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.
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.
Constructor
new(?options:Null<HttpAgentOptions>)
options in socket.connect() are also supported.
The default http.globalAgent that is used by http.request() has all of these values set to their respective defaults.
To configure any of them, a custom http.Agent instance must be created.
Variables
read onlyfreeSockets:DynamicAccess<Array<Socket>>
An object which contains arrays of sockets currently awaiting use by the agent when keepAlive is enabled. Do not modify.
maxFreeSockets:Float
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.
maxSockets:Float
By default set to Infinity.
Determines how many concurrent sockets the agent can have open per origin. Origin is the returned value of getName().
read onlyrequests:DynamicAccess<Array<ClientRequest>>
An object which contains queues of requests that have not yet been assigned to sockets. Do not modify.
read onlysockets:DynamicAccess<Array<Socket>>
An object which contains arrays of sockets currently in use by the agent. Do not modify.
Methods
createConnection(options:SocketConnectOptionsTcp, ?callback:(err:Error, stream:Socket) ‑> Void):Socket
Produces a socket/stream to be used for HTTP requests.
By default, this function is the same as net.createConnection().
However, custom agents may override this method in case greater flexibility is desired.
A socket/stream can be supplied in one of two ways: by returning the socket/stream from this function,
or by passing the socket/stream to callback.
callback has a signature of (err, stream).
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 will no longer be used. Otherwise,
sockets may hang open for quite a long time before the server terminates them.
getName(options:HttpRequestOptions):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.
keepSocketAlive(socket:Socket):Void
reuseSocket(socket:Socket, request:ClientRequest):Void
Called when socket is attached to request after being persisted because of the keep-alive options.
This method can be overridden by a particular Agent subclass.