寻找资源管理/分配系统

Question

What I need is a system I can define simple objects on (say, a "Server" than can have an "Operating System" and "Version" fields, alongside other metadata (IP, MAC address, etc)).
I'd like to be able to request objects from the system in a safe way, such that if I define a "Server", for example, can be used by 3 clients concurrently, then if 4 clients ask for a Server at the same time, one will have to wait until the server is freed.
Furthermore, I need to be able to perform requests in some sort of query-style, for example allocate(type=System, os='Linux', version=2.6) .

Language doesn't matter too much, but Python is an advantage.

I've been googling for something like this for the past few days and came up with nothing, maybe there's a better name for this kind of system that I'm not aware of.

Any recommendations?

Thanks!

Answer 1

Resource limitation in concurrent applications - like your "up to 3 clients" example - is typically implemented by using semaphores (or more precisely, counting semaphores).

You usually initialize a semaphore with some "count" - that's the maximum number of concurrent accesses to that resource - and you decrement this counter every time a client starts using that resource and increment it when a client finishes using it. The implementation of semaphores guarantees the "increment" and "decrement" operations will be atomic.

You can read more about semaphores on Wikipedia . I'm not too familiar with Python but I think these two links can help:

• Python Threading Library
• Semaphore Objects in Python.

Answer 2

For Java there is a very good standard library that has this functionality:

http://java.sun.com/j2se/1.5.0/docs/api/java/util/concurrent/package-summary.html

Just create a class with Semaphore field:

 class Server {
   private static final MAX_AVAILABLE = 100;
   private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);
   // ... put all other fields (OS, version) here...
   private Server () {}

   // add a factory method
   public static Server getServer() throws InterruptedException {
     available.acquire();
     //... do the rest here
   }

 }

Edit:

If you want things to be more "configurable" look into using AOP techniques ie create semaphore-based synchronization aspect.

Edit:

If you want completely standalone system, I guess you can try to use any modern DB (eg PostgreSQL) system that support row-level locking as semaphore. For example, create 3 rows for each representing a server and select them with locking if they are free (eg " select * from server where is_used = 'N' for update "), mark selected server as used, unmark it in the end, commit transaction.