Indexes

B+Tree

LMDB uses a modified design of Append-only B+ Tree and it uses 2 B+ trees : one for maintaining the regular user data pages and one for maintaining the free pages obtained after deletes. LMDB is optimized for short read transactions, long lived read transactions will keep older pages longer in the table and hence blocks write operations. If the workload has too many writes along with long lived read transactions, the performance would be very low.

Isolation Levels

Serializable

LMDB provides Serializable isolation with MVCC, this is possible because of the single-writer semantics. Only a single write transaction can can be alive at a single point of time, hence no races among multiple writers modifying the database.

Data Model

Key/Value

This embedded database is a key-value in the backend, which is stored in the memory-map. The keys are indexed in a B+ tree. LMDB provides transactional guarantees on top of this key-value store. It is not a relational database.

Query Interface

Custom API

LMDB has no SQL layer but applications can directly access the database using API calls provided by LMDB. API support is not just in C but many wrappers for other languages have been developed by open-source contributors. All key-value store operations can be performed using these API calls.

System Architecture

Shared-Memory

LMDB uses shared-memory model i.e. it handles the memory as a single address space and all the threads access this in parallel. It uses copy-on-write semantics.

Query Execution

Tuple-at-a-Time Model

There is no query planning or query execution options as this is an embedded database, since we operate at individual key level, the closest we can classify it is under tuple-at-a-time. The user can program custom querying models on top this embeddded database, which can support other query execution options.

Joins

Not Supported

Stored Procedures

Not Supported

Views

Not Supported

Logging

Shadow Paging

No logging procedures are implemented here, using copy-on-write semantics (with shadow paging) provides durability without any need for logging. Shadow paging allows new writes to a different location and not directly replace the existing pages, hence avoids data-corruption. Also the shadow page reference update is atomic, hence avoids need for logging.

Storage Architecture

In-Memory

LMDB uses mmap, hence it reliquishes most of the caching control to the OS. Memory map allows zero-copies for read/write and no additional buffers for the transaction control. Supports larger-than memory databases, it is bounded by the size of the virtual memory since they use a memory map.

Storage Model

Custom

They use a memory-map to store the database with copy-on-write semantics, hence no specific storage model but the semantics are left to the operating system. The on-disk representation is similar to the memory representation of the database.

Query Compilation

Not Supported

Concurrency Control

Multi-version Concurrency Control (MVCC)

Locking overhead avoided by using MVCC, readers don't block at all and writers don't block readers. Deleted versions are reclaimed by the free space management module of LMDB (essentially stored into a B+ tree for later use).