Question

I have a client application (C++, Windows) that opens sockets, connects to a server, makes requests, receive responses and notifications. It does logging and saves preferences locally. What can be problems if I try to run multiple instances of this application which is prevented presently?

Answer 1

Sounds a little bit like a Web browser 😉

And like a typical Web browser, if your application is implemented correctly, you’ll be able to run multiple instances fine.

Unfortunately, there are ways to botch the implementation, for example:

• Exclusively lock log or configuration files for prolonged periods, thus “stalling” other instances.
• Just plain ignore the concurrent access to files, leading to all sorts of possible corruptions.
• Act not just as a client but as a server as well, and listen to a hard-coded port (so the second instance will fail while attempting to open the same port).
• Incorrectly declare a mutex as “public” (and therefore shared between processes) instead of “private”, leading to slow-downs and possibly deadlocks.
• There is a limit for number of GDI handles per session. If you application uses excessive handles, multiple instances taken together might reach that limit, even when each of them individually observes the 10000 handles-per-process limit.
• Be a CPU hog (e.g. through busy waiting). One CPU hog on a modern multicore CPU might pass unnoticed, but once the number of instances exceeds the number of CPU cores that’s another story!
• Be a memory hog.
• Mismanage UI:
  • Use UI tricks such as “always on top” windows – multiple such windows on the screen at the same time is no fun!
  • Mismanage the taskbar notification area (e.g. display a tray icon for each instance). Will technically “work” but having excessive number of tray icons is not pleasant, especially if application does not also have a “regular” taskbar button.

Etc etc… Essentially whenever there is a shared resource (be it a filesystem, network, CPU, memory, screen or whatever), care must be taken when concurrently using it.