GDBM

GNU dbm is a library for simple databases.  A database is a file that stores
key-value pairs.  Gdbm allows the user to store, retrieve, and delete data
by key.  It furthermore allows a non-sorted traversal of all key-value
pairs.  A gdbm database thus provides the same functionality as a hash.  As
with objects of the Hash class, elements can be accessed with [].
Furthermore, GDBM mixes in the Enumerable module, thus providing convenient
methods such as #find, #collect, #map, etc.

A process is allowed to open several different databases at the same time.
A process can open a database as a "reader" or a "writer".  Whereas a reader
has only read-access to the database, a writer has read- and write-access.
A database can be accessed either by any number of readers or by exactly one
writer at the same time.