mirror of
https://github.com/git/git
synced 2024-11-05 18:59:29 +00:00
4d542687fc
Diff best viewed with --color-diff. Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
105 lines
4.7 KiB
Text
105 lines
4.7 KiB
Text
Simple-IPC API
|
|
==============
|
|
|
|
The Simple-IPC API is a collection of `ipc_` prefixed library routines
|
|
and a basic communication protocol that allows an IPC-client process to
|
|
send an application-specific IPC-request message to an IPC-server
|
|
process and receive an application-specific IPC-response message.
|
|
|
|
Communication occurs over a named pipe on Windows and a Unix domain
|
|
socket on other platforms. IPC-clients and IPC-servers rendezvous at
|
|
a previously agreed-to application-specific pathname (which is outside
|
|
the scope of this design) that is local to the computer system.
|
|
|
|
The IPC-server routines within the server application process create a
|
|
thread pool to listen for connections and receive request messages
|
|
from multiple concurrent IPC-clients. When received, these messages
|
|
are dispatched up to the server application callbacks for handling.
|
|
IPC-server routines then incrementally relay responses back to the
|
|
IPC-client.
|
|
|
|
The IPC-client routines within a client application process connect
|
|
to the IPC-server and send a request message and wait for a response.
|
|
When received, the response is returned back to the caller.
|
|
|
|
For example, the `fsmonitor--daemon` feature will be built as a server
|
|
application on top of the IPC-server library routines. It will have
|
|
threads watching for file system events and a thread pool waiting for
|
|
client connections. Clients, such as `git status`, will request a list
|
|
of file system events since a point in time and the server will
|
|
respond with a list of changed files and directories. The formats of
|
|
the request and response are application-specific; the IPC-client and
|
|
IPC-server routines treat them as opaque byte streams.
|
|
|
|
|
|
Comparison with sub-process model
|
|
---------------------------------
|
|
|
|
The Simple-IPC mechanism differs from the existing `sub-process.c`
|
|
model (Documentation/technical/long-running-process-protocol.txt) and
|
|
used by applications like Git-LFS. In the LFS-style sub-process model,
|
|
the helper is started by the foreground process, communication happens
|
|
via a pair of file descriptors bound to the stdin/stdout of the
|
|
sub-process, the sub-process only serves the current foreground
|
|
process, and the sub-process exits when the foreground process
|
|
terminates.
|
|
|
|
In the Simple-IPC model the server is a very long-running service. It
|
|
can service many clients at the same time and has a private socket or
|
|
named pipe connection to each active client. It might be started
|
|
(on-demand) by the current client process or it might have been
|
|
started by a previous client or by the OS at boot time. The server
|
|
process is not associated with a terminal and it persists after
|
|
clients terminate. Clients do not have access to the stdin/stdout of
|
|
the server process and therefore must communicate over sockets or
|
|
named pipes.
|
|
|
|
|
|
Server startup and shutdown
|
|
---------------------------
|
|
|
|
How an application server based upon IPC-server is started is also
|
|
outside the scope of the Simple-IPC design and is a property of the
|
|
application using it. For example, the server might be started or
|
|
restarted during routine maintenance operations, or it might be
|
|
started as a system service during the system boot-up sequence, or it
|
|
might be started on-demand by a foreground Git command when needed.
|
|
|
|
Similarly, server shutdown is a property of the application using
|
|
the simple-ipc routines. For example, the server might decide to
|
|
shutdown when idle or only upon explicit request.
|
|
|
|
|
|
Simple-IPC protocol
|
|
-------------------
|
|
|
|
The Simple-IPC protocol consists of a single request message from the
|
|
client and an optional response message from the server. Both the
|
|
client and server messages are unlimited in length and are terminated
|
|
with a flush packet.
|
|
|
|
The pkt-line routines (linkgit:gitprotocol-common[5])
|
|
are used to simplify buffer management during message generation,
|
|
transmission, and reception. A flush packet is used to mark the end
|
|
of the message. This allows the sender to incrementally generate and
|
|
transmit the message. It allows the receiver to incrementally receive
|
|
the message in chunks and to know when they have received the entire
|
|
message.
|
|
|
|
The actual byte format of the client request and server response
|
|
messages are application specific. The IPC layer transmits and
|
|
receives them as opaque byte buffers without any concern for the
|
|
content within. It is the job of the calling application layer to
|
|
understand the contents of the request and response messages.
|
|
|
|
|
|
Summary
|
|
-------
|
|
|
|
Conceptually, the Simple-IPC protocol is similar to an HTTP REST
|
|
request. Clients connect, make an application-specific and
|
|
stateless request, receive an application-specific
|
|
response, and disconnect. It is a one round trip facility for
|
|
querying the server. The Simple-IPC routines hide the socket,
|
|
named pipe, and thread pool details and allow the application
|
|
layer to focus on the task at hand.
|