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48 lines
2.6 KiB
Markdown
48 lines
2.6 KiB
Markdown
\page intro Introduction
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\section characteristics Characteristics
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Preface {#preface}
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------------------
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> For the most part, this section is a copy of the corresponding text
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> from LMDB description, but with some edits reflecting the improvements
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> and enhancements were made in MDBX.
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MDBX is a Btree-based database management library modeled loosely on the
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BerkeleyDB API, but much simplified. The entire database (aka "environment")
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is exposed in a memory map, and all data fetches return data directly from
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the mapped memory, so no malloc's or memcpy's occur during data fetches.
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As such, the library is extremely simple because it requires no page caching
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layer of its own, and it is extremely high performance and memory-efficient.
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It is also fully transactional with full ACID semantics, and when the memory
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map is read-only, the database integrity cannot be corrupted by stray pointer
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writes from application code.
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The library is fully thread-aware and supports concurrent read/write access
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from multiple processes and threads. Data pages use a copy-on-write strategy
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so no active data pages are ever overwritten, which also provides resistance
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to corruption and eliminates the need of any special recovery procedures
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after a system crash. Writes are fully serialized; only one write transaction
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may be active at a time, which guarantees that writers can never deadlock.
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The database structure is multi-versioned so readers run with no locks;
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writers cannot block readers, and readers don't block writers.
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Unlike other well-known database mechanisms which use either write-ahead
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transaction logs or append-only data writes, MDBX requires no maintenance
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during operation. Both write-ahead loggers and append-only databases require
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periodic checkpointing and/or compaction of their log or database files
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otherwise they grow without bound. MDBX tracks retired/freed pages within the
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database and re-uses them for new write operations, so the database size does
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not grow without bound in normal use. It is worth noting that the "next"
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version libmdbx (\ref MithrilDB) will solve this problem.
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The memory map can be used as a read-only or read-write map. It is read-only
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by default as this provides total immunity to corruption. Using read-write
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mode offers much higher write performance, but adds the possibility for stray
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application writes thru pointers to silently corrupt the database.
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Of course if your application code is known to be bug-free (...) then this is
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not an issue.
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If this is your first time using a transactional embedded key-value store,
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you may find the \ref starting section below to be helpful.
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