As we know that Berkeley DB provides interfaces supporting deadlock-free, multiple-reader/single writer access to the database.

Now I want to new five threads which want to do the 'write' operation with one database at the same time , but only one thread can write the db at one time, the other four will be hanged. This is the question that which is the next executable thread in the other four threads, what is the order and rule?

Thank you very much!

Berkeley DB will unlock the threads in the order they arrived. That,
of course, depends on how the threads are scheduled on your system.

Michael.

Michael， Thank you very much!

And do you have any manuals about this part, I want to study more detailed.

This is really an implementation detail that isn't covered in our
documentation.

However, if you have a Berkeley DB source tree, look at
__lock_get_internal in lock/lock.c. You will see that there are
holding and waiting queues, and requests are put onto one of those
queues depending on whether the locks conflict.

I think the best reference book to read for this area is still Jim
Gray's "Transaction Processing: Concepts and Techniques".

Regards,
Michael.