Some information is so important that you cannot afford to lose it. Consider the name and password for authenticating users. If a person registers at a site that charges a subscription fee, it would be unfortunate if his subscription details were lost the next time the web server was restarted. In this case, the information must be stored in a non-volatile way, and that usually means on disk. Several options are available, ranging from flat files to DBM files to fully-fledged relational databases. Which one you choose will depend on a number of factors, including:

• The size of each record and the volume of the data to be stored

• The number of concurrent accesses (to the server or even to the same data)

• Data complexity (do all the records fit into one row, or are there relations between different kinds of record?)

• Budget (some database implementations are great but very expensive)

• Failover and backup strategies (how important it is to avoid downtime, how soon the data must be restored in the case of a system failure)