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 class="details">
 440 <span id="revdate">2024-11-20</span>
 441 </div>
 442 </div>
 443 <div id="content">
 444 <div id="preamble">
 445 <div class="sectionbody">
 446 <div class="paragraph">
 447 <p>The Simple-IPC API is a collection of <code>ipc_</code> prefixed library routines
 448 and a basic communication protocol that allows an IPC-client process to
 449 send an application-specific IPC-request message to an IPC-server
 450 process and receive an application-specific IPC-response message.</p>
 451 </div>
 452 <div class="paragraph">
 453 <p>Communication occurs over a named pipe on Windows and a Unix domain
 454 socket on other platforms.  IPC-clients and IPC-servers rendezvous at
 455 a previously agreed-to application-specific pathname (which is outside
 456 the scope of this design) that is local to the computer system.</p>
 457 </div>
 458 <div class="paragraph">
 459 <p>The IPC-server routines within the server application process create a
 460 thread pool to listen for connections and receive request messages
 461 from multiple concurrent IPC-clients.  When received, these messages
 462 are dispatched up to the server application callbacks for handling.
 463 IPC-server routines then incrementally relay responses back to the
 464 IPC-client.</p>
 465 </div>
 466 <div class="paragraph">
 467 <p>The IPC-client routines within a client application process connect
 468 to the IPC-server and send a request message and wait for a response.
 469 When received, the response is returned back to the caller.</p>
 470 </div>
 471 <div class="paragraph">
 472 <p>For example, the <code>fsmonitor--daemon</code> feature will be built as a server
 473 application on top of the IPC-server library routines.  It will have
 474 threads watching for file system events and a thread pool waiting for
 475 client connections.  Clients, such as <code>git</code> <code>status</code>, will request a list
 476 of file system events since a point in time and the server will
 477 respond with a list of changed files and directories.  The formats of
 478 the request and response are application-specific; the IPC-client and
 479 IPC-server routines treat them as opaque byte streams.</p>
 480 </div>
 481 </div>
 482 </div>
 483 <div class="sect1">
 484 <h2 id="_comparison_with_sub_process_model">Comparison with sub-process model</h2>
 485 <div class="sectionbody">
 486 <div class="paragraph">
 487 <p>The Simple-IPC mechanism differs from the existing <code>sub-process.c</code>
 488 model (Documentation/technical/long-running-process-protocol.txt) and
 489 used by applications like Git-LFS.  In the LFS-style sub-process model,
 490 the helper is started by the foreground process, communication happens
 491 via a pair of file descriptors bound to the stdin/stdout of the
 492 sub-process, the sub-process only serves the current foreground
 493 process, and the sub-process exits when the foreground process
 494 terminates.</p>
 495 </div>
 496 <div class="paragraph">
 497 <p>In the Simple-IPC model the server is a very long-running service.  It
 498 can service many clients at the same time and has a private socket or
 499 named pipe connection to each active client.  It might be started
 500 (on-demand) by the current client process or it might have been
 501 started by a previous client or by the OS at boot time.  The server
 502 process is not associated with a terminal and it persists after
 503 clients terminate.  Clients do not have access to the stdin/stdout of
 504 the server process and therefore must communicate over sockets or
 505 named pipes.</p>
 506 </div>
 507 </div>
 508 </div>
 509 <div class="sect1">
 510 <h2 id="_server_startup_and_shutdown">Server startup and shutdown</h2>
 511 <div class="sectionbody">
 512 <div class="paragraph">
 513 <p>How an application server based upon IPC-server is started is also
 514 outside the scope of the Simple-IPC design and is a property of the
 515 application using it.  For example, the server might be started or
 516 restarted during routine maintenance operations, or it might be
 517 started as a system service during the system boot-up sequence, or it
 518 might be started on-demand by a foreground Git command when needed.</p>
 519 </div>
 520 <div class="paragraph">
 521 <p>Similarly, server shutdown is a property of the application using
 522 the simple-ipc routines.  For example, the server might decide to
 523 shutdown when idle or only upon explicit request.</p>
 524 </div>
 525 </div>
 526 </div>
 527 <div class="sect1">
 528 <h2 id="_simple_ipc_protocol">Simple-IPC protocol</h2>
 529 <div class="sectionbody">
 530 <div class="paragraph">
 531 <p>The Simple-IPC protocol consists of a single request message from the
 532 client and an optional response message from the server.  Both the
 533 client and server messages are unlimited in length and are terminated
 534 with a flush packet.</p>
 535 </div>
 536 <div class="paragraph">
 537 <p>The pkt-line routines (<a href="../gitprotocol-common.html">gitprotocol-common(5)</a>)
 538 are used to simplify buffer management during message generation,
 539 transmission, and reception.  A flush packet is used to mark the end
 540 of the message.  This allows the sender to incrementally generate and
 541 transmit the message.  It allows the receiver to incrementally receive
 542 the message in chunks and to know when they have received the entire
 543 message.</p>
 544 </div>
 545 <div class="paragraph">
 546 <p>The actual byte format of the client request and server response
 547 messages are application specific.  The IPC layer transmits and
 548 receives them as opaque byte buffers without any concern for the
 549 content within.  It is the job of the calling application layer to
 550 understand the contents of the request and response messages.</p>
 551 </div>
 552 </div>
 553 </div>
 554 <div class="sect1">
 555 <h2 id="_summary">Summary</h2>
 556 <div class="sectionbody">
 557 <div class="paragraph">
 558 <p>Conceptually, the Simple-IPC protocol is similar to an HTTP REST
 559 request.  Clients connect, make an application-specific and
 560 stateless request, receive an application-specific
 561 response, and disconnect.  It is a one round trip facility for
 562 querying the server.  The Simple-IPC routines hide the socket,
 563 named pipe, and thread pool details and allow the application
 564 layer to focus on the task at hand.</p>
 565 </div>
 566 </div>
 567 </div>
 568 </div>
 569 <div id="footer">
 570 <div id="footer-text">
 571 Last updated 2023-10-24 06:43:46 +0900
 572 </div>
 573 </div>
 574 </body>
 575 </html>