http.Agent

An Agent is responsible for managing connection persistence and reuse for HTTP clients. It maintains a queue of pending requests for a given host and port, reusing a single socket connection for each until the queue is empty, at which time the socket is either destroyed or put into a pool where it is kept to be used again for requests to the same host and port. Whether it is destroyed or pooled depends on the keepAlive option.

Pooled connections have TCP Keep-Alive enabled for them, but servers may still close idle connections, in which case they will be removed from the pool and a new connection will be made when a new HTTP request is made for that host and port. Servers may also refuse to allow multiple requests over the same connection, in which case the connection will have to be remade for every request and cannot be pooled. The Agent will still make the requests to that server, but each one will occur over a new connection.

On a reused HTTP/1.1 keep-alive connection, responses are associated with requests by their order on that connection. HTTP/1.1 keep-alive does not provide per-request response attribution beyond that ordering. Applications that require per-request connection isolation can use a separate Agent, disable keep-alive, or pass agent: false.

When a connection is closed by the client or the server, it is removed from the pool. Any unused sockets in the pool will be unrefed so as not to keep the Node.js process running when there are no outstanding requests. (see socket.unref()).

It is good practice, to destroy() an Agent instance when it is no longer in use, because unused sockets consume OS resources.

Sockets are removed from an agent when the socket emits either a 'close' event or an 'agentRemove' event. When intending to keep one HTTP request open for a long time without keeping it in the agent, something like the following may be done:

http.get(options, (res) => {
  // Do stuff
}).on('socket', (socket) => {
  socket.emit('agentRemove');
});

An agent may also be used for an individual request. By providing {agent: false} as an option to the http.get() or http.request() functions, a one-time use Agent with default options will be used for the client connection.

agent:false:

http.get({
  hostname: 'localhost',
  port: 80,
  path: '/',
  agent: false,  // Create a new agent just for this one request
}, (res) => {
  // Do stuff with response
});

Use agent: false to avoid connection reuse for a request.

new Agent(options?): Agent

options in socket.connect() are also supported.

To configure any of them, a custom http.Agent instance must be created.

import { Agent, request } from 'node:http';
const keepAliveAgent = new Agent({ keepAlive: true });
options.agent = keepAliveAgent;
request(options, onResponseCallback);

const http = require('node:http');
const keepAliveAgent = new http.Agent({ keepAlive: true });
options.agent = keepAliveAgent;
http.request(options, onResponseCallback);

agent.createConnection(options, callback?): stream.Duplex

Produces a socket/stream to be used for HTTP requests.

By default, this function behaves identically to net.createConnection(), synchronously returning the created socket. The optional callback parameter in the signature is not used by this default implementation.

However, custom agents may override this method to provide greater flexibility, for example, to create sockets asynchronously. When overriding createConnection:

1. Synchronous socket creation: The overriding method can return the socket/stream directly.
2. Asynchronous socket creation: The overriding method can accept the callback and pass the created socket/stream to it (e.g., callback(null, newSocket)). If an error occurs during socket creation, it should be passed as the first argument to the callback (e.g., callback(err)).

The agent will call the provided createConnection function with options and this internal callback. The callback provided by the agent has a signature of (err, stream).

agent.keepSocketAlive(socket): void

Called when socket is detached from a request and could be persisted by the Agent. Default behavior is to:

socket.setKeepAlive(true, this.keepAliveMsecs);
socket.unref();
return true;

This method can be overridden by a particular Agent subclass. If this method returns a falsy value, the socket will be destroyed instead of persisting it for use with the next request.

The socket argument can be an instance of net.Socket, a subclass of stream.Duplex.

agent.reuseSocket(socket, request): void

Called when socket is attached to request after being persisted because of the keep-alive options. Default behavior is to:

socket.ref();

This method can be overridden by a particular Agent subclass.

The socket argument can be an instance of net.Socket, a subclass of stream.Duplex.

agent.destroy(): void

Destroy any sockets that are currently in use by the agent.

It is usually not necessary to do this. However, if using an agent with keepAlive enabled, then it is best to explicitly shut down the agent when it is no longer needed. Otherwise, sockets might stay open for quite a long time before the server terminates them.

An object which contains arrays of sockets currently awaiting use by the agent when keepAlive is enabled. Do not modify.

Sockets in the freeSockets list will be automatically destroyed and removed from the array on 'timeout'.

agent.getName(options?): string

Get a unique name for a set of request options, to determine whether a connection can be reused. For an HTTP agent, this returns host:port:localAddress or host:port:localAddress:family. For an HTTPS agent, the name includes the CA, cert, ciphers, and other HTTPS/TLS-specific options that determine socket reusability.

By default set to 256. For agents with keepAlive enabled, this sets the maximum number of sockets that will be left open in the free state.

By default set to Infinity. Determines how many concurrent sockets the agent can have open per origin. Origin is the returned value of agent.getName().

By default set to Infinity. Determines how many concurrent sockets the agent can have open. Unlike maxSockets, this parameter applies across all origins.

An object which contains queues of requests that have not yet been assigned to sockets. Do not modify.

An object which contains arrays of sockets currently in use by the agent. Do not modify.