(Keep It) Simple Stupid Database

KISSDB is about the simplest key/value store you'll ever see, anywhere. It's written in plain vanilla C using only the standard string and FILE I/O functions, and should port to just about anything with a disk or something that acts like one.

It stores keys and values of fixed length in a stupid-simple file format based on fixed-size hash tables. If a hash collision occurrs, a new "page" of hash table is appended to the database. The format is append-only. There is no delete. Puts that replace an existing value, however, will not grow the file as they will overwrite the existing entry.

Hash table size is a space/speed trade-off parameter. Larger hash tables will reduce collisions and speed things up a bit, at the expense of memory and disk space. A good size is usually about 1/2 the average number of entries you expect.

Features:

• Tiny, compiles to ~4k on an x86_64 Linux system
• Small memory footprint (only caches hash tables)
• Very space-efficient (on disk) if small hash tables are used
• Makes a decent effort to be robust on power loss
• Pretty respectably fast, especially given its simplicity
• 64-bit, file size limit is 2^64 bytes
• Ports to anything with a C compiler and stdlib/stdio
• Public domain

Limitations:

• Fixed-size keys and values, must recreate and copy to change any init size parameter
• Add/update only, no delete
• Iteration is supported but key order is undefined
• No search for subsets of keys/values
• No indexes
• No transactions
• No special recovery features if a database gets corrupted
• No built-in thread-safety (guard it with a mutex in MT code)
• No built-in caching of data (only hash tables are cached for lookup speed)
• No endian-awareness (currently), so big-endian DBs won't read on little-endian machines

Alternative key/value stores and embedded databases:

• MDB uses mmap() and is very fast (not quite as tiny/simple/portable)
• CDB is also minimal and fast, probably the closest thing to this (but has a 4gb size limit)
• Kyoto Cabinet is very fast, full-featured, and modern (license required for commercial use)
• SQLite gives you a complete embedded SQL server (public domain, very mature, much larger)
• Others include GDBM, NDBM, Berkeley DB, etc. Use your Googles. :)

KISSDB is good if you want space-efficient relatively fast write-once/read-many storage of keys mapped to values. It's not a good choice if you need searches, indexes, delete, structured storage, or widely varying key/value sizes. It's also probably not a good choice if you need a long-lived database for critical data, since it lacks recovery features and is brittle if its internals are modified. It would be better for a cache of data that can be restored or "re-learned," such as keys, Bitcoin transactions, nodes on a peer-to-peer network, log analysis results, rendered web pages, session cookies, auth tokens, etc.

KISSDB is in the public domain as according to the Creative Commons Public Domain Dedication. One reason it was written was the poverty of simple key/value databases with wide open licensing. Even old ones like GDBM have GPL, not LGPL, licenses.

See comments in kissdb.h for documentation. Makefile can be used to build a test program on systems with gcc.

Author: Adam Ierymenko / ZeroTier Networks LLC