[SSSD] Embedding Lua into SSSD
Hi everyone,
I might be in a strange and careless mood today, but here is something I
wanted to suggest since the time I saw the amount of logic that goes into SSSD
and is implemented in C.
What if we implement some of the complicated logic inside SSSD in Lua [1]?
Lua is a language specifically designed to be embedded. It grew into
popularity after it was embedded into World Of Warcraft. It was also embedded
and is still being embedded into a bunch of other games. However, games is far
from the only application for Lua.
Lua is a simple and lightweight language, yet with lots of flexibility -
grasping the essence of the whole reference manual [2] takes one evening and
it's an entertaining read. The VM is small and fast. In fact some people use
it on microcontrollers [3].
I personally embedded it into two C-based projects, one of which was already
quite big, and another was a rewrite of a smallish, but complicated C project.
In both cases, where before things were tedious and hard, they have become
easy and fun. The first project was on an embedded platform. I also had some
other, but smaller stuff done with it.
Generally, Lua gets put into the middle with outside interfaces and
performance-critical parts implemented in C. With a very simple interface to
and from C one can put boundaries wherever necessary. Yes, the interface is
simpler than Python's.
The process is this: you implement some C libraries (shared/static) which are
loadable from Lua (Lua->C interface) and then create a VM or several, load
some Lua code and call into it (C->Lua interface).
Some of the benefits are:
Clearer logic (higher level language)
Much less memory leaks and very little memory management (mostly in Lua->C
interface).
Faster development cycle (no compilation, edit and retry on a live daemon)
Potentially easy plugin interface (yay for admin scripts!)
Some of the drawbacks are:
More difficult to do low level stuff (so we just don't do that)
Lower performance than C (still good enough for games, and we can have the
critical parts in C easily)
Needs learning and development investment.
Regarding the development investment, due to low cost of C/Lua interface
implementation, it is practical to replace the relevant C code piece-by-piece,
provided pieces are not too small (say, 2KLOC and up).
Lastly, don't take this too seriously. I realise that this is disruptive and
hard. However if you ever feel like SSSD has grown too big and complex, ping
me, or just try Lua yourself :)
Nick
[1] http://www.lua.org/
[2] http://www.lua.org/manual/5.3/manual.html
[3] http://www.eluaproject.net/