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.
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
});options in socket.connect() are also supported.
To configure any of them, a custom {@link Agent} instance must be created.
const http = require('node:http');
const keepAliveAgent = new http.Agent({ keepAlive: true });
options.agent = keepAliveAgent;
http.request(options, onResponseCallback)Since
v0.3.4
Inheritors
Properties
An object which contains arrays of sockets currently awaiting use by the agent when keepAlive is enabled. Do not modify.
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.
Functions
Alias for emitter.on(eventName, listener).
Returns an array listing the events for which the emitter has registered listeners. The values in the array are strings or Symbols.
Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to {@link defaultMaxListeners}.
Returns the number of listeners listening for the event named eventName. If listener is provided, it will return how many times the listener is found in the list of the listeners of the event.
Returns a copy of the array of listeners for the event named eventName.
Alias for emitter.removeListener().
Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.
Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
Adds a one-timelistener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.
Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).
Removes all listeners, or those of the specified eventName.
Removes the specified listener from the listener array for the event named eventName.
By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set to Infinity (or 0) to indicate an unlimited number of listeners.