Question

I’ve heard mumblings around the internets about being able to hot-swap code in Lua similar to how it’s done in Java, Erlang, Lisp, etc. However, 30 minutes of googling for it has turned up nothing. Has anyone read anything substantial about this? Anyone have any experience doing it? Does it work in LuaJIT or only in the reference VM?

I’m more interested in the technique as a shortcut in development/debugging than an upgrade path in a live environment.

Answer 1

Lua, and most scripting languages for that matter, do not support the most generalized form of “hot swapping” as you define it. That is, you cannot guaranteeably change a file on disk and have any changes in it propagate itself into an executing program.

However, Lua, and most scripting languages for that matter, are perfectly capable of controlled forms of hot swapping. Global functions are global functions. Modules simply load global functions (if you use them that way). So if a module loads global functions, you can reload the module again if it is changed, and those global function references will change to the newly loaded functions.

However, Lua, and most scripting languages for that matter, makes no guarantees about this. All that’s happening is the changing of global state data. If someone copied an old function into a local variable, they can still access it. If your module uses local state data, the new version of the module cannot access the old module’s state. If a module creates some kind of object that has member functions, unless those members are fetched from globals, these objects will always refer to the old functions, not the new ones. And so forth.

Also, Lua is not thread safe; you can’t just interrupt a lua_State at some point and try to load a module again. So you would have to set up some specific point in time for it to check stuff out and reload changed files.

So you can do it, but it isn’t “supported” in the sense that it can just happen. You have to work for it, and you have to be careful about how you write things and what you put in local vs. global functions.