cdb, short for "constant database", refers to both a library and data format created by Daniel J. Bernstein.
Additionally, since all modifications create a replacement database, it can take advantage of UNIX filesystem semantics to provide a guarantee of reliability.
[1] cdb is used by djbdns, fastforward, mess822, qmail and ucspi-tcp to provide highly efficient, reliable, and simple data access.
Lookups are designed for exact keys only, though other types of searches could be performed by scanning the entire database.
The fixed-size header at the beginning of the database describes 256 hash tables by listing their position within the file and their length in slots.
However, the rest of the cdb package used to be license-free software, meaning it must be distributed verbatim.