Ok... My application is having a problem... Sometimes, it seems that I run out of available transactions, but when I check the database transactions all of them are idle... When this happens, the only way to solve it seems to be to stop the application and restart it.
I believe the problem may be that some of my transactions are really long. I update many objects and then commit my transactions. And I do this on several threads.
To fix this problem, first I want to understand why it happens. Even though my transactions are long, they should be able to finish eventually, right? Why isn't that the case here? Does it matter that some of these threads are doing changes on the same tables?

It would be nice if we can see your code. But it's usually the problem on how the transactions are handled. Transactions should be atomic. You would have performance issues when another transaction is kick-off within a transaction. If you have several threads manipulating a given data/subset of data in one transaction, i think that should be fine as long as the threads doesn't cause any dirty reads or writes. I don't really know what the goal of multiple threads running in your app and it would be nice if you can write a pseudo-code so it'll she more light on the problem.