Programming a long-running time-based process

Question

I was wondering what the best way to write an application would be. Basically, I have a sports simulation project that is multi-threaded and can execute different game simulations concurrently.

I store my matches in a SQLite database that have a DateTime attached to it.

I want to write an application that checks every hour or so to see if any new matches need to be played and spawns those threads off.

I can't rely on the task scheduler to execute this every hour because there are objects that the different instances of that process would share (specifically a tournament object), that I suspect would be overwritten by a newer process when saved back into the DB. So ideally I need to write some sort of long-running process that sleeps between hours.

I've written my object model so that each object is only loaded once from memory, so as long as all simulation threads are spawned from this one application, they shouldn't be overwriting data.

EDIT: More detail on requirements

Basically, multiple matches need to be able to run concurrently. These matches can be of arbitrary length, so it's not necessary that one finishes before the other begins (in fact, in most cases there will be multiple matches executing at the same time).

What I'm envisioning is a program that runs in the background (a service, I guess?) that sleeps for 60 minutes and then checks the database to see if any games should be started. If there are any to be started, it fires off threads to simulate those games and then goes back to sleep. Hence, the simulation threads are running but the "scheduling" thread is sleeping for another 60 minutes.

The reason I can't (I think) use the default OS task-scheduling interface is that these require the task to be executed to be spurned as a new process. I have developed my database object model such that they are cached by each object class on first load (the memory reference) meaning that each object is only loaded from memory once and that reference is used on all saves. Meaning that when each simulation thread is done and saves off its state, the same reference is used (with updated state) to save off the state. If a different executable is launched every time, presumably a different memory reference will be opened by each process and hence one process could save into the DB and overwrite the state written by the other process.

A service looks like the way to go. Is there a way to make a service just sleep for 60 minutes and wake up and execute a function after that? I feel like making this a standard console application would waste memory, but I don't know if there is an efficient way to do that which I'm not aware of.

Answer 1

If you want to make it really reliable, make it a Service.

But I don't see any problems in making it a normal (Console, WinForms, WPF) application.

Maybe you could expand on the requirements a little.

Answer 2

The reason I can't (I think) use the default OS task-scheduling interface is that these require the task to be executed to be spurned as a new process. I have developed my database object model such that they are cached by each object class on first load (the memory reference) meaning that each object is only loaded from memory once and that reference is used on all saves

If you want everything to remain cached forever, then you do need to have an application that simply runs forever. You can make this a windows service, or a normal windows application.
A windows service is just a normal exe that conforms to the service manager API. If you want to make one, visual studio has a wizard which auto-generates some skeleton code for you. Basically instead of having a Main method you have a Service class with a Run method and everything else is the same.

You could, if you wanted to, use the windows task scheduler to schedule your actions. The way you'd do this is to have your long-running windows service in the background that does nothing. Have it open a TCP socket or named pipe or something and just sit there. Then write a small "stub" exe which just connects to this socket or named pipe and tells the background app to wake up.
This is, of course, a lot harder than just doing a sleep in your background application, but it does let you have a lot more control - you can change the sleep time without restarting the background service, run it on-demand, etc.

---

I would however, consider your design. The fact that you rely on a long-running service is a large point of failure. If your app needs to run for days, and you have a single bug which crashes it, then you have to start again. A much better architecture is to follow the Unix model, where you have small processes which start, do one thing, then finish (in this case, process each game simulation as it's own process so if one dies it doesn't take the master process or the other simulations down).

It seems like the main reason you're trying to have it long-running is to cache your database queries. Do you actually need to do this at all? A lot of the time databases are plenty fast enough (they have their own caches, which are plenty smart). A common mistake I've seen programmers make is to just assume that something like a database is slow, and waste a pile of time optimizing when in actual fact it would be fine