I have a problem and I think that it is rather common: I am about to
release a new program that will use a Firebird database and I am
planning to sell the program at different prices for a different number
of concurrent users.
The program will be more expensive if the users want more concurrent
connections.

So I need a way to limit the number of concurrent users to the database:
I searched the list history but I have found only a few (and rather old)
messages about this topic.

I am experimenting with a solution and I would like to receive comments
about it, just to know if it is a valid one or not.

I have created a custom table, with as many rows as the maximum expected
number of users. For my program this number will be very small. If the
user wants a lot of connections there is an unlimited version.
Each row contains an ID (primary key) and a dummy field.

When a program tries to connect to the database it issues a "SELECT ...
WITH LOCK" statement in a no-wait transaction, trying to lock the first
row. If it gets a lock error the row is locked by another user so the
program tries to lock the second row, and so on.
In this way it should be easy to count the current number of users and
the program can know if the user count limit has been reached or not.
If the program can lock a row it keeps it locked until it terminates.

This solution has the disadvantage of blocking garbage collection so
each program could commit the locking transaction from time to time.
This should solve the garbage collection problem.

If a program crashes I suppose that the row will remain locked from some
time, but I don't know how long it will remain locked. Does anybody know
this locking time?

Do you think that this a reasonable solution, or are there better ones?

Thanks in advance.

Fulvio Senore