cdb, short for "constant database", refers to both a library and data format created by Daniel J. Bernstein. cdb acts as an on-disk associative array, mapping keys to values, and allows multiple values to be stored for a single key. A constant database allows only two operations: creation and reading. Both operations are designed to be very fast and highly reliable. Since the database does not change while it is in use, multiple processes can access a single database without locking. Additionally, since all modifications create a replacement database, it can take advantage of UNIX filesystem semantics to provide a guarantee of reliability.