If a shared lock request is issued, it is granted immediately if the file is not locked or has another shared lock on it. If the file has an exclusive lock on it, the shared lock request is granted as soon as that lock is removed. The lock status becomes SHARED on success.
If an exclusive lock is requested, it is granted as soon as the file becomes unlocked. The lock status becomes EXCLUSIVE on success.
If the DB has a shared lock on it, a process that makes an exclusive lock request will poll until there are no reading or writing processes left. Lots of processes can successfully read the file, since they do not block each other. This means that a process that wants to write to the file may never get a chance to squeeze in, since it needs to obtain an exclusive lock.
Figure 19-2 represents a possible scenario in which everybody can read but no one can write. ("pX" represents different processes running at different times, all acquiring shared locks on the DBM file.)
The result is a starving process that will time out the request, which will fail to update the DB. Ken Williams solved this problem with his Tie::DB_Lock module, discussed later in this chapter.
There are several locking wrappers for DB_File on CPAN right now. Each one implements locking differently and has different goals in mind. It is worth knowing the differences between them, so that you can pick the right one for your application.