Archive for Janeiro 7th, 2012

Hot Swap

Sábado, Janeiro 7th, 2012

One thing I’ve been absolutely jealous of the PLC world is the ability to hotswap DataBlocks, FCs, etc. I mean, while the PLC is running and executing code, you can simply upload a modified version of an FC (basically a function) and… it just works. The PLC (once the upload is completed) changes the function pointer to the new code between cycles, and all is well.

Obviously, PLCs are rather simple functional machines, they basically work with static memory blocks (for variables and functions) and with a very rigid structure (IN/OUT image management, main cycle). So it’s very easy to achive the hotswapping of functions, and even of variable blocks (you basically refer to a variable by it’s address; if you’re not carefull changing a varible block, you might be now pointing to some address that encompasses parts of two variables… it does not complain).

When you mainly roam on Java land, like I do, the sights change dramatically. Sure, you have an immense power on your hands, but think about class hierarchies, objects, constructors, third party libraries… and it’s very far from trivial to implement hotswapping in the Java Virtual Machine.

I’m using Java to build my most important “desktop” applications (actually, it’s a server-client aproach, but the server app is 100% built by me, no third party web or app servers involved), and sometimes I’d like to have hotswapping on my development system. It really bugs me to have to bring down my server app because of a simple bug fix or improvement.

Still, after reading a bit on HotSwap, JRebel and the like, it probably is not that important. I’m not willing to add significant amounts of complexity just to gain hotswap, since I’ve developed things with a very lightweight and encapsulated structure. On recent hardware and JRE, my server app starts up in 8 seconds (database startup included), and the client app in a mere 4 seconds. Even with a full recompile and shutdown times, I’m looking at a power cycle of under 30 seconds. Not bad.