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Dispatcher

Extends: events.EventEmitter

Dispatcher is the core API used to dispatch requests.

Requests are not guaranteed to be dispatched in order of invocation.

Instance Methods

`Dispatcher.close([callback]): Promise`

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

Arguments:

callback (error: Error | null, data: null) => void (optional)

Returns: void | Promise<null> - Only returns a Promise if no callback argument was passed

dispatcher.close() // -> Promise
dispatcher.close(() => {}) // -> void

Example - Request resolves before Client closes

import { createServer } from 'http'
import { Client } from 'undici'
import { once } from '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])`

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

Arguments:

options ConnectOptions
callback (err: Error | null, data: ConnectData | null) => void (optional)

Returns: void | Promise<ConnectData> - Only returns a Promise if no callback argument was passed

Parameter: `ConnectOptions`

path string
headers UndiciHeaders (optional) - Default: null
signal AbortSignal | events.EventEmitter | null (optional) - Default: null
opaque unknown (optional) - This argument parameter is passed through to ConnectData

Parameter: `ConnectData`

statusCode number
headers Record<string, string | string[] | undefined>
socket stream.Duplex
opaque unknown

Example - Connect request with echo

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

const server = createServer((request, response) => {
  throw 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]): Promise`

Destroy the dispatcher abruptly with the given error. All the pending and running requests will be asynchronously aborted and error. Since this operation is asynchronously dispatched there might still be some progress on dispatched requests.

Both arguments are optional; the method can be called in four different ways:

Arguments:

error Error | null (optional)
callback (error: Error | null, data: null) => void (optional)

Returns: void | Promise<void> - Only returns a Promise if no callback argument was passed

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

Example - Request is aborted when Client is destroyed

import { createServer } from 'http'
import { Client } from 'undici'
import { once } from '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)`

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

Arguments:

options DispatchOptions
handler DispatchHandler

Returns: Boolean - false if dispatcher is busy and further dispatch calls won't make any progress until the 'drain' event has been emitted.

Parameter: `DispatchOptions`

origin string | URL
path string
method string
reset boolean (optional) - Default: false - If false, the request will attempt to create a long-living connection by sending the connection: keep-alive header,otherwise will attempt to close it immediately after response by sending connection: close within the request and closing the socket afterwards.
body string | Buffer | Uint8Array | stream.Readable | Iterable | AsyncIterable | null (optional) - Default: null
headers UndiciHeaders | string[] (optional) - Default: null.
query Record<string, any> | null (optional) - Default: null - Query string params to be embedded in the request URL. Note that both keys and values of query are encoded using encodeURIComponent. If for some reason you need to send them unencoded, embed query params into path directly instead.
idempotent boolean (optional) - Default: true if method is 'HEAD' or 'GET' - Whether the requests can be safely retried or not. If false the request won't be sent until all preceding requests in the pipeline has completed.
blocking boolean (optional) - Default: false - Whether the response is expected to take a long time and would end up blocking the pipeline. When this is set to true further pipelining will be avoided on the same connection until headers have been received.
upgrade string | null (optional) - Default: null - Upgrade the request. Should be used to specify the kind of upgrade i.e. 'Websocket'.
bodyTimeout number | null (optional) - The timeout after which a request will time out, in milliseconds. Monitors time between receiving body data. Use 0 to disable it entirely. Defaults to 300 seconds.
headersTimeout number | null (optional) - The amount of time the parser will wait to receive the complete HTTP headers while not sending the request. Defaults to 300 seconds.
throwOnError boolean (optional) - Default: false - Whether Undici should throw an error upon receiving a 4xx or 5xx response from the server.

Parameter: `DispatchHandler`

onConnect (abort: () => void, context: object) => void - Invoked before request is dispatched on socket. May be invoked multiple times when a request is retried when the request at the head of the pipeline fails.
onError (error: Error) => void - Invoked when an error has occurred. May not throw.
onUpgrade (statusCode: number, headers: Buffer[], socket: Duplex) => void (optional) - Invoked when request is upgraded. Required if DispatchOptions.upgrade is defined or DispatchOptions.method === 'CONNECT'.
onHeaders (statusCode: number, headers: Buffer[], resume: () => void, statusText: string) => boolean - Invoked when statusCode and headers have been received. May be invoked multiple times due to 1xx informational headers. Not required for upgrade requests.
onData (chunk: Buffer) => boolean - Invoked when response payload data is received. Not required for upgrade requests.
onComplete (trailers: Buffer[]) => void - Invoked when response payload and trailers have been received and the request has completed. Not required for upgrade requests.
onBodySent (chunk: string | Buffer | Uint8Array) => void - Invoked when a body chunk is sent to the server. Not required. For a stream or iterable body this will be invoked for every chunk. For other body types, it will be invoked once after the body is sent.

Example 1 - Dispatch GET request

import { createServer } from 'http'
import { Client } from 'undici'
import { once } from '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'
  }
}, {
  onConnect: () => {
    console.log('Connected!')
  },
  onError: (error) => {
    console.error(error)
  },
  onHeaders: (statusCode, headers) => {
    console.log(`onHeaders | statusCode: ${statusCode} | headers: ${headers}`)
  },
  onData: (chunk) => {
    console.log('onData: chunk received')
    data.push(chunk)
  },
  onComplete: (trailers) => {
    console.log(`onComplete | trailers: ${trailers}`)
    const res = Buffer.concat(data).toString('utf8')
    console.log(`Data: ${res}`)
    client.close()
    server.close()
  }
})

Example 2 - Dispatch Upgrade Request

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

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

await once(server, 'listening')

server.on('upgrade', (request, socket, head) => {
  console.log('Node.js Server - upgrade event')
  socket.write('HTTP/1.1 101 Web Socket Protocol Handshake\r\n')
  socket.write('Upgrade: WebSocket\r\n')
  socket.write('Connection: Upgrade\r\n')
  socket.write('\r\n')
  socket.end()
})

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

client.dispatch({
  path: '/',
  method: 'GET',
  upgrade: 'websocket'
}, {
  onConnect: () => {
    console.log('Undici Client - onConnect')
  },
  onError: (error) => {
    console.log('onError') // shouldn't print
  },
  onUpgrade: (statusCode, headers, socket) => {
    console.log('Undici Client - onUpgrade')
    console.log(`onUpgrade Headers: ${headers}`)
    socket.on('data', buffer => {
      console.log(buffer.toString('utf8'))
    })
    socket.on('end', () => {
      client.close()
      server.close()
    })
    socket.end()
  }
})

Example 3 - Dispatch POST request

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

const server = createServer((request, response) => {
  request.on('data', (data) => {
    console.log(`Request Data: ${data.toString('utf8')}`)
    const body = JSON.parse(data)
    body.message = 'World'
    response.end(JSON.stringify(body))
  })
}).listen()

await once(server, 'listening')

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

const data = []

client.dispatch({
  path: '/',
  method: 'POST',
  headers: {
    'content-type': 'application/json'
  },
  body: JSON.stringify({ message: 'Hello' })
}, {
  onConnect: () => {
    console.log('Connected!')
  },
  onError: (error) => {
    console.error(error)
  },
  onHeaders: (statusCode, headers) => {
    console.log(`onHeaders | statusCode: ${statusCode} | headers: ${headers}`)
  },
  onData: (chunk) => {
    console.log('onData: chunk received')
    data.push(chunk)
  },
  onComplete: (trailers) => {
    console.log(`onComplete | trailers: ${trailers}`)
    const res = Buffer.concat(data).toString('utf8')
    console.log(`Response Data: ${res}`)
    client.close()
    server.close()
  }
})

`Dispatcher.pipeline(options, handler)`

For easy use with stream.pipeline. The handler argument should return a Readable from which the result will be read. Usually it should just return the body argument unless some kind of transformation needs to be performed based on e.g. headers or statusCode. The handler should validate the response and save any required state. If there is an error, it should be thrown. The function returns a Duplex which writes to the request and reads from the response.

Arguments:

options PipelineOptions
handler (data: PipelineHandlerData) => stream.Readable

Returns: stream.Duplex

Parameter: PipelineOptions

Extends: RequestOptions

objectMode boolean (optional) - Default: false - Set to true if the handler will return an object stream.

Parameter: PipelineHandlerData

statusCode number
headers Record<string, string | string[] | undefined>
opaque unknown
body stream.Readable
context object
onInfo ({statusCode: number, headers: Record<string, string | string[]>}) => void | null (optional) - Default: null - Callback collecting all the info headers (HTTP 100-199) received.

Example 1 - Pipeline Echo

import { Readable, Writable, PassThrough, pipeline } from 'stream'
import { createServer } from 'http'
import { Client } from 'undici'
import { once } from '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])`

Performs a HTTP request.

Non-idempotent requests will not be pipelined in order to avoid indirect failures.

Idempotent requests will be automatically retried if they fail due to 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.

Arguments:

options RequestOptions
callback (error: Error | null, data: ResponseData) => void (optional)

Returns: void | Promise<ResponseData> - Only returns a Promise if no callback argument was passed.

Parameter: `RequestOptions`

Extends: DispatchOptions

opaque unknown (optional) - Default: null - Used for passing through context to ResponseData.
signal AbortSignal | events.EventEmitter | null (optional) - Default: null.
onInfo ({statusCode: number, headers: Record<string, string | string[]>}) => void | null (optional) - Default: null - Callback collecting all the info headers (HTTP 100-199) received.

The RequestOptions.method property should not be value 'CONNECT'.

Parameter: `ResponseData`

statusCode number
headers Record<string, string | string[]> - Note that all header keys are lower-cased, e. g. content-type.
body stream.Readable which also implements the body mixin from the Fetch Standard.
trailers Record<string, string> - This object starts out as empty and will be mutated to contain trailers after body has emitted 'end'.
opaque unknown
context object

body contains the following additional body mixin methods and properties:

text()
json()
arrayBuffer()
body
bodyUsed

body can not be consumed twice. For example, calling text() after json() throws TypeError.

body contains the following additional extensions:

dump({ limit: Integer }), dump the response by reading up to limit bytes without killing the socket (optional) - Default: 262144.

Note that body will still be a Readable even if it is empty, but attempting to deserialize it with json() will result in an exception. Recommended way to ensure there is a body to deserialize is to check if status code is not 204, and content-type header starts with application/json.

Example 1 - Basic GET Request

import { createServer } from 'http'
import { Client } from 'undici'
import { once } from '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, 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('end', () => {
    console.log('trailers', trailers)
  })

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

Example 2 - Aborting a request

Node.js v15+ is required to run this example

import { createServer } from 'http'
import { Client } from 'undici'
import { once } from '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 abortController = new AbortController()

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

abortController.abort()

Alternatively, any EventEmitter that emits an 'abort' event may be used as an abort controller:

import { createServer } from 'http'
import { Client } from 'undici'
import EventEmitter, { once } from '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 ee = new EventEmitter()

try {
  client.request({
    path: '/',
    method: 'GET',
    signal: ee
  })
} catch (error) {
  console.error(error) // should print an RequestAbortedError
  client.close()
  server.close()
}

ee.emit('abort')

Destroying the request or response body will have the same effect.

import { createServer } from 'http'
import { Client } from 'undici'
import { once } from '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 } = await client.request({
    path: '/',
    method: 'GET'
  })
  body.destroy()
} catch (error) {
  console.error(error) // should print an RequestAbortedError
  client.close()
  server.close()
}

`Dispatcher.stream(options, factory[, callback])`

A faster version of Dispatcher.request. This method expects the second argument factory to return a stream.Writable stream which the response will be written to. This improves performance by avoiding creating an intermediate stream.Readable stream when the user expects to directly pipe the response body to a stream.Writable stream.

As demonstrated in Example 1 - Basic GET stream request, it is recommended to use the option.opaque property to avoid creating a closure for the factory method. This pattern works well with Node.js Web Frameworks such as Fastify. See Example 2 - Stream to Fastify Response for more details.

Arguments:

options RequestOptions
factory (data: StreamFactoryData) => stream.Writable
callback (error: Error | null, data: StreamData) => void (optional)

Returns: void | Promise<StreamData> - Only returns a Promise if no callback argument was passed

Parameter: `StreamFactoryData`

statusCode number
headers Record<string, string | string[] | undefined>
opaque unknown
onInfo ({statusCode: number, headers: Record<string, string | string[]>}) => void | null (optional) - Default: null - Callback collecting all the info headers (HTTP 100-199) received.

Parameter: `StreamData`

opaque unknown
trailers Record<string, string>
context object

Example 1 - Basic GET stream request

import { createServer } from 'http'
import { Client } from 'undici'
import { once } from 'events'
import { Writable } from '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)
}

Example 2 - Stream to Fastify Response

In this example, a (fake) request is made to the fastify server using fastify.inject(). This request then executes the fastify route handler which makes a subsequent request to the raw Node.js http server using undici.dispatcher.stream(). The fastify response is passed to the opaque option so that undici can tap into the underlying writable stream using response.raw. This methodology demonstrates how one could use undici and fastify together to create fast-as-possible requests from one backend server to another.

import { createServer } from 'http'
import { Client } from 'undici'
import { once } from 'events'
import fastify from 'fastify'

const nodeServer = createServer((request, response) => {
  response.end('Hello, World! From Node.js HTTP Server')
}).listen()

await once(nodeServer, 'listening')

console.log('Node Server listening')

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

const fastifyServer = fastify()

fastifyServer.route({
  url: '/',
  method: 'GET',
  handler: (request, response) => {
    nodeServerUndiciClient.stream({
      path: '/',
      method: 'GET',
      opaque: response
    }, ({ opaque }) => opaque.raw)
  }
})

await fastifyServer.listen()

console.log('Fastify Server listening')

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

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

  console.log(`response received ${statusCode}`)
  body.setEncoding('utf8')
  body.on('data', console.log)

  nodeServerUndiciClient.close()
  fastifyServerUndiciClient.close()
  fastifyServer.close()
  nodeServer.close()
} catch (error) { }

`Dispatcher.upgrade(options[, callback])`

Upgrade to a different protocol. Visit MDN - HTTP - Protocol upgrade mechanism for more details.

Arguments:

options UpgradeOptions
callback (error: Error | null, data: UpgradeData) => void (optional)

Returns: void | Promise<UpgradeData> - Only returns a Promise if no callback argument was passed

Parameter: `UpgradeOptions`

path string
method string (optional) - Default: 'GET'
headers UndiciHeaders (optional) - Default: null
protocol string (optional) - Default: 'Websocket' - A string of comma separated protocols, in descending preference order.
signal AbortSignal | EventEmitter | null (optional) - Default: null

Parameter: `UpgradeData`

headers http.IncomingHeaders
socket stream.Duplex
opaque unknown

Example 1 - Basic Upgrade Request

import { createServer } from 'http'
import { Client } from 'undici'
import { once } from '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()
}

Instance Events

Event: `'connect'`

Parameters:

origin URL
targets Array<Dispatcher>

Event: `'disconnect'`

Parameters:

origin URL
targets Array<Dispatcher>
error Error

Event: `'connectionError'`

Parameters:

origin URL
targets Array<Dispatcher>
error Error

Emitted when dispatcher fails to connect to origin.

Event: `'drain'`

Parameters:

origin URL

Emitted when dispatcher is no longer busy.

Parameter: `UndiciHeaders`

Record<string, string | string[] | undefined> | string[] | null

Header arguments such as options.headers in Client.dispatch can be specified in two forms; either as an object specified by the Record<string, string | string[] | undefined> (IncomingHttpHeaders) type, or an array of strings. An array representation of a header list must have an even length or an InvalidArgumentError will be thrown.

Keys are lowercase and values are not modified.

Response headers will derive a host from the url of the Client instance if no host header was previously specified.

Example 1 - Object

{
  'content-length': '123',
  'content-type': 'text/plain',
  connection: 'keep-alive',
  host: 'mysite.com',
  accept: '*/*'
}

Example 2 - Array

[
  'content-length', '123',
  'content-type', 'text/plain',
  'connection', 'keep-alive',
  'host', 'mysite.com',
  'accept', '*/*'
]

27 KiB Raw Blame History

Dispatcher

Instance Methods

Dispatcher.close([callback]): Promise

Example - Request resolves before Client closes

Dispatcher.connect(options[, callback])

Parameter: ConnectOptions

Parameter: ConnectData

Example - Connect request with echo

Dispatcher.destroy([error, callback]): Promise

Example - Request is aborted when Client is destroyed

Dispatcher.dispatch(options, handler)

Parameter: DispatchOptions

Parameter: DispatchHandler

Example 1 - Dispatch GET request

Example 2 - Dispatch Upgrade Request

Example 3 - Dispatch POST request

Dispatcher.pipeline(options, handler)

Parameter: PipelineOptions

Parameter: PipelineHandlerData

Example 1 - Pipeline Echo

Dispatcher.request(options[, callback])

Parameter: RequestOptions

Parameter: ResponseData

Example 1 - Basic GET Request

Example 2 - Aborting a request

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

Parameter: StreamFactoryData

Parameter: StreamData

Example 1 - Basic GET stream request

Example 2 - Stream to Fastify Response

Dispatcher.upgrade(options[, callback])

Parameter: UpgradeOptions

Parameter: UpgradeData

Example 1 - Basic Upgrade Request

Instance Events

Event: 'connect'

Event: 'disconnect'

Event: 'connectionError'

Event: 'drain'

Parameter: UndiciHeaders

Example 1 - Object

Example 2 - Array

27 KiB

Raw Blame History

`Dispatcher.close([callback]): Promise`

`Dispatcher.connect(options[, callback])`

Parameter: `ConnectOptions`

Parameter: `ConnectData`

`Dispatcher.destroy([error, callback]): Promise`

`Dispatcher.dispatch(options, handler)`

Parameter: `DispatchOptions`

Parameter: `DispatchHandler`

`Dispatcher.pipeline(options, handler)`

`Dispatcher.request(options[, callback])`

Parameter: `RequestOptions`

Parameter: `ResponseData`

`Dispatcher.stream(options, factory[, callback])`

Parameter: `StreamFactoryData`

Parameter: `StreamData`

`Dispatcher.upgrade(options[, callback])`

Parameter: `UpgradeOptions`

Parameter: `UpgradeData`

Event: `'connect'`

Event: `'disconnect'`

Event: `'connectionError'`

Event: `'drain'`

Parameter: `UndiciHeaders`