Help: internals.wireprotocol

Wire Protocol

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).

Handshake

It is required or common for clients to perform a *handshake* when connecting to a server. The handshake serves the following purposes:

  • Negotiating protocol/transport level options
  • Allows the client to learn about server capabilities to influence future requests
  • Ensures the underlying transport channel is in a *clean* state

An important goal of the handshake is to allow clients to use more modern wire protocol features. By default, clients must assume they are talking to an old version of Mercurial server (possibly even the very first implementation). So, clients should not attempt to call or utilize modern wire protocol features until they have confirmation that the server supports them. The handshake implementation is designed to allow both ends to utilize the latest set of features and capabilities with as few round trips as possible.

The handshake mechanism varies by transport and protocol and is documented in the sections below.

HTTP Protocol

Handshake

The client sends a "capabilities" command request ("?cmd=capabilities") as soon as HTTP requests may be issued.

By default, the server responds with a version 1 capabilities string, which the client parses to learn about the server's abilities. The "Content-Type" for this response is "application/mercurial-0.1" or "application/mercurial-0.2" depending on whether the client advertised support for version "0.2" in its request. (Clients aren't supposed to advertise support for "0.2" until the capabilities response indicates the server's support for that media type. However, a client could conceivably cache this metadata and issue the capabilities request in such a way to elicit an "application/mercurial-0.2" response.)

Clients wishing to switch to a newer API service may send an "X-HgUpgrade-<X>" header containing a space-delimited list of API service names the client is capable of speaking. The request MUST also include an "X-HgProto-<X>" header advertising a known serialization format for the response. "cbor" is currently the only defined serialization format.

If the request contains these headers, the response "Content-Type" MAY be for a different media type. e.g. "application/mercurial-cbor" if the client advertises support for CBOR.

The response MUST be deserializable to a map with the following keys:

apibase
URL path to API services, relative to the repository root. e.g. "api/".
apis
A map of API service names to API descriptors. An API descriptor contains more details about that API. In the case of the HTTP Version 2 Transport, it will be the normal response to a "capabilities" command.

Only the services advertised by the client that are also available on the server are advertised.

v1capabilities
The capabilities string that would be returned by a version 1 response.

The client can then inspect the server-advertised APIs and decide which API to use, including continuing to use the HTTP Version 1 Transport.

HTTP Version 1 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 request media type should be chosen based on server support. If the "httpmediatype" server capability is present, the client should send the newest mutually supported media type. If this capability is absent, the client must assume the server only supports the "application/mercurial-0.1" media type.

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-*" media types indicate a generic Mercurial data type.

The "application/mercurial-0.1" media type is raw Mercurial data. It is the predecessor of the format below.

The "application/mercurial-0.2" media type is compression framed Mercurial data. The first byte of the payload indicates the length of the compression format identifier that follows. Next are N bytes indicating the compression format. e.g. "zlib". The remaining bytes are compressed according to that compression format. The decompressed data behaves the same as with "application/mercurial-0.1".

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/mercurial-cbor" media type indicates a CBOR payload and should be interpreted as identical to "application/cbor".

Behavior of media types is further described in the "Content Negotiation" section below.

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 a "application/mercurial-0.*" media type and the HTTP response body contains the raw string value (after compression decoding, if used). A "Content-Length" header is typically issued, but not required.

A command returning a "stream" response issues a "application/mercurial-0.*" media type and the HTTP response is typically using *chunked transfer* ("Transfer-Encoding: chunked").

HTTP Version 2 Transport

**Experimental - feature under active development**

Version 2 of the HTTP protocol is exposed under the "/api/*" URL space. It's final API name is not yet formalized.

Commands are triggered by sending HTTP POST requests against URLs of the form "<permission>/<command>", where "<permission>" is "ro" or "rw", meaning read-only and read-write, respectively and "<command>" is a named wire protocol command.

Non-POST request methods MUST be rejected by the server with an HTTP 405 response.

Commands that modify repository state in meaningful ways MUST NOT be exposed under the "ro" URL prefix. All available commands MUST be available under the "rw" URL prefix.

Server adminstrators MAY implement blanket HTTP authentication keyed off the URL prefix. For example, a server may require authentication for all "rw/*" URLs and let unauthenticated requests to "ro/*" URL proceed. A server MAY issue an HTTP 401, 403, or 407 response in accordance with RFC 7235. Clients SHOULD recognize the HTTP Basic (RFC 7617) and Digest (RFC 7616) authentication schemes. Clients SHOULD make an attempt to recognize unknown schemes using the "WWW-Authenticate" response header on a 401 response, as defined by RFC 7235.

Read-only commands are accessible under "rw/*" URLs so clients can signal the intent of the operation very early in the connection lifecycle. For example, a "push" operation - which consists of various read-only commands mixed with at least one read-write command - can perform all commands against "rw/*" URLs so that any server-side authentication requirements are discovered upon attempting the first command - not potentially several commands into the exchange. This allows clients to fail faster or prompt for credentials as soon as the exchange takes place. This provides a better end-user experience.

Requests to unknown commands or URLS result in an HTTP 404. TODO formally define response type, how error is communicated, etc.

HTTP request and response bodies use the *Unified Frame-Based Protocol* (defined below) for media exchange. The entirety of the HTTP message body is 0 or more frames as defined by this protocol.

Clients and servers MUST advertise the "TBD" media type via the "Content-Type" request and response headers. In addition, clients MUST advertise this media type value in their "Accept" request header in all requests. TODO finalize the media type. For now, it is defined in wireprotoserver.py.

Servers receiving requests without an "Accept" header SHOULD respond with an HTTP 406.

Servers receiving requests with an invalid "Content-Type" header SHOULD respond with an HTTP 415.

The command to run is specified in the POST payload as defined by the *Unified Frame-Based Protocol*. This is redundant with data already encoded in the URL. This is by design, so server operators can have better understanding about server activity from looking merely at HTTP access logs.

In most circumstances, the command specified in the URL MUST match the command specified in the frame-based payload or the server will respond with an error. The exception to this is the special "multirequest" URL. (See below.) In addition, HTTP requests are limited to one command invocation. The exception is the special "multirequest" URL.

The "multirequest" command endpoints ("ro/multirequest" and "rw/multirequest") are special in that they allow the execution of *any* command and allow the execution of multiple commands. If the HTTP request issues multiple commands across multiple frames, all issued commands will be processed by the server. Per the defined behavior of the *Unified Frame-Based Protocol*, commands may be issued interleaved and responses may come back in a different order than they were issued. Clients MUST be able to deal with this.

SSH Protocol

Handshake

For all clients, the handshake consists of the client sending 1 or more commands to the server using version 1 of the transport. Servers respond to commands they know how to respond to and send an empty response ("0\n") for unknown commands (per standard behavior of version 1 of the transport). Clients then typically look for a response to the newest sent command to determine which transport version to use and what the available features for the connection and server are.

Preceding any response from client-issued commands, the server may print non-protocol output. It is common for SSH servers to print banners, message of the day announcements, etc when clients connect. It is assumed that any such *banner* output will precede any Mercurial server output. So clients must be prepared to handle server output on initial connect that isn't in response to any client-issued command and doesn't conform to Mercurial's wire protocol. This *banner* output should only be on stdout. However, some servers may send output on stderr.

Pre 0.9.1 clients issue a "between" command with the "pairs" argument having the value "0000000000000000000000000000000000000000-0000000000000000000000000000000000000000".

The "between" command has been supported since the original Mercurial SSH server. Requesting the empty range will return a "\n" string response, which will be encoded as "1\n\n" (value length of "1" followed by a newline followed by the value, which happens to be a newline).

For pre 0.9.1 clients and all servers, the exchange looks like:

c: between\n
c: pairs 81\n
c: 0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
s: 1\n
s: \n

0.9.1+ clients send a "hello" command (with no arguments) before the "between" command. The response to this command allows clients to discover server capabilities and settings.

An example exchange between 0.9.1+ clients and a "hello" aware server looks like:

c: hello\n
c: between\n
c: pairs 81\n
c: 0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
s: 324\n
s: capabilities: lookup changegroupsubset branchmap pushkey known getbundle ...\n
s: 1\n
s: \n

And a similar scenario but with servers sending a banner on connect:

c: hello\n
c: between\n
c: pairs 81\n
c: 0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
s: welcome to the server\n
s: if you find any issues, email someone@somewhere.com\n
s: 324\n
s: capabilities: lookup changegroupsubset branchmap pushkey known getbundle ...\n
s: 1\n
s: \n

Note that output from the "hello" command is terminated by a "\n". This is part of the response payload and not part of the wire protocol adding a newline after responses. In other words, the length of the response contains the trailing "\n".

Clients supporting version 2 of the SSH transport send a line beginning with "upgrade" before the "hello" and "between" commands. The line (which isn't a well-formed command line because it doesn't consist of a single command name) serves to both communicate the client's intent to switch to transport version 2 (transports are version 1 by default) as well as to advertise the client's transport-level capabilities so the server may satisfy that request immediately.

The upgrade line has the form:

upgrade <token> <transport capabilities>

That is the literal string "upgrade" followed by a space, followed by a randomly generated string, followed by a space, followed by a string denoting the client's transport capabilities.

The token can be anything. However, a random UUID is recommended. (Use of version 4 UUIDs is recommended because version 1 UUIDs can leak the client's MAC address.)

The transport capabilities string is a URL/percent encoded string containing key-value pairs defining the client's transport-level capabilities. The following capabilities are defined:

proto
A comma-delimited list of transport protocol versions the client supports. e.g. "ssh-v2".

If the server does not recognize the "upgrade" line, it should issue an empty response and continue processing the "hello" and "between" commands. Here is an example handshake between a version 2 aware client and a non version 2 aware server:

c: upgrade 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a proto=ssh-v2 c: hello\n c: between\n c: pairs 81\n c: 0000000000000000000000000000000000000000-0000000000000000000000000000000000000000 s: 0\n s: 324\n s: capabilities: lookup changegroupsubset branchmap pushkey known getbundle ...\n s: 1\n s: \n

(The initial "0\n" line from the server indicates an empty response to the unknown "upgrade .." command/line.)

If the server recognizes the "upgrade" line and is willing to satisfy that upgrade request, it replies to with a payload of the following form:

upgraded <token> <transport name>\n

This line is the literal string "upgraded", a space, the token that was specified by the client in its "upgrade ..." request line, a space, and the name of the transport protocol that was chosen by the server. The transport name MUST match one of the names the client specified in the "proto" field of its "upgrade ..." request line.

If a server issues an "upgraded" response, it MUST also read and ignore the lines associated with the "hello" and "between" command requests that were issued by the server. It is assumed that the negotiated transport will respond with equivalent requested information following the transport handshake.

All data following the "\n" terminating the "upgraded" line is the domain of the negotiated transport. It is common for the data immediately following to contain additional metadata about the state of the transport and the server. However, this isn't strictly speaking part of the transport handshake and isn't covered by this section.

Here is an example handshake between a version 2 aware client and a version 2 aware server:

c: upgrade 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a proto=ssh-v2 c: hello\n c: between\n c: pairs 81\n c: 0000000000000000000000000000000000000000-0000000000000000000000000000000000000000 s: upgraded 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a ssh-v2\n s: <additional transport specific data>

The client-issued token that is echoed in the response provides a more resilient mechanism for differentiating *banner* output from Mercurial output. In version 1, properly formatted banner output could get confused for Mercurial server output. By submitting a randomly generated token that is then present in the response, the client can look for that token in response lines and have reasonable certainty that the line did not originate from a *banner* message.

SSH Version 1 Transport

The SSH transport (version 1) 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).

If the server announces support for the "protocaps" capability, the client should issue a "protocaps" command after the initial handshake to annonunce its own capabilities. The client capabilities are persistent.

SSH Version 2 Transport

**Experimental and under development**

Version 2 of the SSH transport behaves identically to version 1 of the SSH transport with the exception of handshake semantics. See above for how version 2 of the SSH transport is negotiated.

Immediately following the "upgraded" line signaling a switch to version 2 of the SSH protocol, the server automatically sends additional details about the capabilities of the remote server. This has the form:

<integer length of value>\n capabilities: ...\n

e.g.

s: upgraded 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a ssh-v2\n s: 240\n s: capabilities: known getbundle batch ...\n

Following capabilities advertisement, the peers communicate using version 1 of the SSH transport.

Unified Frame-Based Protocol

**Experimental and under development**

The *Unified Frame-Based Protocol* is a communications protocol between Mercurial peers. The protocol aims to be mostly transport agnostic (works similarly on HTTP, SSH, etc).

To operate the protocol, a bi-directional, half-duplex pipe supporting ordered sends and receives is required. That is, each peer has one pipe for sending data and another for receiving.

All data is read and written in atomic units called *frames*. These are conceptually similar to TCP packets. Higher-level functionality is built on the exchange and processing of frames.

All frames are associated with a *stream*. A *stream* provides a unidirectional grouping of frames. Streams facilitate two goals: content encoding and parallelism. There is a dedicated section on streams below.

The protocol is request-response based: the client issues requests to the server, which issues replies to those requests. Server-initiated messaging is not currently supported, but this specification carves out room to implement it.

All frames are associated with a numbered request. Frames can thus be logically grouped by their request ID.

Frames begin with an 8 octet header followed by a variable length payload:

+------------------------------------------------+
|                 Length (24)                    |
+--------------------------------+---------------+
|         Request ID (16)        | Stream ID (8) |
+------------------+-------------+---------------+
| Stream Flags (8) |
+-----------+------+
| Type (4)  |
+-----------+
| Flags (4) |
+===========+===================================================|
|                     Frame Payload (0...)                    ...
+---------------------------------------------------------------+

The length of the frame payload is expressed as an unsigned 24 bit little endian integer. Values larger than 65535 MUST NOT be used unless given permission by the server as part of the negotiated capabilities during the handshake. The frame header is not part of the advertised frame length. The payload length is the over-the-wire length. If there is content encoding applied to the payload as part of the frame's stream, the length is the output of that content encoding, not the input.

The 16-bit "Request ID" field denotes the integer request identifier, stored as an unsigned little endian integer. Odd numbered requests are client-initiated. Even numbered requests are server-initiated. This refers to where the *request* was initiated - not where the *frame* was initiated, so servers will send frames with odd "Request ID" in response to client-initiated requests. Implementations are advised to start ordering request identifiers at "1" and "0", increment by "2", and wrap around if all available numbers have been exhausted.

The 8-bit "Stream ID" field denotes the stream that the frame is associated with. Frames belonging to a stream may have content encoding applied and the receiver may need to decode the raw frame payload to obtain the original data. Odd numbered IDs are client-initiated. Even numbered IDs are server-initiated.

The 8-bit "Stream Flags" field defines stream processing semantics. See the section on streams below.

The 4-bit "Type" field denotes the type of frame being sent.

The 4-bit "Flags" field defines special, per-type attributes for the frame.

The sections below define the frame types and their behavior.

Command Request ("0x01")

This frame contains a request to run a command.

The payload consists of a CBOR map defining the command request. The bytestring keys of that map are:

name
Name of the command that should be executed (bytestring).
args
Map of bytestring keys to various value types containing the named arguments to this command.

Each command defines its own set of argument names and their expected types.

This frame type MUST ONLY be sent from clients to servers: it is illegal for a server to send this frame to a client.

The following flag values are defined for this type:

0x01
New command request. When set, this frame represents the beginning of a new request to run a command. The "Request ID" attached to this frame MUST NOT be active.
0x02
Command request continuation. When set, this frame is a continuation from a previous command request frame for its "Request ID". This flag is set when the CBOR data for a command request does not fit in a single frame.
0x04
Additional frames expected. When set, the command request didn't fit into a single frame and additional CBOR data follows in a subsequent frame.
0x08
Command data frames expected. When set, command data frames are expected to follow the final command request frame for this request.

"0x01" MUST be set on the initial command request frame for a "Request ID".

"0x01" or "0x02" MUST be set to indicate this frame's role in a series of command request frames.

If command data frames are to be sent, "0x08" MUST be set on ALL command request frames.

Command Data ("0x02")

This frame contains raw data for a command.

Most commands can be executed by specifying arguments. However, arguments have an upper bound to their length. For commands that accept data that is beyond this length or whose length isn't known when the command is initially sent, they will need to stream arbitrary data to the server. This frame type facilitates the sending of this data.

The payload of this frame type consists of a stream of raw data to be consumed by the command handler on the server. The format of the data is command specific.

The following flag values are defined for this type:

0x01
Command data continuation. When set, the data for this command continues into a subsequent frame.
0x02
End of data. When set, command data has been fully sent to the server. The command has been fully issued and no new data for this command will be sent. The next frame will belong to a new command.

Command Response Data ("0x03")

This frame contains response data to an issued command.

Response data ALWAYS consists of a series of 1 or more CBOR encoded values. A CBOR value may be using indefinite length encoding. And the bytes constituting the value may span several frames.

The following flag values are defined for this type:

0x01
Data continuation. When set, an additional frame containing response data will follow.
0x02
End of data. When set, the response data has been fully sent and no additional frames for this response will be sent.

The "0x01" flag is mutually exclusive with the "0x02" flag.

Error Occurred ("0x05")

Some kind of error occurred.

There are 3 general kinds of failures that can occur:

  • Command error encountered before any response issued
  • Command error encountered after a response was issued
  • Protocol or stream level error

This frame type is used to capture the latter cases. (The general command error case is handled by the leading CBOR map in "Command Response" frames.)

The payload of this frame contains a CBOR map detailing the error. That map has the following bytestring keys:

type
(bytestring) The overall type of error encountered. Can be one of the following values:
protocol
A protocol-level error occurred. This typically means someone is violating the framing protocol semantics and the server is refusing to proceed.
server
A server-level error occurred. This typically indicates some kind of logic error on the server, likely the fault of the server.
command
A command-level error, likely the fault of the client.
message
(array of maps) A richly formatted message that is intended for human consumption. See the "Human Output Side-Channel" frame section for a description of the format of this data structure.

Human Output Side-Channel ("0x06")

This frame contains a message that is intended to be displayed to people. Whereas most frames communicate machine readable data, this frame communicates textual data that is intended to be shown to humans.

The frame consists of a series of *formatting requests*. Each formatting request consists of a formatting string, arguments for that formatting string, and labels to apply to that formatting string.

A formatting string is a printf()-like string that allows variable substitution within the string. Labels allow the rendered text to be *decorated*. Assuming use of the canonical Mercurial code base, a formatting string can be the input to the "i18n._" function. This allows messages emitted from the server to be localized. So even if the server has different i18n settings, people could see messages in their *native* settings. Similarly, the use of labels allows decorations like coloring and underlining to be applied using the client's configured rendering settings.

Formatting strings are similar to "printf()" strings or how Python's "%" operator works. The only supported formatting sequences are "%s" and "%%". "%s" will be replaced by whatever the string at that position resolves to. "%%" will be replaced by "%". All other 2-byte sequences beginning with "%" represent a literal "%" followed by that character. However, future versions of the wire protocol reserve the right to allow clients to opt in to receiving formatting strings with additional formatters, hence why "%%" is required to represent the literal "%".

The frame payload consists of a CBOR array of CBOR maps. Each map defines an *atom* of text data to print. Each *atom* has the following bytestring keys:

msg
(bytestring) The formatting string. Content MUST be ASCII.
args (optional)
Array of bytestrings defining arguments to the formatting string.
labels (optional)
Array of bytestrings defining labels to apply to this atom.

All data to be printed MUST be encoded into a single frame: this frame does not support spanning data across multiple frames.

All textual data encoded in these frames is assumed to be line delimited. The last atom in the frame SHOULD end with a newline ("\n"). If it doesn't, clients MAY add a newline to facilitate immediate printing.

Progress Update ("0x07")

This frame holds the progress of an operation on the peer. Consumption of these frames allows clients to display progress bars, estimated completion times, etc.

Each frame defines the progress of a single operation on the peer. The payload consists of a CBOR map with the following bytestring keys:

topic
Topic name (string)
pos
Current numeric position within the topic (integer)
total
Total/end numeric position of this topic (unsigned integer)
label (optional)
Unit label (string)
item (optional)
Item name (string)

Progress state is created when a frame is received referencing a *topic* that isn't currently tracked. Progress tracking for that *topic* is finished when a frame is received reporting the current position of that topic as "-1".

Multiple *topics* may be active at any given time.

Rendering of progress information is not mandated or governed by this specification: implementations MAY render progress information however they see fit, including not at all.

The string data describing the topic SHOULD be static strings to facilitate receivers localizing that string data. The emitter MUST normalize all string data to valid UTF-8 and receivers SHOULD validate that received data conforms to UTF-8. The topic name SHOULD be ASCII.

Stream Encoding Settings ("0x08")

This frame type holds information defining the content encoding settings for a *stream*.

This frame type is likely consumed by the protocol layer and is not passed on to applications.

This frame type MUST ONLY occur on frames having the *Beginning of Stream* "Stream Flag" set.

The payload of this frame defines what content encoding has (possibly) been applied to the payloads of subsequent frames in this stream.

The payload begins with an 8-bit integer defining the length of the encoding *profile*, followed by the string name of that profile, which must be an ASCII string. All bytes that follow can be used by that profile for supplemental settings definitions. See the section below on defined encoding profiles.

Stream States and Flags

Streams can be in two states: *open* and *closed*. An *open* stream is active and frames attached to that stream could arrive at any time. A *closed* stream is not active. If a frame attached to a *closed* stream arrives, that frame MUST have an appropriate stream flag set indicating beginning of stream. All streams are in the *closed* state by default.

The "Stream Flags" field denotes a set of bit flags for defining the relationship of this frame within a stream. The following flags are defined:

0x01
Beginning of stream. The first frame in the stream MUST set this flag. When received, the "Stream ID" this frame is attached to becomes "open".
0x02
End of stream. The last frame in a stream MUST set this flag. When received, the "Stream ID" this frame is attached to becomes "closed". Any content encoding context associated with this stream can be destroyed after processing the payload of this frame.
0x04
Apply content encoding. When set, any content encoding settings defined by the stream should be applied when attempting to read the frame. When not set, the frame payload isn't encoded.

Streams

Streams - along with "Request IDs" - facilitate grouping of frames. But the purpose of each is quite different and the groupings they constitute are independent.

A "Request ID" is essentially a tag. It tells you which logical request a frame is associated with.

A *stream* is a sequence of frames grouped for the express purpose of applying a stateful encoding or for denoting sub-groups of frames.

Unlike "Request ID"s which span the request and response, a stream is unidirectional and stream IDs are independent from client to server.

There is no strict hierarchical relationship between "Request IDs" and *streams*. A stream can contain frames having multiple "Request IDs". Frames belonging to the same "Request ID" can span multiple streams.

One goal of streams is to facilitate content encoding. A stream can define an encoding to be applied to frame payloads. For example, the payload transmitted over the wire may contain output from a zstandard compression operation and the receiving end may decompress that payload to obtain the original data.

The other goal of streams is to facilitate concurrent execution. For example, a server could spawn 4 threads to service a request that can be easily parallelized. Each of those 4 threads could write into its own stream. Those streams could then in turn be delivered to 4 threads on the receiving end, with each thread consuming its stream in near isolation. The *main* thread on both ends merely does I/O and encodes/decodes frame headers: the bulk of the work is done by worker threads.

In addition, since content encoding is defined per stream, each *worker thread* could perform potentially CPU bound work concurrently with other threads. This approach of applying encoding at the sub-protocol / stream level eliminates a potential resource constraint on the protocol stream as a whole (it is common for the throughput of a compression engine to be smaller than the throughput of a network).

Having multiple streams - each with their own encoding settings - also facilitates the use of advanced data compression techniques. For example, a transmitter could see that it is generating data faster and slower than the receiving end is consuming it and adjust its compression settings to trade CPU for compression ratio accordingly.

While streams can define a content encoding, not all frames within that stream must use that content encoding. This can be useful when data is being served from caches and being derived dynamically. A cache could pre-compressed data so the server doesn't have to recompress it. The ability to pick and choose which frames are compressed allows servers to easily send data to the wire without involving potentially expensive encoding overhead.

Content Encoding Profiles

Streams can have named content encoding *profiles* associated with them. A profile defines a shared understanding of content encoding settings and behavior.

The following profiles are defined:

TBD

Command Protocol

A client can request that a remote run a command by sending it frames defining that command. This logical stream is composed of 1 or more "Command Request" frames and and 0 or more "Command Data" frames.

All frames composing a single command request MUST be associated with the same "Request ID".

Clients MAY send additional command requests without waiting on the response to a previous command request. If they do so, they MUST ensure that the "Request ID" field of outbound frames does not conflict with that of an active "Request ID" whose response has not yet been fully received.

Servers MAY respond to commands in a different order than they were sent over the wire. Clients MUST be prepared to deal with this. Servers also MAY start executing commands in a different order than they were received, or MAY execute multiple commands concurrently.

If there is a dependency between commands or a race condition between commands executing (e.g. a read-only command that depends on the results of a command that mutates the repository), then clients MUST NOT send frames issuing a command until a response to all dependent commands has been received. TODO think about whether we should express dependencies between commands to avoid roundtrip latency.

A command is defined by a command name, 0 or more command arguments, and optional command data.

Arguments are the recommended mechanism for transferring fixed sets of parameters to a command. Data is appropriate for transferring variable data. Thinking in terms of HTTP, arguments would be headers and data would be the message body.

It is recommended for servers to delay the dispatch of a command until all argument have been received. Servers MAY impose limits on the maximum argument size. TODO define failure mechanism.

Servers MAY dispatch to commands immediately once argument data is available or delay until command data is received in full.

Once a "Command Request" frame is sent, a client must be prepared to receive any of the following frames associated with that request: "Command Response", "Error Response", "Human Output Side-Channel", "Progress Update".

The *main* response for a command will be in "Command Response" frames. The payloads of these frames consist of 1 or more CBOR encoded values. The first CBOR value on the first "Command Response" frame is special and denotes the overall status of the command. This CBOR map contains the following bytestring keys:

status
(bytestring) A well-defined message containing the overall status of this command request. The following values are defined:
ok
The command was received successfully and its response follows.
error
There was an error processing the command. More details about the error are encoded in the "error" key.
error (optional)
A map containing information about an encountered error. The map has the following keys:
message
(array of maps) A message describing the error. The message uses the same format as those in the "Human Output Side-Channel" frame.

Capabilities

Servers advertise supported wire protocol features. This allows clients to probe for server features before blindly calling a command or passing a specific argument.

The server's features are exposed via a *capabilities* string. This is a space-delimited string of tokens/features. Some features are single words like "lookup" or "batch". Others are complicated key-value pairs advertising sub-features. e.g. "httpheader=2048". When complex, non-word values are used, each feature name can define its own encoding of sub-values. Comma-delimited and "x-www-form-urlencoded" values are common.

The following document capabilities defined by the canonical Mercurial server implementation.

batch

Whether the server supports the "batch" command.

This capability/command was introduced in Mercurial 1.9 (released July 2011).

branchmap

Whether the server supports the "branchmap" command.

This capability/command was introduced in Mercurial 1.3 (released July 2009).

bundle2-exp

Precursor to "bundle2" capability that was used before bundle2 was a stable feature.

This capability was introduced in Mercurial 3.0 behind an experimental flag. This capability should not be observed in the wild.

bundle2

Indicates whether the server supports the "bundle2" data exchange format.

The value of the capability is a URL quoted, newline ("\n") delimited list of keys or key-value pairs.

A key is simply a URL encoded string.

A key-value pair is a URL encoded key separated from a URL encoded value by an "=". If the value is a list, elements are delimited by a "," after URL encoding.

For example, say we have the values:

{'HG20': [], 'changegroup': ['01', '02'], 'digests': ['sha1', 'sha512']}

We would first construct a string:

HG20\nchangegroup=01,02\ndigests=sha1,sha512

We would then URL quote this string:

HG20%0Achangegroup%3D01%2C02%0Adigests%3Dsha1%2Csha512

This capability was introduced in Mercurial 3.4 (released May 2015).

changegroupsubset

Whether the server supports the "changegroupsubset" command.

This capability was introduced in Mercurial 0.9.2 (released December 2006).

This capability was introduced at the same time as the "lookup" capability/command.

compression

Declares support for negotiating compression formats.

Presence of this capability indicates the server supports dynamic selection of compression formats based on the client request.

Servers advertising this capability are required to support the "application/mercurial-0.2" media type in response to commands returning streams. Servers may support this media type on any command.

The value of the capability is a comma-delimited list of strings declaring supported compression formats. The order of the compression formats is in server-preferred order, most preferred first.

The identifiers used by the official Mercurial distribution are:

bzip2
bzip2
none
uncompressed / raw data
zlib
zlib (no gzip header)
zstd
zstd

This capability was introduced in Mercurial 4.1 (released February 2017).

getbundle

Whether the server supports the "getbundle" command.

This capability was introduced in Mercurial 1.9 (released July 2011).

httpheader

Whether the server supports receiving command arguments via HTTP request headers.

The value of the capability is an integer describing the max header length that clients should send. Clients should ignore any content after a comma in the value, as this is reserved for future use.

This capability was introduced in Mercurial 1.9 (released July 2011).

httpmediatype

Indicates which HTTP media types ("Content-Type" header) the server is capable of receiving and sending.

The value of the capability is a comma-delimited list of strings identifying support for media type and transmission direction. The following strings may be present:

0.1rx
Indicates server support for receiving "application/mercurial-0.1" media types.
0.1tx
Indicates server support for sending "application/mercurial-0.1" media types.
0.2rx
Indicates server support for receiving "application/mercurial-0.2" media types.
0.2tx
Indicates server support for sending "application/mercurial-0.2" media types.
minrx=X
Minimum media type version the server is capable of receiving. Value is a string like "0.2".

This capability can be used by servers to limit connections from legacy clients not using the latest supported media type. However, only clients with knowledge of this capability will know to consult this value. This capability is present so the client may issue a more user-friendly error when the server has locked out a legacy client.

mintx=X
Minimum media type version the server is capable of sending. Value is a string like "0.1".

Servers advertising support for the "application/mercurial-0.2" media type should also advertise the "compression" capability.

This capability was introduced in Mercurial 4.1 (released February 2017).

httppostargs

**Experimental**

Indicates that the server supports and prefers clients send command arguments via a HTTP POST request as part of the request body.

This capability was introduced in Mercurial 3.8 (released May 2016).

known

Whether the server supports the "known" command.

This capability/command was introduced in Mercurial 1.9 (released July 2011).

lookup

Whether the server supports the "lookup" command.

This capability was introduced in Mercurial 0.9.2 (released December 2006).

This capability was introduced at the same time as the "changegroupsubset" capability/command.

partial-pull

Indicates that the client can deal with partial answers to pull requests by repeating the request.

If this parameter is not advertised, the server will not send pull bundles.

This client capability was introduced in Mercurial 4.6.

protocaps

Whether the server supports the "protocaps" command for SSH V1 transport.

This capability was introduced in Mercurial 4.6.

pushkey

Whether the server supports the "pushkey" and "listkeys" commands.

This capability was introduced in Mercurial 1.6 (released July 2010).

standardbundle

**Unsupported**

This capability was introduced during the Mercurial 0.9.2 development cycle in 2006. It was never present in a release, as it was replaced by the "unbundle" capability. This capability should not be encountered in the wild.

stream-preferred

If present the server prefers that clients clone using the streaming clone protocol ("hg clone --stream") rather than the standard changegroup/bundle based protocol.

This capability was introduced in Mercurial 2.2 (released May 2012).

streamreqs

Indicates whether the server supports *streaming clones* and the *requirements* that clients must support to receive it.

If present, the server supports the "stream_out" command, which transmits raw revlogs from the repository instead of changegroups. This provides a faster cloning mechanism at the expense of more bandwidth used.

The value of this capability is a comma-delimited list of repo format *requirements*. These are requirements that impact the reading of data in the ".hg/store" directory. An example value is "streamreqs=generaldelta,revlogv1" indicating the server repo requires the "revlogv1" and "generaldelta" requirements.

If the only format requirement is "revlogv1", the server may expose the "stream" capability instead of the "streamreqs" capability.

This capability was introduced in Mercurial 1.7 (released November 2010).

stream

Whether the server supports *streaming clones* from "revlogv1" repos.

If present, the server supports the "stream_out" command, which transmits raw revlogs from the repository instead of changegroups. This provides a faster cloning mechanism at the expense of more bandwidth used.

This capability was introduced in Mercurial 0.9.1 (released July 2006).

When initially introduced, the value of the capability was the numeric revlog revision. e.g. "stream=1". This indicates the changegroup is using "revlogv1". This simple integer value wasn't powerful enough, so the "streamreqs" capability was invented to handle cases where the repo requirements have more than just "revlogv1". Newer servers omit the "=1" since it was the only value supported and the value of "1" can be implied by clients.

unbundlehash

Whether the "unbundle" commands supports receiving a hash of all the heads instead of a list.

For more, see the documentation for the "unbundle" command.

This capability was introduced in Mercurial 1.9 (released July 2011).

unbundle

Whether the server supports pushing via the "unbundle" command.

This capability/command has been present since Mercurial 0.9.1 (released July 2006).

Mercurial 0.9.2 (released December 2006) added values to the capability indicating which bundle types the server supports receiving. This value is a comma-delimited list. e.g. "HG10GZ,HG10BZ,HG10UN". The order of values reflects the priority/preference of that type, where the first value is the most preferred type.

Content Negotiation

The wire protocol has some mechanisms to help peers determine what content types and encoding the other side will accept. Historically, these mechanisms have been built into commands themselves because most commands only send a well-defined response type and only certain commands needed to support functionality like compression.

Currently, only the HTTP version 1 transport supports content negotiation at the protocol layer.

HTTP requests advertise supported response formats via the "X-HgProto-<N>" request header, where "<N>" is an integer starting at 1 allowing the logical value to span multiple headers. This value consists of a list of space-delimited parameters. Each parameter denotes a feature or capability.

The following parameters are defined:

0.1
Indicates the client supports receiving "application/mercurial-0.1" responses.
0.2
Indicates the client supports receiving "application/mercurial-0.2" responses.
cbor
Indicates the client supports receiving "application/mercurial-cbor" responses.

(Only intended to be used with version 2 transports.)

comp
Indicates compression formats the client can decode. Value is a list of comma delimited strings identifying compression formats ordered from most preferential to least preferential. e.g. "comp=zstd,zlib,none".

This parameter does not have an effect if only the "0.1" parameter is defined, as support for "application/mercurial-0.2" or greater is required to use arbitrary compression formats.

If this parameter is not advertised, the server interprets this as equivalent to "zlib,none".

Clients may choose to only send this header if the "httpmediatype" server capability is present, as currently all server-side features consulting this header require the client to opt in to new protocol features advertised via the "httpmediatype" capability.

A server that doesn't receive an "X-HgProto-<N>" header should infer a value of "0.1". This is compatible with legacy clients.

A server receiving a request indicating support for multiple media type versions may respond with any of the supported media types. Not all servers may support all media types on all commands.

Commands

This section contains a list of all wire protocol commands implemented by the canonical Mercurial server.

batch

Issue multiple commands while sending a single command request. The purpose of this command is to allow a client to issue multiple commands while avoiding multiple round trips to the server therefore enabling commands to complete quicker.

The command accepts a "cmds" argument that contains a list of commands to execute.

The value of "cmds" is a ";" delimited list of strings. Each string has the form "<command> <arguments>". That is, the command name followed by a space followed by an argument string.

The argument string is a "," delimited list of "<key>=<value>" values corresponding to command arguments. Both the argument name and value are escaped using a special substitution map:

: -> :c
, -> :o
; -> :s
= -> :e

The response type for this command is "string". The value contains a ";" delimited list of responses for each requested command. Each value in this list is escaped using the same substitution map used for arguments.

If an error occurs, the generic error response may be sent.

between

(Legacy command used for discovery in old clients)

Obtain nodes between pairs of nodes.

The "pairs" arguments contains a space-delimited list of "-" delimited hex node pairs. e.g.:

a072279d3f7fd3a4aa7ffa1a5af8efc573e1c896-6dc58916e7c070f678682bfe404d2e2d68291a18

Return type is a "string". Value consists of lines corresponding to each requested range. Each line contains a space-delimited list of hex nodes. A newline "\n" terminates each line, including the last one.

branchmap

Obtain heads in named branches.

Accepts no arguments. Return type is a "string".

Return value contains lines with URL encoded branch names followed by a space followed by a space-delimited list of hex nodes of heads on that branch. e.g.:

default a072279d3f7fd3a4aa7ffa1a5af8efc573e1c896 6dc58916e7c070f678682bfe404d2e2d68291a18
stable baae3bf31522f41dd5e6d7377d0edd8d1cf3fccc

There is no trailing newline.

branches

(Legacy command used for discovery in old clients. Clients with "getbundle" use the "known" and "heads" commands instead.)

Obtain ancestor changesets of specific nodes back to a branch point.

Despite the name, this command has nothing to do with Mercurial named branches. Instead, it is related to DAG branches.

The command accepts a "nodes" argument, which is a string of space-delimited hex nodes.

For each node requested, the server will find the first ancestor node that is a DAG root or is a merge.

Return type is a "string". Return value contains lines with result data for each requested node. Each line contains space-delimited nodes followed by a newline ("\n"). The 4 nodes reported on each line correspond to the requested node, the ancestor node found, and its 2 parent nodes (which may be the null node).

capabilities

Obtain the capabilities string for the repo.

Unlike the "hello" command, the capabilities string is not prefixed. There is no trailing newline.

This command does not accept any arguments. Return type is a "string".

This command was introduced in Mercurial 0.9.1 (released July 2006).

changegroup

(Legacy command: use "getbundle" instead)

Obtain a changegroup version 1 with data for changesets that are descendants of client-specified changesets.

The "roots" arguments contains a list of space-delimited hex nodes.

The server responds with a changegroup version 1 containing all changesets between the requested root/base nodes and the repo's head nodes at the time of the request.

The return type is a "stream".

changegroupsubset

(Legacy command: use "getbundle" instead)

Obtain a changegroup version 1 with data for changesetsets between client specified base and head nodes.

The "bases" argument contains a list of space-delimited hex nodes. The "heads" argument contains a list of space-delimited hex nodes.

The server responds with a changegroup version 1 containing all changesets between the requested base and head nodes at the time of the request.

The return type is a "stream".

clonebundles

Obtains a manifest of bundle URLs available to seed clones.

Each returned line contains a URL followed by metadata. See the documentation in the "clonebundles" extension for more.

The return type is a "string".

getbundle

Obtain a bundle containing repository data.

This command accepts the following arguments:

heads
List of space-delimited hex nodes of heads to retrieve.
common
List of space-delimited hex nodes that the client has in common with the server.
obsmarkers
Boolean indicating whether to include obsolescence markers as part of the response. Only works with bundle2.
bundlecaps
Comma-delimited set of strings defining client bundle capabilities.
listkeys
Comma-delimited list of strings of "pushkey" namespaces. For each namespace listed, a bundle2 part will be included with the content of that namespace.
cg
Boolean indicating whether changegroup data is requested.
cbattempted
Boolean indicating whether the client attempted to use the *clone bundles* feature before performing this request.
bookmarks
Boolean indicating whether bookmark data is requested.
phases
Boolean indicating whether phases data is requested.

The return type on success is a "stream" where the value is bundle. On the HTTP version 1 transport, the response is zlib compressed.

If an error occurs, a generic error response can be sent.

Unless the client sends a false value for the "cg" argument, the returned bundle contains a changegroup with the nodes between the specified "common" and "heads" nodes. Depending on the command arguments, the type and content of the returned bundle can vary significantly.

The default behavior is for the server to send a raw changegroup version "01" response.

If the "bundlecaps" provided by the client contain a value beginning with "HG2", a bundle2 will be returned. The bundle2 data may contain additional repository data, such as "pushkey" namespace values.

heads

Returns a list of space-delimited hex nodes of repository heads followed by a newline. e.g. "a9eeb3adc7ddb5006c088e9eda61791c777cbf7c 31f91a3da534dc849f0d6bfc00a395a97cf218a1\n"

This command does not accept any arguments. The return type is a "string".

hello

Returns lines describing interesting things about the server in an RFC-822 like format.

Currently, the only line defines the server capabilities. It has the form:

capabilities: <value>

See above for more about the capabilities string.

SSH clients typically issue this command as soon as a connection is established.

This command does not accept any arguments. The return type is a "string".

This command was introduced in Mercurial 0.9.1 (released July 2006).

listkeys

List values in a specified "pushkey" namespace.

The "namespace" argument defines the pushkey namespace to operate on.

The return type is a "string". The value is an encoded dictionary of keys.

Key-value pairs are delimited by newlines ("\n"). Within each line, keys and values are separated by a tab ("\t"). Keys and values are both strings.

lookup

Try to resolve a value to a known repository revision.

The "key" argument is converted from bytes to an "encoding.localstr" instance then passed into "localrepository.__getitem__" in an attempt to resolve it.

The return type is a "string".

Upon successful resolution, returns "1 <hex node>\n". On failure, returns "0 <error string>\n". e.g.:

1 273ce12ad8f155317b2c078ec75a4eba507f1fba\n
0 unknown revision 'foo'\n

known

Determine whether multiple nodes are known.

The "nodes" argument is a list of space-delimited hex nodes to check for existence.

The return type is "string".

Returns a string consisting of "0"s and "1"s indicating whether nodes are known. If the Nth node specified in the "nodes" argument is known, a "1" will be returned at byte offset N. If the node isn't known, "0" will be present at byte offset N.

There is no trailing newline.

protocaps

Notify the server about the client capabilities in the SSH V1 transport protocol.

The "caps" argument is a space-delimited list of capabilities.

The server will reply with the string "OK".

pushkey

Set a value using the "pushkey" protocol.

Accepts arguments "namespace", "key", "old", and "new", which correspond to the pushkey namespace to operate on, the key within that namespace to change, the old value (which may be empty), and the new value. All arguments are string types.

The return type is a "string". The value depends on the transport protocol.

The SSH version 1 transport sends a string encoded integer followed by a newline ("\n") which indicates operation result. The server may send additional output on the "stderr" stream that should be displayed to the user.

The HTTP version 1 transport sends a string encoded integer followed by a newline followed by additional server output that should be displayed to the user. This may include output from hooks, etc.

The integer result varies by namespace. "0" means an error has occurred and there should be additional output to display to the user.

stream_out

Obtain *streaming clone* data.

The return type is either a "string" or a "stream", depending on whether the request was fulfilled properly.

A return value of "1\n" indicates the server is not configured to serve this data. If this is seen by the client, they may not have verified the "stream" capability is set before making the request.

A return value of "2\n" indicates the server was unable to lock the repository to generate data.

All other responses are a "stream" of bytes. The first line of this data contains 2 space-delimited integers corresponding to the path count and payload size, respectively:

<path count> <payload size>\n

The "<payload size>" is the total size of path data: it does not include the size of the per-path header lines.

Following that header are "<path count>" entries. Each entry consists of a line with metadata followed by raw revlog data. The line consists of:

<store path>\0<size>\n

The "<store path>" is the encoded store path of the data that follows. "<size>" is the amount of data for this store path/revlog that follows the newline.

There is no trailer to indicate end of data. Instead, the client should stop reading after "<path count>" entries are consumed.

unbundle

Send a bundle containing data (usually changegroup data) to the server.

Accepts the argument "heads", which is a space-delimited list of hex nodes corresponding to server repository heads observed by the client. This is used to detect race conditions and abort push operations before a server performs too much work or a client transfers too much data.

The request payload consists of a bundle to be applied to the repository, similarly to as if 'hg unbundle' were called.

In most scenarios, a special "push response" type is returned. This type contains an integer describing the change in heads as a result of the operation. A value of "0" indicates nothing changed. "1" means the number of heads remained the same. Values "2" and larger indicate the number of added heads minus 1. e.g. "3" means 2 heads were added. Negative values indicate the number of fewer heads, also off by 1. e.g. "-2" means there is 1 fewer head.

The encoding of the "push response" type varies by transport.

For the SSH version 1 transport, this type is composed of 2 "string" responses: an empty response ("0\n") followed by the integer result value. e.g. "1\n2". So the full response might be "0\n1\n2".

For the HTTP version 1 transport, the response is a "string" type composed of an integer result value followed by a newline ("\n") followed by string content holding server output that should be displayed on the client (output hooks, etc).

In some cases, the server may respond with a "bundle2" bundle. In this case, the response type is "stream". For the HTTP version 1 transport, the response is zlib compressed.

The server may also respond with a generic error type, which contains a string indicating the failure.

Frame-Based Protocol Commands

**Experimental and under active development**

This section documents the wire protocol commands exposed to transports using the frame-based protocol. The set of commands exposed through these transports is distinct from the set of commands exposed to legacy transports.

The frame-based protocol uses CBOR to encode command execution requests. All command arguments must be mapped to a specific or set of CBOR data types.

The response to many commands is also CBOR. There is no common response format: each command defines its own response format.

TODO require node type be specified, as N bytes of binary node value could be ambiguous once SHA-1 is replaced.

branchmap

Obtain heads in named branches.

Receives no arguments.

The response is a map with bytestring keys defining the branch name. Values are arrays of bytestring defining raw changeset nodes.

capabilities

Obtain the server's capabilities.

Receives no arguments.

This command is typically called only as part of the handshake during initial connection establishment.

The response is a map with bytestring keys defining server information.

The defined keys are:

commands
A map defining available wire protocol commands on this server.

Keys in the map are the names of commands that can be invoked. Values are maps defining information about that command. The bytestring keys are:

args
A map of argument names and their expected types.

Types are defined as a representative value for the expected type. e.g. an argument expecting a boolean type will have its value set to true. An integer type will have its value set to 42. The actual values are arbitrary and may not have meaning. permissions An array of permissions required to execute this command.

compression
An array of maps defining available compression format support.

The array is sorted from most preferred to least preferred.

Each entry has the following bytestring keys:

name
Name of the compression engine. e.g. "zstd" or "zlib".
framingmediatypes
An array of bytestrings defining the supported framing protocol media types. Servers will not accept media types not in this list.
rawrepoformats
An array of storage formats the repository is using. This set of requirements can be used to determine whether a client can read a *raw* copy of file data available.

heads

Obtain DAG heads in the repository.

The command accepts the following arguments:

publiconly (optional)
(boolean) If set, operate on the DAG for public phase changesets only. Non-public (i.e. draft) phase DAG heads will not be returned.

The response is a CBOR array of bytestrings defining changeset nodes of DAG heads. The array can be empty if the repository is empty or no changesets satisfied the request.

TODO consider exposing phase of heads in response

known

Determine whether a series of changeset nodes is known to the server.

The command accepts the following arguments:

nodes
(array of bytestrings) List of changeset nodes whose presence to query.

The response is a bytestring where each byte contains a 0 or 1 for the corresponding requested node at the same index.

TODO use a bit array for even more compact response

listkeys

List values in a specified "pushkey" namespace.

The command receives the following arguments:

namespace
(bytestring) Pushkey namespace to query.

The response is a map with bytestring keys and values.

TODO consider using binary to represent nodes in certain pushkey namespaces.

lookup

Try to resolve a value to a changeset revision.

Unlike "known" which operates on changeset nodes, lookup operates on node fragments and other names that a user may use.

The command receives the following arguments:

key
(bytestring) Value to try to resolve.

On success, returns a bytestring containing the resolved node.

pushkey

Set a value using the "pushkey" protocol.

The command receives the following arguments:

namespace
(bytestring) Pushkey namespace to operate on.
key
(bytestring) The pushkey key to set.
old
(bytestring) Old value for this key.
new
(bytestring) New value for this key.

TODO consider using binary to represent nodes is certain pushkey namespaces. TODO better define response type and meaning.