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doc: update http.md for consistency #10715

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doc: update http.md for consistency #10715

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f9c053d
doc: update http.md for consistency
lance Jan 9, 2017
cfeedac
doc: incorporate feedback into http.md
lance Jan 9, 2017
a776624
doc: attempt to clarify behavior of http.Agent
lance Jan 17, 2017
e92d4ae
doc: minor wording change/clarification to http.md
lance Jan 17, 2017
4d7148b
doc: clarify that pooled sockets are unref()ed.
lance Jan 18, 2017
bf0b7d1
doc: fix wording for agent option in http.request
lance Jan 18, 2017
dab4842
doc: fix link to net.html#net_socket_unref
lance Jan 18, 2017
8972714
doc: minor adjustments to keepAlive usage
lance Jan 18, 2017
05a8bcc
doc: incorporate feedback on http.Agent docs
lance Jan 24, 2017
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93 changes: 52 additions & 41 deletions doc/api/http.md
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Original file line number Diff line number Diff line change
Expand Up @@ -48,26 +48,32 @@ list like the following:
added: v0.3.4
-->

The HTTP Agent is used for pooling sockets used in HTTP client
requests.

The HTTP Agent also defaults client requests to using
`Connection: keep-alive`. If no pending HTTP requests are waiting on a
socket to become free the socket is closed. This means that Node.js's
pool has the benefit of keep-alive when under load but still does not
require developers to manually close the HTTP clients using
KeepAlive.

If you opt into using HTTP KeepAlive, you can create an Agent object
with that flag set to `true`. (See the [constructor options][].)
Then, the Agent will keep unused sockets in a pool for later use. They
will be explicitly marked so as to not keep the Node.js process running.
However, it is still a good idea to explicitly [`destroy()`][] KeepAlive
agents when they are no longer in use, so that the Sockets will be shut
down.

Sockets are removed from the agent's pool when the socket emits either
a `'close'` event or a special `'agentRemove'` event. This means that if
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](#http_new_agent_options).

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's pool when the socket emits either
a `'close'` event or an `'agentRemove'` event. This means that if
you intend to keep one HTTP request open for a long time and don't
want it to stay in the pool you can do something along the lines of:

Expand All @@ -79,7 +85,11 @@ http.get(options, (res) => {
});
```

Alternatively, you could just opt out of pooling entirely using
You may also use an agent 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`:

```js
Expand All @@ -100,11 +110,13 @@ added: v0.3.4

* `options` {Object} Set of configurable options to set on the agent.
Can have the following fields:
* `keepAlive` {Boolean} Keep sockets around in a pool to be used by
other requests in the future. Default = `false`
* `keepAliveMsecs` {Integer} When using HTTP KeepAlive, how often
to send TCP KeepAlive packets over sockets being kept alive.
Default = `1000`. Only relevant if `keepAlive` is set to `true`.
* `keepAlive` {Boolean} Keep sockets around even when there are no
outstanding requests, so they can be used for future requests without
having to reestablish a TCP connection. Default = `false`
* `keepAliveMsecs` {Integer} When using the `keepAlive` option, specifies
the [initial delay](#net_socket_setkeepalive_enable_initialdelay)
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@Krinkle Krinkle Feb 1, 2017 •
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This reference is broken. There is no such anchor on the http page. This should link to net.html instead.

Fixed in pull #11108.

for TCP Keep-Alive packets. Ignored when the
`keepAlive` option is `false` or `undefined`. Default = `1000`.
* `maxSockets` {Number} Maximum number of sockets to allow per
host. Default = `Infinity`.
* `maxFreeSockets` {Number} Maximum number of sockets to leave open
Expand All @@ -114,7 +126,7 @@ added: v0.3.4
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, you must create your own [`http.Agent`][] object.
To configure any of them, you must create your own [`http.Agent`][] instance.

```js
const http = require('http');
Expand All @@ -136,7 +148,7 @@ added: v0.11.4
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.
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`.
Expand All @@ -151,7 +163,7 @@ added: v0.11.4
Destroy any sockets that are currently in use by the agent.

It is usually not necessary to do this. However, if you are using an
agent with KeepAlive enabled, then it is best to explicitly shut down
agent with `keepAlive` enabled, then it is best to explicitly shut down
the agent when you know that it will no longer be used. Otherwise,
sockets may hang open for quite a long time before the server
terminates them.
Expand All @@ -164,7 +176,7 @@ added: v0.11.4
* {Object}

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

### agent.getName(options)
<!-- YAML
Expand All @@ -179,8 +191,8 @@ added: v0.11.4
* Returns: {String}

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

Expand All @@ -191,7 +203,7 @@ added: v0.11.7

* {Number}

By default set to 256. For Agents supporting HTTP KeepAlive, this
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.

Expand Down Expand Up @@ -224,7 +236,7 @@ added: v0.3.6
* {Object}

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

## Class: http.ClientRequest
<!-- YAML
Expand Down Expand Up @@ -652,7 +664,7 @@ added: v0.1.0
* `response` {http.ServerResponse}

Emitted each time there is a request. Note that there may be multiple requests
per connection (in the case of keep-alive connections).
per connection (in the case of HTTP Keep-Alive connections).
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correct usage

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You're saying this is the correct usage, right? Not that I should correct the usage here as well. I think this has the intended meaning.

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It looks correct to me.

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Yes, sorry, I am saying this is the correct usage, in contrast to a number of other places where I commented that it wasn't.


### Event: 'upgrade'
<!-- YAML
Expand Down Expand Up @@ -1490,7 +1502,7 @@ added: v0.5.9

* {http.Agent}

Global instance of Agent which is used as the default for all HTTP client
Global instance of `Agent` which is used as the default for all HTTP client
requests.

## http.request(options[, callback])
Expand Down Expand Up @@ -1520,15 +1532,13 @@ added: v0.3.6
* `headers` {Object} An object containing request headers.
* `auth` {String} Basic authentication i.e. `'user:password'` to compute an
Authorization header.
* `agent` {http.Agent|Boolean} Controls [`Agent`][] behavior. When an Agent
is used request will default to `Connection: keep-alive`. Possible values:
* `agent` {http.Agent|Boolean} Controls [`Agent`][] behavior. Possible values:
* `undefined` (default): use [`http.globalAgent`][] for this host and port.
* `Agent` object: explicitly use the passed in `Agent`.
* `false`: opts out of connection pooling with an Agent, defaults request to
`Connection: close`.
* `false`: causes a new `Agent` with default values to be used.
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This is really, really different behaviour from the original text. Are you absolutely sure that it is impossible to send a one-shot HTTP request with node, that disables HTTP Keep-Alive, aka sends Connection: close? Node.js docs have been wrong before, but this strikes me as a surprising bug.

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See my note above, but yes, I think this is the case based on this code: https://github.com/nodejs/node/blob/master/lib/_http_client.js#L63-L75

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I also stumbled upon this while working on #10818 and I was like 😲

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Isn't Connection: close the default, since you have to explicitly opt-in via keepAlive: true when creating a new Agent? So if the defaults are used, then that means keepAlive: false is used ...

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@mscdex good point.

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@mscdex @lpinca not really, no. As I read the code, all OutgoingMessages use Connection: keep-alive whether the keepAlive option is specified or not. This is what allows the behavior I noted above - that multiple, queued requests will reuse the same socket. So Connection: keep-alive is always used, and once the queue of requests has been emptied, then only does OutgoingMessage send Connection: close.

You can see how sockets are reused even when keepAlive option is false here.

As I've said before, I could be mistaken and reading the code wrong, but nobody has actually given me another interpretation of the code itself that corresponds to the old documentation. My text changes are based on what I see in the code. I've tried to provide that analysis here and here. If you think it's wrong or mistaken, please let me know how and where. The code is spaghettiish and quite confusing.

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@mscdex mscdex Jan 20, 2017 •
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@lance The socket is only reused if 'free' is emitted as you can see in your link.

Here's the entire client request flow:

  1. http.request() merely creates a new ClientRequest()
  2. ClientRequest() creates a new Agent instance using the default options when agent: false is passed
    a. As we see in the Agent() constructor, the default value for keepAlive is false
    b. We also see that in the Agent() constructor that the default value for maxSockets is Agent.defaultMaxSockets which is Infinity.
  3. Later in ClientRequest(), we check if the agent is using keepAlive and if the maxSockets is set to Infinity. If so, then we disable reuse of sockets by setting req.shouldKeepAlive to false.
    a. As a side note, if the server is sending Connection: close or something similar, then we definitely do not reuse the socket, even if agent.keepAlive is true.
  4. When we go to send the initial HTTP request line and headers, we see that if the user has not explicitly set a Connection header, that the default behavior is to check req.shouldKeepAlive (among other values) to determine whether to send Connection: keep-alive or not. Importantly, if req.shouldKeepAlive is false, then Connection: close is sent.
  5. When the response from the server is complete, req.shouldKeepAlive is checked and responseKeepAlive() is called because req.shouldKeepAlive is false.
  6. Once inside responseKeepAlive(), we immediately check if req.shouldKeepAlive is false. If so, we destroy the socket if it hasn't already been destroyed. You can see in the else block that when req.shouldKeepAlive is true, that we emit 'free', which leads to the execution of the code that you pointed out.

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@mscdex thank you for clarifying. Stay tuned.

* `createConnection` {Function} A function that produces a socket/stream to
use for the request when the `agent` option is not used. This can be used to
avoid creating a custom Agent class just to override the default
avoid creating a custom `Agent` class just to override the default
`createConnection` function. See [`agent.createConnection()`][] for more
details.
* `timeout` {Integer}: A number specifying the socket timeout in milliseconds.
Expand Down Expand Up @@ -1649,3 +1659,4 @@ There are a few special headers that should be noted.
[constructor options]: #http_new_agent_options
[Readable Stream]: stream.html#stream_class_stream_readable
[Writable Stream]: stream.html#stream_class_stream_writable
[socket.unref()]: net.html#net_socket_unref