Monday, 5 February 2018

The interesting case of Lua in RTC world

An interesting pattern that caught my attention is the role that Lua is gaining in the RTC (Real-Time Communications) world.

Lua is a small-footprint programming language, powerful while keeping a simple syntax.

I’ve been using Lua to script dialplan actions for FreeSWITCH since about 2014. It has provided me with a way to define relatively complex logic and speed up the definition of FS’ behaviour.

Delegating this type of logic to a scripting language had several advantages, such as:
  • It’s easier to read and understand than native dialplans or native routing logic.
  • Makes unit testing of the dialplan possible/easier.
  • Allows changing some pieces of logic easily, in many cases preventing expensive reload of modules or restart of applications.

I’ve been using Lua for Kamailio as well. Kamailio is an open source programmable SIP Proxy. In a specific case, some bits of the routing logic required regex processing and was expecting to change often: an ideal case for an external script to do that work.
When the logic changes, it’s sufficient to instruct Kamailio to reload the script, and from that moment on the new requests being processed will use the new logic.

Recent versions of Kamailio though add a framework called KEMI. This opens up new possibilities, and also provides support for many other scripting languages, python being the most popular, JS, Squirrel. Still, Lua appears to provide the fastest implementations (with no observable performance degradation) while others have limitations. Python, as you can imagine, provides a rich set of functions and libraries, but it’s not as performant and the reload mechanism currently has some issues.

Wireshark, a tool to capture and analyse network activity, exposes a useful API for Lua. You can use the API to define your own Wireshark dissector (which you’ll need to install as a plugin). This has performance limitations in comparison with dissectors written in C - and so it’s recommended for prototyping only - but still can solve your problem perfectly. Out of need, I wrote a Wireshark dissector for HEP, a binary protocol used in the Homer environment. Homer is an open source framework for the monitoring and analysis of Real-Time Communications.

Last weekend a new interesting case was presented by Lorenzo Miniero at FOSDEM. The target application was Janus, an open source framework to build WebRTC gateways.

Janus allows to build applications by defining the transport and business logic as plugins, on top of the core that implements the WebRTC stack.
It’s written in C and so far users needed to write plugins with that language. The Janus developers have introduced the possibility to write plugins in Lua.

In his presentation Lorenzo explains also in detail what approach is best to use for a Real-Time application to interact with single threaded language like Lua in an asynchronous context.

Just a funny note: Lua uses a double dash for commenting out a line: '--'. Be careful when you watch diffs in a terminal because a removed comment will start with '- --‘ and may not the easiest thing to interpret (experiences may vary depending on the terminal, of course!).

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