 'listening')

const client = new Client(`http://localhost:${server.address().port}`)

const bufs = []

try {
  await client.stream({
    path: '/',
    method: 'GET',
    opaque: { bufs }
  }, ({ statusCode, headers, opaque: { bufs } }) => {
    console.log(`response received ${statusCode}`)
    console.log('headers', headers)
    return new Writable({
      write (chunk, encoding, callback) {
        bufs.push(chunk)
        callback()
      }
    })
  })

  console.log(Buffer.concat(bufs).toString('utf-8'))

  client.close()
  server.close()
} catch (error) {
  console.error(error)
}

dispatcher.upgrade(options[, callback])

  • options {Object}
    • path {string} The request path.
    • method {string} The HTTP method. Default: 'GET'.
    • headers {UndiciHeaders} Request headers. Default: null.
    • protocol {string} A comma-separated list of protocols, in descending preference order. Default: 'Websocket'.
    • signal {AbortSignal|EventEmitter|null} A signal used to abort the request. Default: null.
    • responseHeaders {string|null} Set to 'raw' to return the response headers as a raw array. Default: null.
  • callback {Function} (optional) Invoked when the upgrade completes.
    • error {Error|null}
    • data {Object}
      • headers {Object} The response headers.
      • socket {Duplex} The upgraded socket.
      • opaque {any}
  • Returns: {Promise} A Promise is returned only when callback is omitted. It resolves with the data object described above.

Upgrades to a different protocol. See MDN: Protocol upgrade mechanism for more details.

import { createServer } from 'node:http'
import { Client } from 'undici'
import { once } from 'node:events'

const server = createServer((request, response) => {
  response.statusCode = 101
  response.setHeader('connection', 'upgrade')
  response.setHeader('upgrade', request.headers.upgrade)
  response.end()
}).listen()

await once(server, 'listening')

const client = new Client(`http://localhost:${server.address().port}`)

try {
  const { headers, socket } = await client.upgrade({ path: '/' })
  socket.on('end', () => {
    console.log(`upgrade: ${headers.upgrade}`) // upgrade: Websocket
    client.close()
    server.close()
  })
  socket.end()
} catch (error) {
  console.error(error)
  client.close()
  server.close()
}

dispatcher.compose(interceptors[, interceptor])

  • interceptors {Array|Function} An array of interceptor functions, or the first of several interceptors passed as separate arguments.
  • ...interceptor {Function} Additional interceptors when passed as separate arguments.
  • Returns: {Dispatcher} A new dispatcher that applies the interceptors over the current dispatcher's dispatch() method.

Composes a new dispatcher from the current dispatcher and the given interceptors. Each interceptor is a function that takes a dispatch method and returns a dispatch-like function with the same signature ((options, handler)).

The order of the interceptors matters: the last interceptor in the chain is the first to be called. Forking the chain of interceptors can lead to unexpected results.

compose([interceptor1, interceptor2, interceptor3])

Request flow:
Request -> interceptor3 -> interceptor2 -> interceptor1 -> dispatcher.dispatch
           (called first)  (called second) (called last)

import { Client, RedirectHandler } from 'undici'

const redirectInterceptor = dispatch => {
  return (opts, handler) => {
    const { maxRedirections } = opts

    if (!maxRedirections) {
      return dispatch(opts, handler)
    }

    const redirectHandler = new RedirectHandler(dispatch, maxRedirections, opts, handler)
    opts = { ...opts, maxRedirections: 0 } // Stop the sub-dispatcher from also redirecting.
    return dispatch(opts, redirectHandler)
  }
}

const client = new Client('http://localhost:3000').compose(redirectInterceptor)

await client.request({ path: '/', method: 'GET' })

Interceptors can be chained by calling compose() repeatedly:

import { Client } from 'undici'

const client = new Client('http://localhost:3000')
  .compose(redirectInterceptor)
  .compose(retryInterceptor)

await client.request({ path: '/', method: 'GET' })

For the full list of built-in interceptors provided by undici, see Interceptors. For an example of a custom interceptor that wraps handler callbacks, see Custom interceptors.

Event: 'connect'

  • origin {URL}
  • targets {Array}

Emitted when the dispatcher has connected to the origin.

Event: 'disconnect'

  • origin {URL}
  • targets {Array}
  • error {Error}

Emitted when the dispatcher has been disconnected from the origin.

For HTTP/2, this event is also emitted when the dispatcher receives a GOAWAY frame, with an error whose message is HTTP/2: "GOAWAY" frame received and whose code is UND_ERR_INFO. Because of the binary-framing nature of the protocol, a request may hang if a frame is received between the HEADER and DATA frames. It is recommended to handle this event and close the dispatcher to create a new HTTP/2 session.

Event: 'connectionError'

  • origin {URL}
  • targets {Array}
  • error {Error}

Emitted when the dispatcher fails to connect to the origin.

Event: 'drain'

  • origin {URL}

Emitted when the dispatcher is no longer busy and further dispatch() calls can make progress.

Pre-built interceptors

For the full reference of built-in interceptors (dump, retry, redirect, decompress, responseError, dns, cache, deduplicate) and their options, see Interceptors.