Client-Server application using thread pool - implementation suggestions

[wysota]

1 - Should I create one connection to the database per thread (possibly in the run() function, or the constructor of the class) or one connection is sufficient (created in the WServer class)?

You need to have a dedicated connection for each thread. I'm not sure if creating them in run() if you're going to use QRunnable is a good solution. As far as I understand it a single thread can run more than one runnable. If that was the case, it would be best to store connections outside the runnable so that if a runnable is ran on a thread that used to handle some other runnable, it can pick up the connection the previous runnable created. That's purely for efficiency reasons so you can ignore it and have the connection created and destroyed within the runnable.

2 - Can I access members variables of the WServer class in the run function of the QRunnable class instances?? (With the access protected with QMutex). For instance, if one of the clients request the following information: what is the number of clients connected in this moment?? The run method needs to access client_list->size()

Well... that depends. If you design it properly then yes. What might be troubling is that you might cause a deadlock with improper locking. The controlling thread should be operative all the time. But if you don't intend to block it, you'll be safe.