== Writing HTTP servers and clients

Vert.x allows you to easily write non blocking HTTP clients and servers.

Vert.x supports the HTTP/1.0, HTTP/1.1 and HTTP/2 protocols.

The base API for HTTP is the same for HTTP/1.x and HTTP/2, specific API features are available for dealing with the
HTTP/2 protocol.

=== Creating an HTTP Server

The simplest way to create an HTTP server, using all default options is as follows:

[source,$lang]
----
{@link examples.HTTPExamples#example1}
----

=== Configuring an HTTP server

If you don't want the default, a server can be configured by passing in a {@link io.vertx.core.http.HttpServerOptions}
instance when creating it:

[source,$lang]
----
{@link examples.HTTPExamples#example2}
----

=== Configuring an HTTP/2 server

Vert.x supports HTTP/2 over TLS `h2` and over TCP `h2c`.

- `h2` identifies the HTTP/2 protocol when used over TLS negotiated by https://en.wikipedia.org/wiki/Application-Layer_Protocol_Negotiation[Application-Layer Protocol Negotiation] (ALPN)
- `h2c` identifies the HTTP/2 protocol when using in clear text over TCP, such connections are established either with
an HTTP/1.1 upgraded request or directly

To handle `h2` requests, TLS must be enabled along with {@link io.vertx.core.http.HttpServerOptions#setUseAlpn(boolean)}:

[source,$lang]
----
{@link examples.HTTP2Examples#example0}
----

ALPN is a TLS extension that negotiates the protocol before the client and the server start to exchange data.

Clients that don't support ALPN will still be able to do a _classic_ SSL handshake.

ALPN will usually agree on the `h2` protocol, although `http/1.1` can be used if the server or the client decides
so.

To handle `h2c` requests, TLS must be disabled, the server will upgrade to HTTP/2 any request HTTP/1.1 that wants to
upgrade to HTTP/2. It will also accept a direct `h2c` connection beginning with the `PRI * HTTP/2.0\r\nSM\r\n` preface.

WARNING: most browsers won't support `h2c`, so for serving web sites you should use `h2` and not `h2c`.

When a server accepts an HTTP/2 connection, it sends to the client its {@link io.vertx.core.http.HttpServerOptions#getInitialSettings initial settings}.
The settings define how the client can use the connection, the default initial settings for a server are:

- {@link io.vertx.core.http.Http2Settings#getMaxConcurrentStreams}: `100` as recommended by the HTTP/2 RFC
- the default HTTP/2 settings values for the others

NOTE: Worker Verticles are not compatible with HTTP/2

=== Logging network server activity

For debugging purposes, network activity can be logged.

[source,$lang]
----
{@link examples.HTTPExamples#exampleServerLogging}
----

See the chapter on <<logging_network_activity, logging network activity>> for a detailed explanation.

=== Start the Server Listening

To tell the server to listen for incoming requests you use one of the {@link io.vertx.core.http.HttpServer#listen}
alternatives.

To tell the server to listen at the host and port as specified in the options:

[source,$lang]
----
{@link examples.HTTPExamples#example3}
----

Or to specify the host and port in the call to listen, ignoring what is configured in the options:

[source,$lang]
----
{@link examples.HTTPExamples#example4}
----

The default host is `0.0.0.0` which means 'listen on all available addresses' and the default port is `80`.

The actual bind is asynchronous so the server might not actually be listening until some time *after* the call to
listen has returned.

If you want to be notified when the server is actually listening you can provide a handler to the `listen` call.
For example:

[source,$lang]
----
{@link examples.HTTPExamples#example5}
----

=== Getting notified of incoming requests

To be notified when a request arrives you need to set a {@link io.vertx.core.http.HttpServer#requestHandler}:

[source,$lang]
----
{@link examples.HTTPExamples#example6}
----

=== Handling requests

When a request arrives, the request handler is called passing in an instance of {@link io.vertx.core.http.HttpServerRequest}.
This object represents the server side HTTP request.

The handler is called when the headers of the request have been fully read.

If the request contains a body, that body will arrive at the server some time after the request handler has been called.

The server request object allows you to retrieve the {@link io.vertx.core.http.HttpServerRequest#uri},
{@link io.vertx.core.http.HttpServerRequest#path}, {@link io.vertx.core.http.HttpServerRequest#params} and
{@link io.vertx.core.http.HttpServerRequest#headers}, amongst other things.

Each server request object is associated with one server response object. You use
{@link io.vertx.core.http.HttpServerRequest#response} to get a reference to the {@link io.vertx.core.http.HttpServerResponse}
object.

Here's a simple example of a server handling a request and replying with "hello world" to it.

[source,$lang]
----
{@link examples.HTTPExamples#example7_1}
----

==== Request version

The version of HTTP specified in the request can be retrieved with {@link io.vertx.core.http.HttpServerRequest#version}

==== Request method

Use {@link io.vertx.core.http.HttpServerRequest#method} to retrieve the HTTP method of the request.
(i.e. whether it's GET, POST, PUT, DELETE, HEAD, OPTIONS, etc).

==== Request URI

Use {@link io.vertx.core.http.HttpServerRequest#uri} to retrieve the URI of the request.

Note that this is the actual URI as passed in the HTTP request, and it's almost always a relative URI.

The URI is as defined in http://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html[Section 5.1.2 of the HTTP specification - Request-URI]

==== Request path

Use {@link io.vertx.core.http.HttpServerRequest#path} to return the path part of the URI

For example, if the request URI was:

 a/b/c/page.html?param1=abc&param2=xyz

Then the path would be

 /a/b/c/page.html

==== Request query

Use {@link io.vertx.core.http.HttpServerRequest#query} to return the query part of the URI

For example, if the request URI was:

 a/b/c/page.html?param1=abc&param2=xyz

Then the query would be

 param1=abc&param2=xyz

==== Request headers

Use {@link io.vertx.core.http.HttpServerRequest#headers} to return the headers of the HTTP request.

This returns an instance of {@link io.vertx.core.MultiMap} - which is like a normal Map or Hash but allows multiple
values for the same key - this is because HTTP allows multiple header values with the same key.

It also has case-insensitive keys, that means you can do the following:

[source,$lang]
----
{@link examples.HTTPExamples#example8}
----

==== Request host

Use {@link io.vertx.core.http.HttpServerRequest#host} to return the host of the HTTP request.

For HTTP/1.x requests the `host` header is returned, for HTTP/1 requests the `:authority` pseudo header is returned.

==== Request parameters

Use {@link io.vertx.core.http.HttpServerRequest#params} to return the parameters of the HTTP request.

Just like {@link io.vertx.core.http.HttpServerRequest#headers} this returns an instance of {@link io.vertx.core.MultiMap}
as there can be more than one parameter with the same name.

Request parameters are sent on the request URI, after the path. For example if the URI was:

 /page.html?param1=abc&param2=xyz

Then the parameters would contain the following:

----
param1: 'abc'
param2: 'xyz
----

Note that these request parameters are retrieved from the URL of the request. If you have form attributes that
have been sent as part of the submission of an HTML form submitted in the body of a `multi-part/form-data` request
then they will not appear in the params here.

==== Remote address

The address of the sender of the request can be retrieved with {@link io.vertx.core.http.HttpServerRequest#remoteAddress}.

==== Absolute URI

The URI passed in an HTTP request is usually relative. If you wish to retrieve the absolute URI corresponding
to the request, you can get it with {@link io.vertx.core.http.HttpServerRequest#absoluteURI}

==== End handler

The {@link io.vertx.core.http.HttpServerRequest#endHandler} of the request is invoked when the entire request,
including any body has been fully read.

==== Reading Data from the Request Body

Often an HTTP request contains a body that we want to read. As previously mentioned the request handler is called
when just the headers of the request have arrived so the request object does not have a body at that point.

This is because the body may be very large (e.g. a file upload) and we don't generally want to buffer the entire
body in memory before handing it to you, as that could cause the server to exhaust available memory.

To receive the body, you can use the {@link io.vertx.core.http.HttpServerRequest#handler}  on the request,
this will get called every time a chunk of the request body arrives. Here's an example:

[source,$lang]
----
{@link examples.HTTPExamples#example9}
----

The object passed into the handler is a {@link io.vertx.core.buffer.Buffer}, and the handler can be called
multiple times as data arrives from the network, depending on the size of the body.

In some cases (e.g. if the body is small) you will want to aggregate the entire body in memory, so you could do
the aggregation yourself as follows:

[source,$lang]
----
{@link examples.HTTPExamples#example10}
----

This is such a common case, that Vert.x provides a {@link io.vertx.core.http.HttpServerRequest#bodyHandler} to do this
for you. The body handler is called once when all the body has been received:

[source,$lang]
----
{@link examples.HTTPExamples#example11}
----

==== Streaming requests

The request object is a {@link io.vertx.core.streams.ReadStream} so you can pipe the request body to any
{@link io.vertx.core.streams.WriteStream} instance.

See the chapter on <<streams, streams>> for a detailed explanation.

==== Handling HTML forms

HTML forms can be submitted with either a content type of `application/x-www-form-urlencoded` or `multipart/form-data`.

For url encoded forms, the form attributes are encoded in the url, just like normal query parameters.

For multi-part forms they are encoded in the request body, and as such are not available until the entire body
has been read from the wire.

Multi-part forms can also contain file uploads.

If you want to retrieve the attributes of a multi-part form you should tell Vert.x that you expect to receive
such a form *before* any of the body is read by calling {@link io.vertx.core.http.HttpServerRequest#setExpectMultipart}
with true, and then you should retrieve the actual attributes using {@link io.vertx.core.http.HttpServerRequest#formAttributes}
once the entire body has been read:

[source,$lang]
----
{@link examples.HTTPExamples#example12}
----

==== Handling form file uploads

Vert.x can also handle file uploads which are encoded in a multi-part request body.

To receive file uploads you tell Vert.x to expect a multi-part form and set an
{@link io.vertx.core.http.HttpServerRequest#uploadHandler} on the request.

This handler will be called once for every
upload that arrives on the server.

The object passed into the handler is a {@link io.vertx.core.http.HttpServerFileUpload} instance.

[source,$lang]
----
{@link examples.HTTPExamples#example13}
----

File uploads can be large we don't provide the entire upload in a single buffer as that might result in memory
exhaustion, instead, the upload data is received in chunks:

[source,$lang]
----
{@link examples.HTTPExamples#example14}
----

The upload object is a {@link io.vertx.core.streams.ReadStream} so you can pipe the request body to any
{@link io.vertx.core.streams.WriteStream} instance. See the chapter on <<streams, streams>> for a
detailed explanation.

If you just want to upload the file to disk somewhere you can use {@link io.vertx.core.http.HttpServerFileUpload#streamToFileSystem}:

[source,$lang]
----
{@link examples.HTTPExamples#example15}
----

WARNING: Make sure you check the filename in a production system to avoid malicious clients uploading files
to arbitrary places on your filesystem. See <<Security notes, security notes>> for more information.

==== Handling cookies

You use {@link io.vertx.core.http.HttpServerRequest#getCookie(String)} to retrieve
a cookie by name, or use {@link io.vertx.core.http.HttpServerRequest#cookieMap()} to retrieve all the cookies.

To remove a cookie, use {@link io.vertx.core.http.HttpServerResponse#removeCookie(String)}.

To add a cookie use {@link io.vertx.core.http.HttpServerResponse#addCookie(Cookie)}.

The set of cookies will be written back in the response automatically when the response headers are written so the
browser can store them.

Cookies are described by instances of {@link io.vertx.core.http.Cookie}. This allows you to retrieve the name,
value, domain, path and other normal cookie properties.

Same Site Cookies let servers require that a cookie shouldn't be sent with cross-site (where Site is defined by the
registrable domain) requests, which provides some protection against cross-site request forgery attacks. This kind
of cookies are enabled using the setter: {@link io.vertx.core.http.Cookie#setSameSite(CookieSameSite)}.

Same site cookies can have one of 3 values:

* None - The browser will send cookies with both cross-site requests and same-site requests.
* Strict - he browser will only send cookies for same-site requests (requests originating from the site that set the
  cookie). If the request originated from a different URL than the URL of the current location, none of the cookies
  tagged with the Strict attribute will be included.
* Lax - Same-site cookies are withheld on cross-site subrequests, such as calls to load images or frames, but will be
  sent when a user navigates to the URL from an external site; for example, by following a link.

Here's an example of querying and adding cookies:

[source,$lang]
----
{@link examples.HTTPExamples#exampleHandlingCookies}
----

==== Handling compressed body

Vert.x can handle compressed body payloads which are encoded by the client with the _deflate_ or _gzip_
algorithms.

To enable decompression set {@link io.vertx.core.http.HttpServerOptions#setDecompressionSupported(boolean)} on the
options when creating the server.

By default decompression is disabled.

==== Receiving custom HTTP/2 frames

HTTP/2 is a framed protocol with various frames for the HTTP request/response model. The protocol allows other kind
of frames to be sent and received.

To receive custom frames, you can use the {@link io.vertx.core.http.HttpServerRequest#customFrameHandler} on the request,
this will get called every time a custom frame arrives. Here's an example:

[source,$lang]
----
{@link examples.HTTP2Examples#example1}
----

HTTP/2 frames are not subject to flow control - the frame handler will be called immediatly when a
custom frame is received whether the request is paused or is not

==== Non standard HTTP methods

The {@link io.vertx.core.http.HttpMethod#OTHER} HTTP method is used for non standard methods, in this case
{@link io.vertx.core.http.HttpServerRequest#rawMethod()} returns the HTTP method as sent by the client.

=== Sending back responses

The server response object is an instance of {@link io.vertx.core.http.HttpServerResponse} and is obtained from the
request with {@link io.vertx.core.http.HttpServerRequest#response}.

You use the response object to write a response back to the HTTP client.

==== Setting status code and message

The default HTTP status code for a response is `200`, representing `OK`.

Use {@link io.vertx.core.http.HttpServerResponse#setStatusCode} to set a different code.

You can also specify a custom status message with {@link io.vertx.core.http.HttpServerResponse#setStatusMessage}.

If you don't specify a status message, the default one corresponding to the status code will be used.

NOTE: for HTTP/2 the status won't be present in the response since the protocol won't transmit the message
to the client

==== Writing HTTP responses

To write data to an HTTP response, you use one of the {@link io.vertx.core.http.HttpServerResponse#write} operations.

These can be invoked multiple times before the response is ended. They can be invoked in a few ways:

With a single buffer:

[source,$lang]
----
{@link examples.HTTPExamples#example16}
----

With a string. In this case the string will encoded using UTF-8 and the result written to the wire.

[source,$lang]
----
{@link examples.HTTPExamples#example17}
----

With a string and an encoding. In this case the string will encoded using the specified encoding and the
result written to the wire.

[source,$lang]
----
{@link examples.HTTPExamples#example18}
----

Writing to a response is asynchronous and always returns immediately after the write has been queued.

If you are just writing a single string or buffer to the HTTP response you can write it and end the response in a
single call to the {@link io.vertx.core.http.HttpServerResponse#end(String)}

The first call to write results in the response header being written to the response. Consequently, if you are
not using HTTP chunking then you must set the `Content-Length` header before writing to the response, since it will
be too late otherwise. If you are using HTTP chunking you do not have to worry.

==== Ending HTTP responses

Once you have finished with the HTTP response you should {@link io.vertx.core.http.HttpServerResponse#end} it.

This can be done in several ways:

With no arguments, the response is simply ended.

[source,$lang]
----
{@link examples.HTTPExamples#example19}
----

It can also be called with a string or buffer in the same way `write` is called. In this case it's just the same as
calling write with a string or buffer followed by calling end with no arguments. For example:

[source,$lang]
----
{@link examples.HTTPExamples#example20}
----

==== Closing the underlying connection

You can close the underlying TCP connection with {@link io.vertx.core.http.HttpServerResponse#close}.

Non keep-alive connections will be automatically closed by Vert.x when the response is ended.

Keep-alive connections are not automatically closed by Vert.x by default. If you want keep-alive connections to be
closed after an idle time, then you configure {@link io.vertx.core.http.HttpServerOptions#setIdleTimeout}.

HTTP/2 connections send a {@literal GOAWAY} frame before closing the response.

==== Setting response headers

HTTP response headers can be added to the response by adding them directly to the
{@link io.vertx.core.http.HttpServerResponse#headers}:

[source,$lang]
----
{@link examples.HTTPExamples#example21}
----

Or you can use {@link io.vertx.core.http.HttpServerResponse#putHeader}

[source,$lang]
----
{@link examples.HTTPExamples#example22}
----

Headers must all be added before any parts of the response body are written.

==== Chunked HTTP responses and trailers

Vert.x supports http://en.wikipedia.org/wiki/Chunked_transfer_encoding[HTTP Chunked Transfer Encoding].

This allows the HTTP response body to be written in chunks, and is normally used when a large response body is
being streamed to a client and the total size is not known in advance.

You put the HTTP response into chunked mode as follows:

[source,$lang]
----
{@link examples.HTTPExamples#example23}
----

Default is non-chunked. When in chunked mode, each call to one of the {@link io.vertx.core.http.HttpServerResponse#write}
methods will result in a new HTTP chunk being written out.

When in chunked mode you can also write HTTP response trailers to the response. These are actually written in
the final chunk of the response.

NOTE: chunked response has no effect for an HTTP/2 stream

To add trailers to the response, add them directly to the {@link io.vertx.core.http.HttpServerResponse#trailers}.

[source,$lang]
----
{@link examples.HTTPExamples#example24}
----

Or use {@link io.vertx.core.http.HttpServerResponse#putTrailer}.

[source,$lang]
----
{@link examples.HTTPExamples#example25}
----

==== Serving files directly from disk or the classpath

If you were writing a web server, one way to serve a file from disk would be to open it as an {@link io.vertx.core.file.AsyncFile}
and pipe it to the HTTP response.

Or you could load it it one go using {@link io.vertx.core.file.FileSystem#readFile} and write it straight to the response.

Alternatively, Vert.x provides a method which allows you to serve a file from disk or the filesystem to an HTTP response
in one operation.
Where supported by the underlying operating system this may result in the OS directly transferring bytes from the
file to the socket without being copied through user-space at all.

This is done by using {@link io.vertx.core.http.HttpServerResponse#sendFile}, and is usually more efficient for large
files, but may be slower for small files.

Here's a very simple web server that serves files from the file system using sendFile:

[source,$lang]
----
{@link examples.HTTPExamples#example26}
----

Sending a file is asynchronous and may not complete until some time after the call has returned. If you want to
be notified when the file has been writen you can use {@link io.vertx.core.http.HttpServerResponse#sendFile(String,io.vertx.core.Handler)}

Please see the chapter about <<classpath, serving files from the classpath>> for restrictions about the classpath resolution or disabling it.

NOTE: If you use `sendFile` while using HTTPS it will copy through user-space, since if the kernel is copying data
directly from disk to socket it doesn't give us an opportunity to apply any encryption.

WARNING: If you're going to write web servers directly using Vert.x be careful that users cannot exploit the
path to access files outside the directory from which you want to serve them or the classpath It may be safer instead to use
Vert.x Web.

When there is a need to serve just a segment of a file, say starting from a given byte, you can achieve this by doing:

[source,$lang]
----
{@link examples.HTTPExamples#example26b}
----

You are not required to supply the length if you want to send a file starting from an offset until the end, in this
case you can just do:

[source,$lang]
----
{@link examples.HTTPExamples#example26c}
----

==== Piping responses

The server response is a {@link io.vertx.core.streams.WriteStream} instance so you can pipe to it from any
{@link io.vertx.core.streams.ReadStream}, e.g. {@link io.vertx.core.file.AsyncFile}, {@link io.vertx.core.net.NetSocket},
{@link io.vertx.core.http.WebSocket} or {@link io.vertx.core.http.HttpServerRequest}.

Here's an example which echoes the request body back in the response for any PUT methods.
It uses a pipe for the body, so it will work even if the HTTP request body is much larger than can fit in memory
at any one time:

[source,$lang]
----
{@link examples.HTTPExamples#example27}
----

==== Writing HTTP/2 frames

HTTP/2 is a framed protocol with various frames for the HTTP request/response model. The protocol allows other kind
of frames to be sent and received.

To send such frames, you can use the {@link io.vertx.core.http.HttpServerResponse#writeCustomFrame} on the response.
Here's an example:

[source,$lang]
----
{@link examples.HTTP2Examples#example2}
----

These frames are sent immediately and are not subject to flow control - when such frame is sent there it may be done
before other {@literal DATA} frames.

==== Stream reset

HTTP/1.x does not allow a clean reset of a request or a response stream, for example when a client uploads
a resource already present on the server, the server needs to accept the entire response.

HTTP/2 supports stream reset at any time during the request/response:

[source,$lang]
----
{@link examples.HTTP2Examples#example3}
----

By default the `NO_ERROR` (0) error code is sent, another code can sent instead:

[source,$lang]
----
{@link examples.HTTP2Examples#example4}
----

The HTTP/2 specification defines the list of http://httpwg.org/specs/rfc7540.html#ErrorCodes[error codes] one can use.

The request handler are notified of stream reset events with the {@link io.vertx.core.http.HttpServerRequest#exceptionHandler request handler} and
{@link io.vertx.core.http.HttpServerResponse#exceptionHandler response handler}:

[source,$lang]
----
{@link examples.HTTP2Examples#example5}
----

==== Server push

Server push is a new feature of HTTP/2 that enables sending multiple responses in parallel for a single client request.

When a server process a request, it can push a request/response to the client:

[source,$lang]
----
{@link examples.HTTP2Examples#example6}
----

When the server is ready to push the response, the push response handler is called and the handler can send the response.

The push response handler may receive a failure, for instance the client may cancel the push because it already has `main.js` in its
cache and does not want it anymore.

The {@link io.vertx.core.http.HttpServerResponse#push} method must be called before the initiating response ends, however
the pushed response can be written after.

==== Handling exceptions

You can set an {@link io.vertx.core.http.HttpServer#exceptionHandler(io.vertx.core.Handler)} to receive any
exceptions that happens before the connection is passed to the {@link io.vertx.core.http.HttpServer#requestHandler(io.vertx.core.Handler)}
or to the {@link io.vertx.core.http.HttpServer#webSocketHandler(io.vertx.core.Handler)}, e.g during the TLS handshake.

=== HTTP Compression

Vert.x comes with support for HTTP Compression out of the box.

This means you are able to automatically compress the body of the responses before they are sent back to the client.

If the client does not support HTTP compression the responses are sent back without compressing the body.

This allows to handle Client that support HTTP Compression and those that not support it at the same time.

To enable compression use can configure it with {@link io.vertx.core.http.HttpServerOptions#setCompressionSupported}.

By default compression is not enabled.

When HTTP compression is enabled the server will check if the client includes an `Accept-Encoding` header which
includes the supported compressions. Commonly used are deflate and gzip. Both are supported by Vert.x.

If such a header is found the server will automatically compress the body of the response with one of the supported
compressions and send it back to the client.

Whenever the response needs to be sent without compression you can set the header `content-encoding` to `identity`:

[source,$lang]
----
{@link examples.HTTPExamples#setIdentityContentEncodingHeader}
----

Be aware that compression may be able to reduce network traffic but is more CPU-intensive.

To address this latter issue Vert.x allows you to tune the 'compression level' parameter that is native of the gzip/deflate compression algorithms.

Compression level allows to configure gizp/deflate algorithms in terms of the compression ratio of the resulting data and the computational cost of the compress/decompress operation.

The compression level is an integer value ranged from '1' to '9', where '1' means lower compression ratio but fastest algorithm and '9' means maximum compression ratio available but a slower algorithm.

Using compression levels higher that 1-2 usually allows to save just some bytes in size - the gain is not linear, and depends on the specific data to be compressed
- but it comports a non-trascurable cost in term of CPU cycles required to the server while generating the compressed response data
( Note that at moment Vert.x doesn't support any form caching of compressed response data, even for static files, so the compression is done on-the-fly
at every request body generation ) and in the same way it affects client(s) while decoding (inflating) received responses, operation that becomes more CPU-intensive
the more the level increases.

By default - if compression is enabled via {@link io.vertx.core.http.HttpServerOptions#setCompressionSupported} - Vert.x will use '6' as compression level,
but the parameter can be configured to address any case with {@link io.vertx.core.http.HttpServerOptions#setCompressionLevel}.

=== Creating an HTTP client

You create an {@link io.vertx.core.http.HttpClient} instance with default options as follows:

[source,$lang]
----
{@link examples.HTTPExamples#example28}
----

If you want to configure options for the client, you create it as follows:

[source,$lang]
----
{@link examples.HTTPExamples#example29}
----

Vert.x supports HTTP/2 over TLS `h2` and over TCP `h2c`.

By default the http client performs HTTP/1.1 requests, to perform HTTP/2 requests the {@link io.vertx.core.http.HttpClientOptions#setProtocolVersion}
must be set to {@link io.vertx.core.http.HttpVersion#HTTP_2}.

For `h2` requests, TLS must be enabled with _Application-Layer Protocol Negotiation_:

[source,$lang]
----
{@link examples.HTTP2Examples#example7}
----

For `h2c` requests, TLS must be disabled, the client will do an HTTP/1.1 requests and try an upgrade to HTTP/2:

[source,$lang]
----
{@link examples.HTTP2Examples#example8}
----

`h2c` connections can also be established directly, i.e connection started with a prior knowledge, when
{@link io.vertx.core.http.HttpClientOptions#setHttp2ClearTextUpgrade(boolean)} options is set to false: after the
connection is established, the client will send the HTTP/2 connection preface and expect to receive
the same preface from the server.

The http server may not support HTTP/2, the actual version can be checked
with {@link io.vertx.core.http.HttpClientResponse#version()} when the response arrives.

When a clients connects to an HTTP/2 server, it sends to the server its {@link io.vertx.core.http.HttpClientOptions#getInitialSettings initial settings}.
The settings define how the server can use the connection, the default initial settings for a client are the default
values defined by the HTTP/2 RFC.

=== Logging network client activity

For debugging purposes, network activity can be logged.

[source,$lang]
----
{@link examples.HTTPExamples#exampleClientLogging}
----

See the chapter on <<logging_network_activity, logging network activity>> for a detailed explanation.

=== Making requests

The http client is very flexible and there are various ways you can make requests with it.


Often you want to make many requests to the same host/port with an http client. To avoid you repeating the host/port
every time you make a request you can configure the client with a default host/port:

[source,$lang]
----
{@link examples.HTTPExamples#example30}
----

Alternatively if you find yourself making lots of requests to different host/ports with the same client you can
simply specify the host/port when doing the request.

[source,$lang]
----
{@link examples.HTTPExamples#example31}
----

Both methods of specifying host/port are supported for all the different ways of making requests with the client.

==== Simple requests with no request body

Often, you'll want to make HTTP requests with no request body. This is usually the case with HTTP GET, OPTIONS and
HEAD requests.

The simplest way to do this with the Vert.x http client is using the methods suffixed with `Now`. For example
{@link io.vertx.core.http.HttpClient#getNow}.

These methods create the http request and send it in a single method call and allow you to provide a handler that will be
called with the http response when it comes back.

[source,$lang]
----
{@link examples.HTTPExamples#example32}
----

==== Writing general requests

At other times you don't know the request method you want to send until run-time. For that use case we provide
general purpose request methods such as {@link io.vertx.core.http.HttpClient#request} which allow you to specify
the HTTP method at run-time:

[source,$lang]
----
{@link examples.HTTPExamples#example33}
----

==== Writing request bodies

Sometimes you'll want to write requests which have a body, or perhaps you want to write headers to a request
before sending it.

To do this you can call one of the specific request methods such as {@link io.vertx.core.http.HttpClient#post} or
one of the general purpose request methods such as {@link io.vertx.core.http.HttpClient#request}.

These methods don't send the request immediately, but instead return an instance of {@link io.vertx.core.http.HttpClientRequest}
which can be used to write to the request body or write headers.

Here are some examples of writing a POST request with a body:
m
[source,$lang]
----
{@link examples.HTTPExamples#example34}
----

Methods exist to write strings in UTF-8 encoding and in any specific encoding and to write buffers:

[source,$lang]
----
{@link examples.HTTPExamples#example35}
----

If you are just writing a single string or buffer to the HTTP request you can write it and end the request in a
single call to the `end` function.

[source,$lang]
----
{@link examples.HTTPExamples#example36}
----

When you're writing to a request, the first call to `write` will result in the request headers being written
out to the wire.

The actual write is asynchronous and might not occur until some time after the call has returned.

Non-chunked HTTP requests with a request body require a `Content-Length` header to be provided.

Consequently, if you are not using chunked HTTP then you must set the `Content-Length` header before writing
to the request, as it will be too late otherwise.

If you are calling one of the `end` methods that take a string or buffer then Vert.x will automatically calculate
and set the `Content-Length` header before writing the request body.

If you are using HTTP chunking a a `Content-Length` header is not required, so you do not have to calculate the size
up-front.

==== Writing request headers

You can write headers to a request using the {@link io.vertx.core.http.HttpClientRequest#headers()} multi-map as follows:

[source,$lang]
----
{@link examples.HTTPExamples#example37}
----

The headers are an instance of {@link io.vertx.core.MultiMap} which provides operations for adding, setting and removing
entries. Http headers allow more than one value for a specific key.

You can also write headers using {@link io.vertx.core.http.HttpClientRequest#putHeader}

[source,$lang]
----
{@link examples.HTTPExamples#example38}
----

If you wish to write headers to the request you must do so before any part of the request body is written.

==== Non standard HTTP methods

The {@link io.vertx.core.http.HttpMethod#OTHER} HTTP method is used for non standard methods, when this method
is used, {@link io.vertx.core.http.HttpClientRequest#setRawMethod(java.lang.String)} must be used to
set the raw method to send to the server.

==== Ending HTTP requests

Once you have finished with the HTTP request you must end it with one of the {@link io.vertx.core.http.HttpClientRequest#end}
operations.

Ending a request causes any headers to be written, if they have not already been written and the request to be marked
as complete.

Requests can be ended in several ways. With no arguments the request is simply ended:

[source,$lang]
----
{@link examples.HTTPExamples#example39}
----

Or a string or buffer can be provided in the call to `end`. This is like calling `write` with the string or buffer
before calling `end` with no arguments

[source,$lang]
----
{@link examples.HTTPExamples#example40}
----

==== Chunked HTTP requests

Vert.x supports http://en.wikipedia.org/wiki/Chunked_transfer_encoding[HTTP Chunked Transfer Encoding] for requests.

This allows the HTTP request body to be written in chunks, and is normally used when a large request body is being streamed
to the server, whose size is not known in advance.

You put the HTTP request into chunked mode using {@link io.vertx.core.http.HttpClientRequest#setChunked(boolean)}.

In chunked mode each call to write will cause a new chunk to be written to the wire. In chunked mode there is
no need to set the `Content-Length` of the request up-front.

[source,$lang]
----
{@link examples.HTTPExamples#example41}
----

==== Request timeouts

You can set a timeout for a specific http request using {@link io.vertx.core.http.HttpClientRequest#setTimeout(long)}.

If the request does not return any data within the timeout period an exception will be passed to the exception handler
(if provided) and the request will be closed.

==== Handling exceptions

You can handle exceptions corresponding to a request by setting an exception handler on the
{@link io.vertx.core.http.HttpClientRequest} instance:

[source,$lang]
----
{@link examples.HTTPExamples#example42}
----

This does not handle non _2xx_ response that need to be handled in the
{@link io.vertx.core.http.HttpClientResponse} code:

[source, $lang]
----
{@link examples.HTTPExamples#statusCodeHandling}
----

IMPORTANT: `XXXNow` methods cannot receive an exception handler.

==== Specifying a handler on the client request

Instead of providing a response handler in the call to create the client request object, alternatively, you can
not provide a handler when the request is created and set it later on the request object itself, using
{@link io.vertx.core.http.HttpClientRequest#handler(io.vertx.core.Handler)}, for example:

[source,$lang]
----
{@link examples.HTTPExamples#example43}
----

==== Using the request as a stream

The {@link io.vertx.core.http.HttpClientRequest} instance is also a {@link io.vertx.core.streams.WriteStream} which means
you can pump to it from any {@link io.vertx.core.streams.ReadStream} instance.

For, example, you could pump a file on disk to a http request body as follows:

[source,$lang]
----
{@link examples.HTTPExamples#example44}
----

==== Writing HTTP/2 frames

HTTP/2 is a framed protocol with various frames for the HTTP request/response model. The protocol allows other kind
of frames to be sent and received.

To send such frames, you can use the {@link io.vertx.core.http.HttpClientRequest#write} on the request. Here's an example:

[source,$lang]
----
{@link examples.HTTP2Examples#example9}
----

==== Stream reset

HTTP/1.x does not allow a clean reset of a request or a response stream, for example when a client uploads a resource already
present on the server, the server needs to accept the entire response.

HTTP/2 supports stream reset at any time during the request/response:

[source,$lang]
----
{@link examples.HTTP2Examples#example10}
----

By default the NO_ERROR (0) error code is sent, another code can sent instead:

[source,$lang]
----
{@link examples.HTTP2Examples#example11}
----

The HTTP/2 specification defines the list of http://httpwg.org/specs/rfc7540.html#ErrorCodes[error codes] one can use.

The request handler are notified of stream reset events with the {@link io.vertx.core.http.HttpClientRequest#exceptionHandler request handler} and
{@link io.vertx.core.http.HttpClientResponse#exceptionHandler response handler}:

[source,$lang]
----
{@link examples.HTTP2Examples#example12}
----

=== Handling HTTP responses

You receive an instance of {@link io.vertx.core.http.HttpClientResponse} into the handler that you specify in of
the request methods or by setting a handler directly on the {@link io.vertx.core.http.HttpClientRequest} object.

You can query the status code and the status message of the response with {@link io.vertx.core.http.HttpClientResponse#statusCode}
and {@link io.vertx.core.http.HttpClientResponse#statusMessage}.

[source,$lang]
----
{@link examples.HTTPExamples#example45}
----

==== Using the response as a stream

The {@link io.vertx.core.http.HttpClientResponse} instance is also a {@link io.vertx.core.streams.ReadStream} which means
you can pipe it to any {@link io.vertx.core.streams.WriteStream} instance.

==== Response headers and trailers

Http responses can contain headers. Use {@link io.vertx.core.http.HttpClientResponse#headers} to get the headers.

The object returned is a {@link io.vertx.core.MultiMap} as HTTP headers can contain multiple values for single keys.

[source,$lang]
----
{@link examples.HTTPExamples#example46}
----

Chunked HTTP responses can also contain trailers - these are sent in the last chunk of the response body.

You use {@link io.vertx.core.http.HttpClientResponse#trailers} to get the trailers. Trailers are also a {@link io.vertx.core.MultiMap}.

==== Reading the request body

The response handler is called when the headers of the response have been read from the wire.

If the response has a body this might arrive in several pieces some time after the headers have been read. We
don't wait for all the body to arrive before calling the response handler as the response could be very large and we
might be waiting a long time, or run out of memory for large responses.

As parts of the response body arrive, the {@link io.vertx.core.http.HttpClientResponse#handler} is called with
a {@link io.vertx.core.buffer.Buffer} representing the piece of the body:

[source,$lang]
----
{@link examples.HTTPExamples#example47}
----

If you know the response body is not very large and want to aggregate it all in memory before handling it, you can
either aggregate it yourself:

[source,$lang]
----
{@link examples.HTTPExamples#example48}
----

Or you can use the convenience {@link io.vertx.core.http.HttpClientResponse#bodyHandler(io.vertx.core.Handler)} which
is called with the entire body when the response has been fully read:

[source,$lang]
----
{@link examples.HTTPExamples#example49}
----

==== Response end handler

The response {@link io.vertx.core.http.HttpClientResponse#endHandler} is called when the entire response body has been read
or immediately after the headers have been read and the response handler has been called if there is no body.

==== Reading cookies from the response

You can retrieve the list of cookies from a response using {@link io.vertx.core.http.HttpClientResponse#cookies()}.

Alternatively you can just parse the `Set-Cookie` headers yourself in the response.

==== 30x redirection handling

The client can be configured to follow HTTP redirections provided by the `Location` response header when the client receives:

* a `301`, `302`, `307` or `308` status code along with a HTTP GET or HEAD method
* a `303` status code, in addition the directed request perform an HTTP GET methodn

Here's an example:

[source,$lang]
----
{@link examples.HTTPExamples#exampleFollowRedirect01}
----

The maximum redirects is `16` by default and can be changed with {@link io.vertx.core.http.HttpClientOptions#setMaxRedirects(int)}.

[source,$lang]
----
{@link examples.HTTPExamples#exampleFollowRedirect02}
----

One size does not fit all and the default redirection policy may not be adapted to your needs.

The default redirection policy can changed with a custom implementation:

[source,$lang]
----
{@link examples.HTTPExamples#exampleFollowRedirect03}
----

The policy handles the original {@link io.vertx.core.http.HttpClientResponse} received and returns either `null`
or a `Future<HttpClientRequest>`.

- when `null` is returned, the original response is processed
- when a future is returned, the request will be sent on its successful completion
- when a future is returned, the exception handler set on the request is called on its failure

The returned request must be unsent so the original request handlers can be sent and the client can send it after.

Most of the original request settings will be propagated to the new request:

* request headers, unless if you have set some headers (including {@link io.vertx.core.http.HttpClientRequest#setHost})
* request body unless the returned request uses a `GET` method
* response handler
* request exception handler
* request timeout

==== 100-Continue handling

According to the http://www.w3.org/Protocols/rfc2616/rfc2616-sec8.html[HTTP 1.1 specification] a client can set a
header `Expect: 100-Continue` and send the request header before sending the rest of the request body.

The server can then respond with an interim response status `Status: 100 (Continue)` to signify to the client that
it is ok to send the rest of the body.

The idea here is it allows the server to authorise and accept/reject the request before large amounts of data are sent.
Sending large amounts of data if the request might not be accepted is a waste of bandwidth and ties up the server
in reading data that it will just discard.

Vert.x allows you to set a {@link io.vertx.core.http.HttpClientRequest#continueHandler(io.vertx.core.Handler)} on the
client request object

This will be called if the server sends back a `Status: 100 (Continue)` response to signify that it is ok to send
the rest of the request.

This is used in conjunction with {@link io.vertx.core.http.HttpClientRequest#sendHead()}to send the head of the request.

Here's an example:

[source,$lang]
----
{@link examples.HTTPExamples#example50}
----

On the server side a Vert.x http server can be configured to automatically send back 100 Continue interim responses
when it receives an `Expect: 100-Continue` header.

This is done by setting the option {@link io.vertx.core.http.HttpServerOptions#setHandle100ContinueAutomatically(boolean)}.

If you'd prefer to decide whether to send back continue responses manually, then this property should be set to
`false` (the default), then you can inspect the headers and call {@link io.vertx.core.http.HttpServerResponse#writeContinue()}
to have the client continue sending the body:

[source,$lang]
----
{@link examples.HTTPExamples#example50_1}
----

You can also reject the request by sending back a failure status code directly: in this case the body
should either be ignored or the connection should be closed (100-Continue is a performance hint and
cannot be a logical protocol constraint):

[source,$lang]
----
{@link examples.HTTPExamples#example50_2}
----

==== Client push

Server push is a new feature of HTTP/2 that enables sending multiple responses in parallel for a single client request.

A push handler can be set on a request to receive the request/response pushed by the server:

[source,$lang]
----
{@link examples.HTTP2Examples#example13}
----

If the client does not want to receive a pushed request, it can reset the stream:

[source,$lang]
----
{@link examples.HTTP2Examples#example14}
----

When no handler is set, any stream pushed will be automatically cancelled by the client with
a stream reset (`8` error code).

==== Receiving custom HTTP/2 frames

HTTP/2 is a framed protocol with various frames for the HTTP request/response model. The protocol allows other kind of
frames to be sent and received.

To receive custom frames, you can use the customFrameHandler on the request, this will get called every time a custom
frame arrives. Here's an example:

[source,$lang]
----
{@link examples.HTTP2Examples#example15}
----

=== Enabling compression on the client

The http client comes with support for HTTP Compression out of the box.

This means the client can let the remote http server know that it supports compression, and will be able to handle
compressed response bodies.

An http server is free to either compress with one of the supported compression algorithms or to send the body back
without compressing it at all. So this is only a hint for the Http server which it may ignore at will.

To tell the http server which compression is supported by the client it will include an `Accept-Encoding` header with
the supported compression algorithm as value. Multiple compression algorithms are supported. In case of Vert.x this
will result in the following header added:

 Accept-Encoding: gzip, deflate

The server will choose then from one of these. You can detect if a server ompressed the body by checking for the
`Content-Encoding` header in the response sent back from it.

If the body of the response was compressed via gzip it will include for example the following header:

 Content-Encoding: gzip

To enable compression set {@link io.vertx.core.http.HttpClientOptions#setTryUseCompression(boolean)} on the options
used when creating the client.

By default compression is disabled.

=== HTTP/1.x pooling and keep alive

Http keep alive allows http connections to be used for more than one request. This can be a more efficient use of
connections when you're making multiple requests to the same server.

For HTTP/1.x versions, the http client supports pooling of connections, allowing you to reuse connections between requests.

For pooling to work, keep alive must be true using {@link io.vertx.core.http.HttpClientOptions#setKeepAlive(boolean)}
on the options used when configuring the client. The default value is true.

When keep alive is enabled. Vert.x will add a `Connection: Keep-Alive` header to each HTTP/1.0 request sent.
When keep alive is disabled. Vert.x will add a `Connection: Close` header to each HTTP/1.1 request sent to signal
that the connection will be closed after completion of the response.

The maximum number of connections to pool *for each server* is configured using {@link io.vertx.core.http.HttpClientOptions#setMaxPoolSize(int)}

When making a request with pooling enabled, Vert.x will create a new connection if there are less than the maximum number of
connections already created for that server, otherwise it will add the request to a queue.

Keep alive connections will be closed by the client automatically after a timeout. The timeout can be specified
by the server using the `keep-alive` header:

----
 keep-alive: timeout=30
----

You can set the default timeout using {@link io.vertx.core.http.HttpClientOptions#setKeepAliveTimeout(int)} - any
connections not used within this timeout will be closed. Please note the timeout value is in seconds not milliseconds.

=== HTTP/1.1 pipe-lining

The client also supports pipe-lining of requests on a connection.

Pipe-lining means another request is sent on the same connection before the response from the preceding one has
returned. Pipe-lining is not appropriate for all requests.

To enable pipe-lining, it must be enabled using {@link io.vertx.core.http.HttpClientOptions#setPipelining(boolean)}.
By default pipe-lining is disabled.

When pipe-lining is enabled requests will be written to connections without waiting for previous responses to return.

The number of pipe-lined requests over a single connection is limited by {@link io.vertx.core.http.HttpClientOptions#setPipeliningLimit}.
This option defines the maximum number of http requests sent to the server awaiting for a response. This limit ensures the
fairness of the distribution of the client requests over the connections to the same server.

=== HTTP/2 multiplexing

HTTP/2 advocates to use a single connection to a server, by default the http client uses a single
connection for each server, all the streams to the same server are multiplexed over the same connection.

When the clients needs to use more than a single connection and use pooling, the {@link io.vertx.core.http.HttpClientOptions#setHttp2MaxPoolSize(int)}
shall be used.

When it is desirable to limit the number of multiplexed streams per connection and use a connection
pool instead of a single connection, {@link io.vertx.core.http.HttpClientOptions#setHttp2MultiplexingLimit(int)}
can be used.

[source,$lang]
----
{@link examples.HTTP2Examples#useMaxStreams}
----

The multiplexing limit for a connection is a setting set on the client that limits the number of streams
of a single connection. The effective value can be even lower if the server sets a lower limit
with the {@link io.vertx.core.http.Http2Settings#setMaxConcurrentStreams SETTINGS_MAX_CONCURRENT_STREAMS} setting.

HTTP/2 connections will not be closed by the client automatically. To close them you can call {@link io.vertx.core.http.HttpConnection#close()}
or close the client instance.

Alternatively you can set idle timeout using {@link io.vertx.core.http.HttpClientOptions#setIdleTimeout(int)} - any
connections not used within this timeout will be closed. Please note the idle timeout value is in seconds not milliseconds.

=== HTTP connections

The {@link io.vertx.core.http.HttpConnection} offers the API for dealing with HTTP connection events, lifecycle
and settings.

HTTP/2 implements fully the {@link io.vertx.core.http.HttpConnection} API.

HTTP/1.x implements partially the {@link io.vertx.core.http.HttpConnection} API: only the close operation,
the close handler and exception handler are implemented. This protocol does not provide semantics for
the other operations.

==== Server connections

The {@link io.vertx.core.http.HttpServerRequest#connection()} method returns the request connection on the server:

[source,$lang]
----
{@link examples.HTTP2Examples#example16}
----

A connection handler can be set on the server to be notified of any incoming connection:

[source,$lang]
----
{@link examples.HTTP2Examples#example17}
----

==== Client connections

The {@link io.vertx.core.http.HttpClientRequest#connection()} method returns the request connection on the client:

[source,$lang]
----
{@link examples.HTTP2Examples#example18}
----

A connection handler can be set on the client to be notified when a connection has been established happens:

[source,$lang]
----
{@link examples.HTTP2Examples#example19}
----

==== Connection settings

The configuration of an HTTP/2 is configured by the {@link io.vertx.core.http.Http2Settings} data object.

Each endpoint must respect the settings sent by the other side of the connection.

When a connection is established, the client and the server exchange initial settings. Initial settings
are configured by {@link io.vertx.core.http.HttpClientOptions#setInitialSettings} on the client and
{@link io.vertx.core.http.HttpServerOptions#setInitialSettings} on the server.

The settings can be changed at any time after the connection is established:

[source,$lang]
----
{@link examples.HTTP2Examples#example20}
----

As the remote side should acknowledge on reception of the settings update, it's possible to give a callback
to be notified of the acknowledgment:

[source,$lang]
----
{@link examples.HTTP2Examples#example21}
----

Conversely the {@link io.vertx.core.http.HttpConnection#remoteSettingsHandler(io.vertx.core.Handler)} is notified
when the new remote settings are received:

[source,$lang]
----
{@link examples.HTTP2Examples#example22}
----

NOTE: this only applies to the HTTP/2 protocol

==== Connection ping

HTTP/2 connection ping is useful for determining the connection round-trip time or check the connection
validity: {@link io.vertx.core.http.HttpConnection#ping} sends a {@literal PING} frame to the remote
endpoint:

[source,$lang]
----
{@link examples.HTTP2Examples#example23}
----

Vert.x will send automatically an acknowledgement when a {@literal PING} frame is received,
an handler can be set to be notified for each ping received:

[source,$lang]
----
{@link examples.HTTP2Examples#example24}
----

The handler is just notified, the acknowledgement is sent whatsoever. Such feature is aimed for
implementing  protocols on top of HTTP/2.

NOTE: this only applies to the HTTP/2 protocol

==== Connection shutdown and go away

Calling {@link io.vertx.core.http.HttpConnection#shutdown()} will send a {@literal GOAWAY} frame to the
remote side of the connection, asking it to stop creating streams: a client will stop doing new requests
and a server will stop pushing responses. After the {@literal GOAWAY} frame is sent, the connection
waits some time (30 seconds by default) until all current streams closed and close the connection:

[source,$lang]
----
{@link examples.HTTP2Examples#example25}
----

The {@link io.vertx.core.http.HttpConnection#shutdownHandler} notifies when all streams have been closed, the
connection is not yet closed.

It's possible to just send a {@literal GOAWAY} frame, the main difference with a shutdown is that
it will just tell the remote side of the connection to stop creating new streams without scheduling a connection
close:

[source,$lang]
----
{@link examples.HTTP2Examples#example26}
----

Conversely, it is also possible to be notified when {@literal GOAWAY} are received:

[source,$lang]
----
{@link examples.HTTP2Examples#example27}
----

The {@link io.vertx.core.http.HttpConnection#shutdownHandler} will be called when all current streams
have been closed and the connection can be closed:

[source,$lang]
----
{@link examples.HTTP2Examples#example28}
----

This applies also when a {@literal GOAWAY} is received.

NOTE: this only applies to the HTTP/2 protocol

==== Connection close

Connection {@link io.vertx.core.http.HttpConnection#close} closes the connection:

- it closes the socket for HTTP/1.x
- a shutdown with no delay for HTTP/2, the {@literal GOAWAY} frame will still be sent before the connection is closed. *

The {@link io.vertx.core.http.HttpConnection#closeHandler} notifies when a connection is closed.

=== HttpClient usage

The HttpClient can be used in a Verticle or embedded.

When used in a Verticle, the Verticle *should use its own client instance*.

More generally a client should not be shared between different Vert.x contexts as it can lead to unexpected behavior.

For example a keep-alive connection will call the client handlers on the context of the request that opened the connection, subsequent requests will use
the same context.

When this happen Vert.x detects it and log a warn:

----
Reusing a connection with a different context: an HttpClient is probably shared between different Verticles
----

The HttpClient can be embedded in a non Vert.x thread like a unit test or a plain java `main`: the client handlers
will be called by different Vert.x threads and contexts, such contexts are created as needed. For production this
usage is not recommended.

=== Server sharing

When several HTTP servers listen on the same port, vert.x orchestrates the request handling using a
round-robin strategy.

Let's take a verticle creating a HTTP server such as:

.io.vertx.examples.http.sharing.HttpServerVerticle
[source,$lang]
----
{@link examples.HTTPExamples#serversharing(io.vertx.core.Vertx)}
----

This service is listening on the port 8080. So, when this verticle is instantiated multiple times as with:
`vertx run io.vertx.examples.http.sharing.HttpServerVerticle -instances 2`, what's happening ? If both
verticles would bind to the same port, you would receive a socket exception. Fortunately, vert.x is handling
this case for you. When you deploy another server on the same host and port as an existing server it doesn't
actually try and create a new server listening on the same host/port. It binds only once to the socket. When
receiving a request it calls the server handlers following a round robin strategy.

Let's now imagine a client such as:
[source,$lang]
----
{@link examples.HTTPExamples#serversharingclient(io.vertx.core.Vertx)}
----

Vert.x delegates the requests to one of the server sequentially:

[source]
----
Hello from i.v.e.h.s.HttpServerVerticle@1
Hello from i.v.e.h.s.HttpServerVerticle@2
Hello from i.v.e.h.s.HttpServerVerticle@1
Hello from i.v.e.h.s.HttpServerVerticle@2
...
----

Consequently the servers can scale over available cores while each Vert.x verticle instance remains strictly
single threaded, and you don't have to do any special tricks like writing load-balancers in order to scale your
server on your multi-core machine.

=== Using HTTPS with Vert.x

Vert.x http servers and clients can be configured to use HTTPS in exactly the same way as net servers.

Please see <<ssl, configuring net servers to use SSL>> for more information.

SSL can also be enabled/disabled per request with {@link io.vertx.core.http.RequestOptions} or when
specifying a scheme with {@link io.vertx.core.http.HttpClient#requestAbs(io.vertx.core.http.HttpMethod,java.lang.String)}
method.

[source,$lang]
----
{@link examples.HTTPExamples#setSSLPerRequest(io.vertx.core.http.HttpClient)}
----

The {@link io.vertx.core.http.HttpClientOptions#setSsl(boolean)} setting acts as the default client setting.

The {@link io.vertx.core.http.RequestOptions#setSsl(Boolean)} overrides the default client setting

* setting the value to `false` will disable SSL/TLS even if the client is configured to use SSL/TLS
* setting the value to `true` will enable SSL/TLS  even if the client is configured to not use SSL/TLS, the actual
client SSL/TLS (such as trust, key/certificate, ciphers, ALPN, ...) will be reused

Likewise {@link io.vertx.core.http.HttpClient#requestAbs(io.vertx.core.http.HttpMethod,java.lang.String)} scheme
also overrides the default client setting.

==== Server Name Indication (SNI)

Vert.x http servers can be configured to use SNI in exactly the same way as {@linkplain io.vertx.core.net net servers}.

Vert.x http client will present the actual hostname as _server name_ during the TLS handshake.

=== WebSockets

http://en.wikipedia.org/wiki/WebSocket[WebSockets] are a web technology that allows a full duplex socket-like
connection between HTTP servers and HTTP clients (typically browsers).

Vert.x supports WebSockets on both the client and server-side.

==== WebSockets on the server

There are two ways of handling WebSockets on the server side.

===== WebSocket handler

The first way involves providing a {@link io.vertx.core.http.HttpServer#webSocketHandler(io.vertx.core.Handler)}
on the server instance.

When a WebSocket connection is made to the server, the handler will be called, passing in an instance of
{@link io.vertx.core.http.ServerWebSocket}.

[source,$lang]
----
{@link examples.HTTPExamples#example51}
----

You can choose to reject the WebSocket by calling {@link io.vertx.core.http.ServerWebSocket#reject()}.

[source,$lang]
----
{@link examples.HTTPExamples#example52}
----

You can perform an asynchronous handshake by calling {@link io.vertx.core.http.ServerWebSocket#setHandshake} with a `Future`:

[source,$lang]
----
{@link examples.HTTPExamples#exampleAsynchronousHandshake}
----

NOTE: the WebSocket will be automatically accepted after the handler is called unless the WebSocket's handshake has been set

===== Upgrading to WebSocket

The second way of handling WebSockets is to handle the HTTP Upgrade request that was sent from the client, and
call {@link io.vertx.core.http.HttpServerRequest#toWebSocket(io.vertx.core.Handler)} on the server request.

[source,$lang]
----
{@link examples.HTTPExamples#example53}
----

===== The server WebSocket

The {@link io.vertx.core.http.ServerWebSocket} instance enables you to retrieve the {@link io.vertx.core.http.ServerWebSocket#headers() headers},
{@link io.vertx.core.http.ServerWebSocket#path() path}, {@link io.vertx.core.http.ServerWebSocket#query() query} and
{@link io.vertx.core.http.ServerWebSocket#uri() URI} of the HTTP request of the WebSocket handshake.

==== WebSockets on the client

The Vert.x {@link io.vertx.core.http.HttpClient} supports WebSockets.

You can connect a WebSocket to a server using one of the {@link io.vertx.core.http.HttpClient#webSocket} operations and
providing a handler.

The handler will be called with an instance of {@link io.vertx.core.http.WebSocket} when the connection has been made:

[source,$lang]
----
{@link examples.HTTPExamples#example54}
----

==== Writing messages to WebSockets

If you wish to write a single WebSocket message to the WebSocket you can do this with
{@link io.vertx.core.http.WebSocket#writeBinaryMessage(io.vertx.core.buffer.Buffer)} or
{@link io.vertx.core.http.WebSocket#writeTextMessage(java.lang.String)} :

[source,$lang]
----
{@link examples.HTTPExamples#example55}
----

If the WebSocket message is larger than the maximum WebSocket frame size as configured with
{@link io.vertx.core.http.HttpClientOptions#setMaxWebSocketFrameSize(int)}
then Vert.x will split it into multiple WebSocket frames before sending it on the wire.

==== Writing frames to WebSockets

A WebSocket message can be composed of multiple frames. In this case the first frame is either a _binary_ or _text_ frame
followed by zero or more _continuation_ frames.

The last frame in the message is marked as _final_.

To send a message consisting of multiple frames you create frames using
{@link io.vertx.core.http.WebSocketFrame#binaryFrame(io.vertx.core.buffer.Buffer,boolean)}
, {@link io.vertx.core.http.WebSocketFrame#textFrame(java.lang.String,boolean)} or
{@link io.vertx.core.http.WebSocketFrame#continuationFrame(io.vertx.core.buffer.Buffer,boolean)} and write them
to the WebSocket using {@link io.vertx.core.http.WebSocket#writeFrame(io.vertx.core.http.WebSocketFrame)}.

Here's an example for binary frames:

[source,$lang]
----
{@link examples.HTTPExamples#example56}
----

In many cases you just want to send a WebSocket message that consists of a single final frame, so we provide a couple
of shortcut methods to do that with {@link io.vertx.core.http.WebSocket#writeFinalBinaryFrame(io.vertx.core.buffer.Buffer)}
and {@link io.vertx.core.http.WebSocket#writeFinalTextFrame(String)}.

Here's an example:

[source,$lang]
----
{@link examples.HTTPExamples#example56_1}
----

==== Reading frames from WebSockets

To read frames from a WebSocket you use the {@link io.vertx.core.http.WebSocket#frameHandler(io.vertx.core.Handler)}.

The frame handler will be called with instances of {@link io.vertx.core.http.WebSocketFrame} when a frame arrives,
for example:

[source,$lang]
----
{@link examples.HTTPExamples#example57}
----

==== Closing WebSockets

Use {@link io.vertx.core.http.WebSocket#close()} to close the WebSocket connection when you have finished with it.

==== Piping WebSockets

The {@link io.vertx.core.http.WebSocket} instance is also a {@link io.vertx.core.streams.ReadStream} and a
{@link io.vertx.core.streams.WriteStream} so it can be used with pipes.

When using a WebSocket as a write stream or a read stream it can only be used with WebSockets connections that are
used with binary frames that are no split over multiple frames.

==== Event bus handlers

Every WebSocket automatically registers two handler on the event bus, and when any data are received in this handler,
it writes them to itself. Those are local subscriptions not routed on the cluster.

This enables you to write data to a WebSocket which is potentially in a completely different verticle sending data
to the address of that handler.

The addresses of the handlers are given by {@link io.vertx.core.http.WebSocket#binaryHandlerID()} and
{@link io.vertx.core.http.WebSocket#textHandlerID()}.

=== Using a proxy for HTTP/HTTPS connections

The http client supports accessing http/https URLs via a HTTP proxy (e.g. Squid) or _SOCKS4a_ or _SOCKS5_ proxy.
The CONNECT protocol uses HTTP/1.x but can connect to HTTP/1.x and HTTP/2 servers.

Connecting to h2c (unencrypted HTTP/2 servers) is likely not supported by http proxies since they will support
HTTP/1.1 only.

The proxy can be configured in the {@link io.vertx.core.http.HttpClientOptions} by setting a
{@link io.vertx.core.net.ProxyOptions} object containing proxy type, hostname, port and optionally username and password.

Here's an example of using an HTTP proxy:

[source,$lang]
----
{@link examples.HTTPExamples#example58}
----

When the client connects to an http URL, it connects to the proxy server and provides the full URL in the
HTTP request ("GET http://www.somehost.com/path/file.html HTTP/1.1").

When the client connects to an https URL, it asks the proxy to create a tunnel to the remote host with
the CONNECT method.

For a SOCKS5 proxy:

[source,$lang]
----
{@link examples.HTTPExamples#example59}
----

The DNS resolution is always done on the proxy server, to achieve the functionality of a SOCKS4 client, it is necessary
to resolve the DNS address locally.

==== Handling of other protocols

The HTTP proxy implementation supports getting ftp:// urls if the proxy supports
that, which isn't available in non-proxy getAbs requests.

[source,$lang]
----
{@link examples.HTTPExamples#example60}
----

Support for other protocols is not available since java.net.URL does not
support them (gopher:// for example).

=== Automatic clean-up in verticles

If you're creating http servers and clients from inside verticles, those servers and clients will be automatically closed
when the verticle is undeployed.
