\page intro Introduction
\section characteristics Characteristics

Preface {#preface}
------------------

> For the most part, this section is a copy of the corresponding text
> from LMDB description, but with some edits reflecting the improvements
> and enhancements were made in MDBX.

MDBX is a Btree-based database management library modeled loosely on the
BerkeleyDB API, but much simplified. The entire database (aka "environment")
is exposed in a memory map, and all data fetches return data directly from
the mapped memory, so no malloc's or memcpy's occur during data fetches.
As such, the library is extremely simple because it requires no page caching
layer of its own, and it is extremely high performance and memory-efficient.
It is also fully transactional with full ACID semantics, and when the memory
map is read-only, the database integrity cannot be corrupted by stray pointer
writes from application code.

The library is fully thread-aware and supports concurrent read/write access
from multiple processes and threads. Data pages use a copy-on-write strategy
so no active data pages are ever overwritten, which also provides resistance
to corruption and eliminates the need of any special recovery procedures
after a system crash. Writes are fully serialized; only one write transaction
may be active at a time, which guarantees that writers can never deadlock.
The database structure is multi-versioned so readers run with no locks;
writers cannot block readers, and readers don't block writers.

Unlike other well-known database mechanisms which use either write-ahead
transaction logs or append-only data writes, MDBX requires no maintenance
during operation. Both write-ahead loggers and append-only databases require
periodic checkpointing and/or compaction of their log or database files
otherwise they grow without bound. MDBX tracks retired/freed pages within the
database and re-uses them for new write operations, so the database size does
not grow without bound in normal use. It is worth noting that the "next"
version libmdbx (MithrilDB) will solve this problem.

The memory map can be used as a read-only or read-write map. It is read-only
by default as this provides total immunity to corruption. Using read-write
mode offers much higher write performance, but adds the possibility for stray
application writes thru pointers to silently corrupt the database.
Of course if your application code is known to be bug-free (...) then this is
not an issue.

If this is your first time using a transactional embedded key-value store,
you may find the \ref starting section below to be helpful.