| Simple-IPC API |
| ============== |
| |
| The Simple-IPC API is a collection of `ipc_` prefixed library routines |
| and a basic communication protocol that allow 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 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 application at hand. |