19.5 Tie::DB_Lock

Tie::DB_Lock ties hashes to databases using shared and exclusive locks. This module, written by Ken Williams, solves the problems discussed earlier.

The main difference with this module is that Tie::DB_Lock copies a DBM file on read. Reading processes do not have to keep the file locked while they read it, and writing processes can still access the file while others are reading. This works best when you have lots of long-duration reading processes and a few short bursts of writing.

The drawback of this module is the heavy I/O performed when every reader makes a fresh copy of the DB. With big DBM files this can be quite a disadvantage and can slow down the server considerably.

An alternative would be to have one copy of the DBM image shared by all the reading processes. This would cut the number of files that are copied and put the responsibility of copying the read-only file on the writer, not the reader. However, some care would be required to make sure that readers are not disturbed when a new read-only copy is put into place.

    Part I: mod_perl Administration
    Part II: mod_perl Performance
    Part VI: Appendixes