Lightning Memory-Mapped Database or LMDB is a software library that provides a high-performance embedded transactional database in the form of a key-value store.

Lightning Memory-Mapped Database is written in C with API bindings for several programming languages.

Lightning Memory-Mapped Database stores arbitrary key/data pairs as byte arrays, has a range-based search capability, supports multiple data items for a single key and has a special mode for appending records at the end of the database (MDB_APPEND) which gives a dramatic write performance increase over other similar stores.

Lightning Memory-Mapped Database is not a relational database but like Berkeley DB and other similar databases such as dbm is strictly a key-value store.

Lightning Memory-Mapped Database design was first discussed in a 2009 post to the OpenLDAP developer mailing list, in the context of exploring solutions to the cache management difficulty caused by the project's dependence on Berkeley DB. A specific goal was to replace the multiple layers of configuration and caching inherent to Berkeley DB's design with a single, automatically managed cache under the control of the host operating system.

OpenLDAP can use Lightning Memory-Mapped Database for which Lightning Memory-Mapped Database was originally developed via back-mdb.

Author Howard Chu made clear that Lightning Memory-Mapped Database is part of the OpenLDAP project, which had its BSD style license before he joined, and it will stay like it.

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