view mercurial/help/internals/wireprotocol.txt @ 29866:b42c26b0a785

help: document wire protocol transport protocols

The HTTP and SSH transport protocols are documented. This
includes how commands and arguments are serialized as well as
response types.

The Mercurial wire protocol is a request-response based protocol
with multiple wire representations.

Each request is modeled as a command name, a dictionary of arguments, and
optional raw input. Command arguments and their types are intrinsic
properties of commands. So is the response type of the command. This means
clients can't always send arbitrary arguments to servers and servers can't
return multiple response types.

The protocol is synchronous and does not support multiplexing (concurrent
commands).

Transport Protocols
===================

HTTP Transport
--------------

Commands are issued as HTTP/1.0 or HTTP/1.1 requests. Commands are
sent to the base URL of the repository with the command name sent in
the ``cmd`` query string parameter. e.g.
``https://example.com/repo?cmd=capabilities``. The HTTP method is ``GET``
or ``POST`` depending on the command and whether there is a request
body.

Command arguments can be sent multiple ways.

The simplest is part of the URL query string using ``x-www-form-urlencoded``
encoding (see Python's ``urllib.urlencode()``. However, many servers impose
length limitations on the URL. So this mechanism is typically only used if
the server doesn't support other mechanisms.

If the server supports the ``httpheader`` capability, command arguments can
be sent in HTTP request headers named ``X-HgArg-<N>`` where ``<N>`` is an
integer starting at 1. A ``x-www-form-urlencoded`` representation of the
arguments is obtained. This full string is then split into chunks and sent
in numbered ``X-HgArg-<N>`` headers. The maximum length of each HTTP header
is defined by the server in the ``httpheader`` capability value, which defaults
to ``1024``. The server reassembles the encoded arguments string by
concatenating the ``X-HgArg-<N>`` headers then URL decodes them into a
dictionary.

The list of ``X-HgArg-<N>`` headers should be added to the ``Vary`` request
header to instruct caches to take these headers into consideration when caching
requests.

If the server supports the ``httppostargs`` capability, the client
may send command arguments in the HTTP request body as part of an
HTTP POST request. The command arguments will be URL encoded just like
they would for sending them via HTTP headers. However, no splitting is
performed: the raw arguments are included in the HTTP request body.

The client sends a ``X-HgArgs-Post`` header with the string length of the
encoded arguments data. Additional data may be included in the HTTP
request body immediately following the argument data. The offset of the
non-argument data is defined by the ``X-HgArgs-Post`` header. The
``X-HgArgs-Post`` header is not required if there is no argument data.

Additional command data can be sent as part of the HTTP request body. The
default ``Content-Type`` when sending data is ``application/mercurial-0.1``.
A ``Content-Length`` header is currently always sent.

Example HTTP requests::

    GET /repo?cmd=capabilities
    X-HgArg-1: foo=bar&baz=hello%20world

The ``Content-Type`` HTTP response header identifies the response as coming
from Mercurial and can also be used to signal an error has occurred.

The ``application/mercurial-0.1`` media type indicates a generic Mercurial
response. It matches the media type sent by the client.

The ``application/hg-error`` media type indicates a generic error occurred.
The content of the HTTP response body typically holds text describing the
error.

The ``application/hg-changegroup`` media type indicates a changegroup response
type.

Clients also accept the ``text/plain`` media type. All other media
types should cause the client to error.

Clients should issue a ``User-Agent`` request header that identifies the client.
The server should not use the ``User-Agent`` for feature detection.

A command returning a ``string`` response issues the
``application/mercurial-0.1`` media type and the HTTP response body contains
the raw string value. A ``Content-Length`` header is typically issued.

A command returning a ``stream`` response issues the
``application/mercurial-0.1`` media type and the HTTP response is typically
using *chunked transfer* (``Transfer-Encoding: chunked``).

SSH Transport
=============

The SSH transport is a custom text-based protocol suitable for use over any
bi-directional stream transport. It is most commonly used with SSH.

A SSH transport server can be started with ``hg serve --stdio``. The stdin,
stderr, and stdout file descriptors of the started process are used to exchange
data. When Mercurial connects to a remote server over SSH, it actually starts
a ``hg serve --stdio`` process on the remote server.

Commands are issued by sending the command name followed by a trailing newline
``\n`` to the server. e.g. ``capabilities\n``.

Command arguments are sent in the following format::

    <argument> <length>\n<value>

That is, the argument string name followed by a space followed by the
integer length of the value (expressed as a string) followed by a newline
(``\n``) followed by the raw argument value.

Dictionary arguments are encoded differently::

    <argument> <# elements>\n
    <key1> <length1>\n<value1>
    <key2> <length2>\n<value2>
    ...

Non-argument data is sent immediately after the final argument value. It is
encoded in chunks::

    <length>\n<data>

Each command declares a list of supported arguments and their types. If a
client sends an unknown argument to the server, the server should abort
immediately. The special argument ``*`` in a command's definition indicates
that all argument names are allowed.

The definition of supported arguments and types is initially made when a
new command is implemented. The client and server must initially independently
agree on the arguments and their types. This initial set of arguments can be
supplemented through the presence of *capabilities* advertised by the server.

Each command has a defined expected response type.

A ``string`` response type is a length framed value. The response consists of
the string encoded integer length of a value followed by a newline (``\n``)
followed by the value. Empty values are allowed (and are represented as
``0\n``).

A ``stream`` response type consists of raw bytes of data. There is no framing.

A generic error response type is also supported. It consists of a an error
message written to ``stderr`` followed by ``\n-\n``. In addition, ``\n`` is
written to ``stdout``.

If the server receives an unknown command, it will send an empty ``string``
response.

The server terminates if it receives an empty command (a ``\n`` character).