Ensuring task execution order in threadpool

Question

I have been reading about the thread-pool pattern and I can\'t seem to find the usual solution for the following problem.

I sometimes want tasks to be executed serial

Answer 1

Use two Active Objects. In two words: active object pattern consists from priority queue and 1 or many working threads those can get tasks from queue and process its.

So use one active object with one working thread: all tasks those would be places to queue would be processed sequentially. Use second active object with number of working thread more then 1. In this case working threads would get and process tasks from queue in any order.

Luck.