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 735 <body class="article">
 736 <div id="header">
 737 <h1>Simple-IPC API</h1>
 738 <span id="revdate">2024-02-06</span>
 739 </div>
 740 <div id="content">
 741 <div id="preamble">
 742 <div class="sectionbody">
 743 <div class="paragraph"><p>The Simple-IPC API is a collection of <code>ipc_</code> prefixed library routines
 744 and a basic communication protocol that allows an IPC-client process to
 745 send an application-specific IPC-request message to an IPC-server
 746 process and receive an application-specific IPC-response message.</p></div>
 747 <div class="paragraph"><p>Communication occurs over a named pipe on Windows and a Unix domain
 748 socket on other platforms.  IPC-clients and IPC-servers rendezvous at
 749 a previously agreed-to application-specific pathname (which is outside
 750 the scope of this design) that is local to the computer system.</p></div>
 751 <div class="paragraph"><p>The IPC-server routines within the server application process create a
 752 thread pool to listen for connections and receive request messages
 753 from multiple concurrent IPC-clients.  When received, these messages
 754 are dispatched up to the server application callbacks for handling.
 755 IPC-server routines then incrementally relay responses back to the
 756 IPC-client.</p></div>
 757 <div class="paragraph"><p>The IPC-client routines within a client application process connect
 758 to the IPC-server and send a request message and wait for a response.
 759 When received, the response is returned back to the caller.</p></div>
 760 <div class="paragraph"><p>For example, the <code>fsmonitor--daemon</code> feature will be built as a server
 761 application on top of the IPC-server library routines.  It will have
 762 threads watching for file system events and a thread pool waiting for
 763 client connections.  Clients, such as <code>git status</code>, will request a list
 764 of file system events since a point in time and the server will
 765 respond with a list of changed files and directories.  The formats of
 766 the request and response are application-specific; the IPC-client and
 767 IPC-server routines treat them as opaque byte streams.</p></div>
 768 </div>
 769 </div>
 770 <div class="sect1">
 771 <h2 id="_comparison_with_sub_process_model">Comparison with sub-process model</h2>
 772 <div class="sectionbody">
 773 <div class="paragraph"><p>The Simple-IPC mechanism differs from the existing <code>sub-process.c</code>
 774 model (Documentation/technical/long-running-process-protocol.txt) and
 775 used by applications like Git-LFS.  In the LFS-style sub-process model,
 776 the helper is started by the foreground process, communication happens
 777 via a pair of file descriptors bound to the stdin/stdout of the
 778 sub-process, the sub-process only serves the current foreground
 779 process, and the sub-process exits when the foreground process
 780 terminates.</p></div>
 781 <div class="paragraph"><p>In the Simple-IPC model the server is a very long-running service.  It
 782 can service many clients at the same time and has a private socket or
 783 named pipe connection to each active client.  It might be started
 784 (on-demand) by the current client process or it might have been
 785 started by a previous client or by the OS at boot time.  The server
 786 process is not associated with a terminal and it persists after
 787 clients terminate.  Clients do not have access to the stdin/stdout of
 788 the server process and therefore must communicate over sockets or
 789 named pipes.</p></div>
 790 </div>
 791 </div>
 792 <div class="sect1">
 793 <h2 id="_server_startup_and_shutdown">Server startup and shutdown</h2>
 794 <div class="sectionbody">
 795 <div class="paragraph"><p>How an application server based upon IPC-server is started is also
 796 outside the scope of the Simple-IPC design and is a property of the
 797 application using it.  For example, the server might be started or
 798 restarted during routine maintenance operations, or it might be
 799 started as a system service during the system boot-up sequence, or it
 800 might be started on-demand by a foreground Git command when needed.</p></div>
 801 <div class="paragraph"><p>Similarly, server shutdown is a property of the application using
 802 the simple-ipc routines.  For example, the server might decide to
 803 shutdown when idle or only upon explicit request.</p></div>
 804 </div>
 805 </div>
 806 <div class="sect1">
 807 <h2 id="_simple_ipc_protocol">Simple-IPC protocol</h2>
 808 <div class="sectionbody">
 809 <div class="paragraph"><p>The Simple-IPC protocol consists of a single request message from the
 810 client and an optional response message from the server.  Both the
 811 client and server messages are unlimited in length and are terminated
 812 with a flush packet.</p></div>
 813 <div class="paragraph"><p>The pkt-line routines (<a href="../gitprotocol-common.html">gitprotocol-common(5)</a>)
 814 are used to simplify buffer management during message generation,
 815 transmission, and reception.  A flush packet is used to mark the end
 816 of the message.  This allows the sender to incrementally generate and
 817 transmit the message.  It allows the receiver to incrementally receive
 818 the message in chunks and to know when they have received the entire
 819 message.</p></div>
 820 <div class="paragraph"><p>The actual byte format of the client request and server response
 821 messages are application specific.  The IPC layer transmits and
 822 receives them as opaque byte buffers without any concern for the
 823 content within.  It is the job of the calling application layer to
 824 understand the contents of the request and response messages.</p></div>
 825 </div>
 826 </div>
 827 <div class="sect1">
 828 <h2 id="_summary">Summary</h2>
 829 <div class="sectionbody">
 830 <div class="paragraph"><p>Conceptually, the Simple-IPC protocol is similar to an HTTP REST
 831 request.  Clients connect, make an application-specific and
 832 stateless request, receive an application-specific
 833 response, and disconnect.  It is a one round trip facility for
 834 querying the server.  The Simple-IPC routines hide the socket,
 835 named pipe, and thread pool details and allow the application
 836 layer to focus on the task at hand.</p></div>
 837 </div>
 838 </div>
 839 </div>
 840 <div id="footnotes"><hr /></div>
 841 <div id="footer">
 842 <div id="footer-text">
 843 Last updated
 844  2023-10-23 14:43:46 PDT
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