141 lines
4 KiB
Text
141 lines
4 KiB
Text
IPC - in cinit and in general
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=============================
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Nico Schottelius <nico-linux-cinit__@__schottelius.org>
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0.1, Initial Version from 2006-07-09
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:Author Initials: NS
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IPC - Inter process communication
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Introduction
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------------
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This document describes the IPC methods used and/or tested for
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cinit.
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It does not describe in detail, how the different methods work
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(this is already done many times, there's great documentation
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available online), but more the advantages and disadvantages
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(especially for an init system).
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What is IPC?
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~~~~~~~~~~~~
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IPC describes methods to communicate between different processes
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(programs).
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IPC as described by SUSV3 (The Single UNIX Specification Version 3)
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aka IEEE Std 1003.1, 2004 Edition aka POSIX only defines
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MSQ, SHM and Semaphores as IPC. This document also covers
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Sockets and FIFOs.
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What is not (yet) covered by this document?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Speed of different IPC methods, different behaviour on different
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unices.
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Why do you need IPC for an init system?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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cinit IPC history + analysis
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----------------------------
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FIFOs
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~~~~~
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First in - first out
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File on the filesystem
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Always need two files for two way communication
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Hints
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^^^^^
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The first idea for IPC in cinit was to use two FIFOs, like minit does.
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Wrong assumption:
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runit also uses fifos, but two fifos for each services. That way
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more parallel transmissions is possible.
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You have to pay attention: Maximum number of opened files!
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(On Linux 2.6 this is 1024, which makes a maximum of 512 services.
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This limit is most likely never reached, but you have to keep it
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in mind).
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Sockets
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~~~~~~~
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Clean and beautiful. They allow easy two way communication.
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If you created a socket, fs is r/o, you cannot use it, although
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there is SO_REUSE.
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First method: Using memory mapped part (tmpfs).
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Second method: use interal communication (pipes!) and external after
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/etc/cinit/ becomes writable.
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Current IPC configuration
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--------------------------
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- switchable (conf/ipc_method)
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- each ipc implementation needs:
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ipc.h -> for global variables and ipc specific things
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prefix variables with ipc_
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int cinit_ipc_init(void); -> general initialization
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return 1 on success, 0 on failure
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int cinit_ipc_listen(void); -> begin to listen for messages
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int cinit_ipc_send(void *data) -> send data to a client
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Abstraction layer: cinit_ipc_*
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------------------------------
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You can choose or even reimplement ipc code for cinit. You only have to create
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a directory below src/ipc/ and create the following necessary functions:
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int cinit_ipc_init(void);
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~~~~~~~~~~~~~~~~~~~~~~~~~
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Initialise the IPC functions in cinit.
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int cinit_ipc_listen(void);
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Never ending looping function that listens for commands and passes the
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retrieved command to read_command().
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other
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~~~~~~
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int cinit_ipc_sclose(void); /* fork of cinit come from outside! */
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int cinit_ipc_ssend(void *data); /* send to a client from the server */
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void cinit_ipc_destroy(void); /* destroy ipc handler in cinit */
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cinit_get_data(int ident, int size, void *data) => read size bytes from client
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with ident
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/*****************************************************************************
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* Functions: in clients
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*/
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int cinit_ipc_logon(void); /* logon to init (client init) */
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int cinit_ipc_connect(void); /* connect to init */
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int cinit_ipc_csend(struct cinit_message *data); /* send to the server from a client */
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--------------------------------------------------------------------------------
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TO SORT:
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1. message queues
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o clients schreiben rein
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o datenpaket gross genug fuer antwort?
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2. shared memory
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o client kann direkt rauslesen, was gestartet werden muss
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o client kann direkt abhaengigkeiten starten, muss diese aber
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eintragen
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Ausprobieren:
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- Mutexe
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- Message Queues
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-
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Ideen:
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Message queue
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1. Client geht auf die normale
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als id = pid (cast!)
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dann rueck via anderer queue
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