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The smart protocol provides a way to send a requests and corresponding
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responses to communicate with a remote bzr process.
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At the bottom level there is either a socket, pipes, or an HTTP
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request/response. We call this layer the *medium*. It is responsible for
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carrying bytes between a client and server. For sockets, we have the idea
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that you have multiple requests and get a read error because the other
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side did shutdown. For pipes we have read pipe which will have a zero
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read which marks end-of-file. For HTTP server environment there is no
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end-of-stream because each request coming into the server is independent.
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So we need a wrapper around pipes and sockets to seperate out requests
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from substrate and this will give us a single model which is consistent
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for HTTP, sockets and pipes.
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On top of the medium is the *protocol*. This is the layer that
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deserialises bytes into the structured data that requests and responses
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Request/Response processing
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---------------------------
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On top of the protocol is the logic for processing requests (on the
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server) or responses (on the client).
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MEDIUM (factory for protocol, reads bytes & pushes to protocol,
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uses protocol to detect end-of-request, sends written
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bytes to client) e.g. socket, pipe, HTTP request handler.
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PROTOCOL(serialization, deserialization) accepts bytes for one
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request, decodes according to internal state, pushes
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structured data to handler. accepts structured data from
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handler and encodes and writes to the medium. factory for
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HANDLER (domain logic) accepts structured data, operates state
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machine until the request can be satisfied,
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sends structured data to the protocol.
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Request handlers are registered in the `bzrlib.smart.request` module.
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CLIENT domain logic, accepts domain requests, generated structured
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data, reads structured data from responses and turns into
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domain data. Sends structured data to the protocol.
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Operates state machines until the request can be delivered
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(e.g. reading from a bundle generated in bzrlib to deliver a
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This is RemoteBzrDir, RemoteRepository, etc.
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PROTOCOL (serialization, deserialization) accepts structured data for one
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request, encodes and writes to the medium. Reads bytes from the
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medium, decodes and allows the client to read structured data.
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MEDIUM accepts bytes from the protocol & delivers to the remote server.
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Allows the protocol to read bytes e.g. socket, pipe, HTTP request.
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The domain logic is in `bzrlib.remote`: `RemoteBzrDir`, `RemoteBranch`,
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There is also an plain file-level transport that calls remote methods to
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manipulate files on the server in `bzrlib.transport.remote`.
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Version one of the protocol was introduced in Bazaar 0.11.
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The protocol (for both requests and responses) is described by::
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REQUEST := MESSAGE_V1
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RESPONSE := MESSAGE_V1
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MESSAGE_V1 := ARGS BODY
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ARGS := ARG [MORE_ARGS] NEWLINE
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MORE_ARGS := SEP ARG [MORE_ARGS]
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BODY := LENGTH NEWLINE BODY_BYTES TRAILER
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LENGTH := decimal integer
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TRAILER := "done" NEWLINE
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That is, a tuple of arguments separated by Ctrl-A and terminated with a
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newline, followed by length prefixed body with a constant trailer. Note
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that although arguments are not 8-bit safe (they cannot include 0x01 or
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0x0a bytes without breaking the protocol encoding), the body is.
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Version two was introduced in Bazaar 0.16.
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The request protocol is::
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REQUEST_V2 := "bzr request 2" NEWLINE MESSAGE_V2
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The response protocol is::
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RESPONSE_V2 := "bzr response 2" NEWLINE MESSAGE_V2
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Future versions should follow this structure, like version two does::
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FUTURE_MESSAGE := VERSION_STRING NEWLINE REST_OF_MESSAGE
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This is so that clients and servers can read bytes up to the first newline
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byte to determine what version a message is.
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For compatibility will all versions (past and future) of bzr clients,
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servers that receive a request in an unknown protocol version should
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respond with a single-line error terminated with 0x0a (NEWLINE), rather
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than structured response prefixed with a version string.
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Version two of the message protocol is::
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MESSAGE_V2 := ARGS BODY
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BODY_V2 := BODY | STREAMED_BODY
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That is, a version one length-prefixed body, or a version two streamed
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Version two with streamed bodies
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--------------------------------
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An extension to version two allows streamed bodies. A streamed body looks
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a lot like HTTP's chunked encoding::
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STREAMED_BODY := "chunked" NEWLINE CHUNKS TERMINATOR
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CHUNKS := CHUNK [CHUNKS]
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CHUNK := CHUNK_LENGTH CHUNK_CONTENT
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CHUNK_LENGTH := HEX_DIGITS NEWLINE
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CHUNK_CONTENT := bytes
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TERMINATOR := SUCCESS_TERMINATOR | ERROR_TERMINATOR
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SUCCESS_TERMINATOR := 'END' NEWLINE
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ERROR_TERMINATOR := 'ERR' NEWLINE CHUNKS SUCCESS_TERMINATOR
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That is, the body consists of a series of chunks. Each chunk starts with
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a length prefix in hexadecimal digits, followed by an ASCII newline byte.
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The end of the body is signaled by '``END\\n``', or by '``ERR\\n``'
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followed by error args, one per chunk. Note that these args are 8-bit
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safe, unlike request args.
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A streamed body starts with the string "chunked" so that legacy clients
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and servers will not mistake the first chunk as the start of a version one
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The type of body (length-prefixed or chunked) in a response is always the
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same for a given request method. Only new request methods introduced in
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Bazaar 0.91 and later use streamed bodies.
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Paths are passed across the network. The client needs to see a namespace
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that includes any repository that might need to be referenced, and the
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client needs to know about a root directory beyond which it cannot ascend.
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Servers run over ssh will typically want to be able to access any path the
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user can access. Public servers on the other hand (which might be over
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http, ssh or tcp) will typically want to restrict access to only a
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particular directory and its children, so will want to do a software
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virtual root at that level. In other words they'll want to rewrite
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incoming paths to be under that level (and prevent escaping using ../
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tricks). The default implementation in bzrlib does this using the
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`bzrlib.transport.chroot` module.
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URLs that include ~ should probably be passed across to the server
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verbatim and the server can expand them. This will proably not be
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meaningful when limited to a directory? See `bug 109143`_.
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.. _bug 109143: https://bugs.launchpad.net/bzr/+bug/109143
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The first argument of a request specifies the request method.
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The available request methods are registered in `bzrlib.smart.request`.
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**XXX**: ideally the request methods should be documented here.
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Contributions welcome!
added
removed
doc/developers/network-protocol.txt
