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libkvstore

A general-purpose wrapper around key-value stores.

• LMDB-style API
• Trasactional
• ACID
• Written in C
• No frills

If you're unsure of when to use something like libkvstore:

• When you're currently using flat files but you want something easier, faster and less error-prone
• When you're currently using SQL but feel like you're fighting the query planner or dynamically generating queries
• When you want to support multiple storage engines with different tradeoffs
• When you want a transactional interface to LevelDB

libkvstore is low level enough to give you direct control over precisely how data is indexed and read, but high level enough that you can use it for general application programming without too much pain. If SQLite is fopen, then libkvstore is open(2).

Supported back-ends:

• LMDB (built-in)
• LevelDB (built-in)

Semi-supported back-ends:

• RocksDB (external)
• HyperLevelDB (external)
• lsmdb (external)

Possible future back-ends:

• An original distributed back-end supporting both strong and eventual consistency
• WiredTiger
• BerkeleyDB?
• CockroachDB?
• SQLite?
• SQLite4 LSM?

Building

./configure
make
sudo make install

You can change the default back-end with DB=leveldb make (but this is overridden if a specific back-end is named at run-time). Use make test to test.

API

Please refer to the LMDB documentation for general information.

Notable differences from LMDB's API:

• Use kvs_env_create_base to create a database environment with a particular named back-end, or kvs_env_create to use the default.
• mdb_cursor_get is split into several functions:
  • kvs_cursor_current: returns key and value at the cursor's current location.
  • kvs_cursor_seek: seeks to key. Direction can be positive (>=), negative (<=), or 0 (==).
  • kvs_cursor_next: steps forward (dir is positive) or backward (dir is negative).
  • kvs_cursor_first: seeks to first (dir is positive) or last (dir is negative) element.
  • kvs_cursor_get is still supported.
• Range functions have been added:
  • kvs_cursor_seekr seeks within a range
  • kvs_cursor_firstr finds the first or last key in a range
  • kvs_cursor_nextr finds the next or previous key in a range
• DBIs are not supported, There is only a single keyspace. (Use ranges for partitioning.)
• DUPSORT mode is not supported. Each key can only have one value. (Suffix your keys and use ranges.)
• Many of the more specialized options are unsupported.
• KVS_NOOVERWRITE is a large performance hit for write-optimized back-ends, so try to avoid it.
• Transactions have a shared cursor which can be used to avoid frequently creating and destroying cursors. Note that "shared" means "not re-entrant."
• A low level schema layer is included. It's optional and subject to change.
• Concurrent access between several processes is supported by some back-ends (LMDB) and not others (LevelDB).
• Puts with NULL data (rather than just empty data) are explicitly allowed.
• kvs_cmd lets you implement your own high level commands for efficient logical replication.

Known Issues

• libkvstore is still under heavy development and the API is still in flux.
• The test suite is nowhere near thorough.
• The HyperLevelDB and LSMDB back-ends are currently broken.
• Custom comparators are currently unsupported in any of the included back-ends due to limitations with LMDB (which is even used in the LevelDB back-end).
• Disk formats are not explicitly detected. If your application supports multiple back-ends, you may need to track which one is used manually.

License: MIT