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libipc-old/docs/libipc.md

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2020-12-09 15:33:54 +01:00
## Before starting
This file is a presentation.
Better get the point tools here: https://git.baguette.netlib.re/Baguette/pointtools
Have fun!
## Why libIPC?
Network code separation
* networking: performed by dedicated services
* other applications: think all communications are local
Library code separation
* make libraries language independent
* micro-services are great, each of them does its part
why not on base systems?
Applications are better than libraries
* implementations change
* languages change
* API… not so much (not as often at least)
## Available libraries
* libevent
* DBUS
* even more complicated stuff
* RPC-style, like Corba
#pause
* ... or bare libc api
* pipes
* sockets (unix, inet, inet6)
* shared memory
## libevent
* works with epoll and kqueue
* great performances
* works on Linux and *BSD
* a bit complicated at first
## DBUS
* not well suited for our needs
(a polite way to say: what a bloody mess)
Is it designed *NOT* to be used?
* over-engineered
* complex
* documentation isn't great
* no code example
And kinda obsolete: a big chunk of the documentation is
about message format. Just use CBOR already!
They even admit they did a poor job on the C part:
> There is a low-level C binding, but that is probably too detailed
> and cumbersome for anything but writing other bindings.
Oh. And C++. YOU SHALL NOT PASS!
This is a Linux requirement nowadays, wth?
## Bare libc api
All have great performances
shared memory and semaphores
* (kinda) complicated api
* not about exchanging messages
pipes, sockets
* lack a conventional message format
... but that's about it
Great to start with!
## LibIPC history
1. based on pipes
* because we gotta go fast!
* ... but implementation was a bit of a mess
#pause
2. rewrite to work with unix sockets
* performances are excellent, no need for absolute best
* way nicer implementation
* select(2) for listening on file descriptors
#pause
* ... wait, does select(2) support more than 1024 connections?
#pause
3. rewrite using poll(2)
* many bugfixes later, way more tested than before
* implementation now kinda production-ready
* enough for altideal.com at least
## Current implementation of libIPC
implementation is simple: < 2000 lines of C code
usage is simple
1. init connection or server
2. loop over events
bindings are available in Crystal
* as well as fancy mappings: JSON and CBOR class serialization
#pause
epoll (Linux) and kqueue (*BSD) were avoided
* 'cause callbacks hell => harder to read and to write code
#pause
* but we need them for better performances with many connections
though, API should stay the same for non threaded applications (simple implementation)
#pause
LibIPC doesn't handle parallelism, yet
## How libIPC works
LibIPC has a high level API for the user
1. init a connection (client) or create an unix socket (service)
example: ipc_server_init (context, "service")
#pause
2. loop, wait for events
listening to file descriptors (libIPC ones or not)
example:
while(1) {
wait_event (context, &event, &timer)
switch (event.type) {
case IPC_EVENT_TYPE_CONNECTION : ...
case IPC_EVENT_TYPE_DISCONNECTION : ...
case IPC_EVENT_TYPE_MESSAGE: {
struct ipc_message *m = event.m;
...
}
}
}
## How libIPC works
3. send messages
struct ipc_message m
m.payload = ...
m.length = strlen(m.payload)
m.fd = event.fd
m.type = ...
m.user_type = ...
ipc_write (context, &m)
#pause
4. add and remove fd from the context
ipc_add_fd (context, fd)
ipc_del_fd (context, fd)
## How libIPC works
LibIPC also helps to create "protocol daemons" like TCPd with
automatic switching between file descriptors
LibIPC takes callbacks to obtain libipc payloads inside arbitrary message structure
Example: websocketd.
Clients exchange data with a libipc service through websockets messages.
websocketd binds both the client and its service file descriptors,
then provides the libipc a callback to extract libipc messages from
the websocket messages sent by the client.
Same thing the other way.
ipc_switching_callbacks (context, client_fd, cb_in, cb_out)
## libIPC internal structures
Main goal: simplest possible structures
Examples:
Message
struct ipc_message {
char type; => Internal message type, used by protocol daemons.
char user_type; => User-defined message type (arbitrary).
int fd; => File descriptor concerned about this message.
uint32_t length; => Payload length.
char *payload; => Actual payload.
};
Context of the whole networking state
struct ipc_ctx {
struct ipc_connection_info *cinfos; => Keeps track of connections.
struct pollfd *pollfd; => List of "pollfd" structures within cinfos,
so we can pass it to poll(2).
size_t size; => Size of the connection list.
struct ipc_messages tx; => Messages to send.
struct ipc_switchings switchdb; => Relations between fd.
};
## Future of libIPC
LibIPC will be rewritten in Zig
* simpler to read and write
* data structures are simpler to create with comptime
* less code redundancy (more generic functions)
* already existing error management as written in current libIPC
* already existing loggin system
* no more C's pitfalls
* way better at exposing bugs
* thanks to a better type system
* way safer: cannot ignore errors
* so I won't
* simpler to cross-compile: same standard library for every OSs
* simpler (and more secure) memory management
Also: this won't change existing bindings! No excuses!
## Why not use it?
Current limitations
* performances (libIPC is based on poll(2), not epoll nor kqueue)
* it really isn't an issue until you have hundreds or thousands of clients
* parallelism is not permitted
nothing in libIPC is thread-safe
The future Zig implementation will overcome these issues.
## Questions?
Ask! `karchnu at karchnu.fr`