technical/api-simple-ipc.html

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 435 </head>
 436 <body class="article">
 437 <div id="header">
 438 <h1>Simple-IPC API</h1>
 439 </div>
 440 <div id="content">
 441 <div id="preamble">
 442 <div class="sectionbody">
 443 <div class="paragraph">
 444 <p>The Simple-IPC API is a collection of <code>ipc_</code> prefixed library routines
 445 and a basic communication protocol that allows an IPC-client process to
 446 send an application-specific IPC-request message to an IPC-server
 447 process and receive an application-specific IPC-response message.</p>
 448 </div>
 449 <div class="paragraph">
 450 <p>Communication occurs over a named pipe on Windows and a Unix domain
 451 socket on other platforms.  IPC-clients and IPC-servers rendezvous at
 452 a previously agreed-to application-specific pathname (which is outside
 453 the scope of this design) that is local to the computer system.</p>
 454 </div>
 455 <div class="paragraph">
 456 <p>The IPC-server routines within the server application process create a
 457 thread pool to listen for connections and receive request messages
 458 from multiple concurrent IPC-clients.  When received, these messages
 459 are dispatched up to the server application callbacks for handling.
 460 IPC-server routines then incrementally relay responses back to the
 461 IPC-client.</p>
 462 </div>
 463 <div class="paragraph">
 464 <p>The IPC-client routines within a client application process connect
 465 to the IPC-server and send a request message and wait for a response.
 466 When received, the response is returned back to the caller.</p>
 467 </div>
 468 <div class="paragraph">
 469 <p>For example, the <code>fsmonitor--daemon</code> feature will be built as a server
 470 application on top of the IPC-server library routines.  It will have
 471 threads watching for file system events and a thread pool waiting for
 472 client connections.  Clients, such as <code>git</code> <code>status</code>, will request a list
 473 of file system events since a point in time and the server will
 474 respond with a list of changed files and directories.  The formats of
 475 the request and response are application-specific; the IPC-client and
 476 IPC-server routines treat them as opaque byte streams.</p>
 477 </div>
 478 </div>
 479 </div>
 480 <div class="sect1">
 481 <h2 id="_comparison_with_sub_process_model">Comparison with sub-process model</h2>
 482 <div class="sectionbody">
 483 <div class="paragraph">
 484 <p>The Simple-IPC mechanism differs from the existing <code>sub-process.c</code>
 485 model (Documentation/technical/long-running-process-protocol.txt) and
 486 used by applications like Git-LFS.  In the LFS-style sub-process model,
 487 the helper is started by the foreground process, communication happens
 488 via a pair of file descriptors bound to the stdin/stdout of the
 489 sub-process, the sub-process only serves the current foreground
 490 process, and the sub-process exits when the foreground process
 491 terminates.</p>
 492 </div>
 493 <div class="paragraph">
 494 <p>In the Simple-IPC model the server is a very long-running service.  It
 495 can service many clients at the same time and has a private socket or
 496 named pipe connection to each active client.  It might be started
 497 (on-demand) by the current client process or it might have been
 498 started by a previous client or by the OS at boot time.  The server
 499 process is not associated with a terminal and it persists after
 500 clients terminate.  Clients do not have access to the stdin/stdout of
 501 the server process and therefore must communicate over sockets or
 502 named pipes.</p>
 503 </div>
 504 </div>
 505 </div>
 506 <div class="sect1">
 507 <h2 id="_server_startup_and_shutdown">Server startup and shutdown</h2>
 508 <div class="sectionbody">
 509 <div class="paragraph">
 510 <p>How an application server based upon IPC-server is started is also
 511 outside the scope of the Simple-IPC design and is a property of the
 512 application using it.  For example, the server might be started or
 513 restarted during routine maintenance operations, or it might be
 514 started as a system service during the system boot-up sequence, or it
 515 might be started on-demand by a foreground Git command when needed.</p>
 516 </div>
 517 <div class="paragraph">
 518 <p>Similarly, server shutdown is a property of the application using
 519 the simple-ipc routines.  For example, the server might decide to
 520 shutdown when idle or only upon explicit request.</p>
 521 </div>
 522 </div>
 523 </div>
 524 <div class="sect1">
 525 <h2 id="_simple_ipc_protocol">Simple-IPC protocol</h2>
 526 <div class="sectionbody">
 527 <div class="paragraph">
 528 <p>The Simple-IPC protocol consists of a single request message from the
 529 client and an optional response message from the server.  Both the
 530 client and server messages are unlimited in length and are terminated
 531 with a flush packet.</p>
 532 </div>
 533 <div class="paragraph">
 534 <p>The pkt-line routines (<a href="../gitprotocol-common.html">gitprotocol-common(5)</a>)
 535 are used to simplify buffer management during message generation,
 536 transmission, and reception.  A flush packet is used to mark the end
 537 of the message.  This allows the sender to incrementally generate and
 538 transmit the message.  It allows the receiver to incrementally receive
 539 the message in chunks and to know when they have received the entire
 540 message.</p>
 541 </div>
 542 <div class="paragraph">
 543 <p>The actual byte format of the client request and server response
 544 messages are application specific.  The IPC layer transmits and
 545 receives them as opaque byte buffers without any concern for the
 546 content within.  It is the job of the calling application layer to
 547 understand the contents of the request and response messages.</p>
 548 </div>
 549 </div>
 550 </div>
 551 <div class="sect1">
 552 <h2 id="_summary">Summary</h2>
 553 <div class="sectionbody">
 554 <div class="paragraph">
 555 <p>Conceptually, the Simple-IPC protocol is similar to an HTTP REST
 556 request.  Clients connect, make an application-specific and
 557 stateless request, receive an application-specific
 558 response, and disconnect.  It is a one round trip facility for
 559 querying the server.  The Simple-IPC routines hide the socket,
 560 named pipe, and thread pool details and allow the application
 561 layer to focus on the task at hand.</p>
 562 </div>
 563 </div>
 564 </div>
 565 </div>
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 567 <div id="footer-text">
 568 Last updated 2023-10-23 14:43:46 -0700
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