lmdb-erl/README.md

EMDB

EMDB is a NIF library for the Memory-Mapped Database database, aka. MDB.

The main purpose of this package is to provide a very fast Riak backend.

But this module could also be used as a general key-value store to replace:

• DETS
• TokyoCabinet: TCERL
• QDBM
• Bitcask
• eLevelDB
• BerkleyDB
• ...

Requirements

• Erlang R14B04+
• GCC 4.2+ or MS VisualStudio 2010+

Build

$ make

API

The following functions were implemented:

• open/1: equivalent to emdb:open(DirName, 10485760).
• open/2: equivalent to emdb:open(DirName, 10485760, 0).
• open/3: creates a new MDB database. This call also re-open an already existing one. Arguments are:
  • DirName: database directory name
  • MapSize: database map size (see map.hrl)
  • EnvFlags: database environment flags (see map.hrl). The possible values are defined in emdb.hrl.
• close/2: closes the database
• put/2: inserts Key with value Val into the database. Assumes that the key is not present, 'key_exit' is returned otherwise.
• get/1: retrieves the value stored with Key in the database.
• del/1: Removes the key-value with key Key from database.
• update/2: inserts Key with value Val into the database if the key is not present, otherwise updates Key to value Val.
• drop/1: deletes all key-value pairs in the database.

Usage

$ make

$ ./start.sh

%% create a new database
1> {ok, Handle} = emdb:open("/tmp/emdb1").

%% insert the key <<"a">> with value <<"1">>
2> ok = Handle:put(<<"a">>, <<"1">>).

%% try to re-insert the same key <<"a">>
3> key_exist = Handle:put(<<"a">>, <<"2">>).

%% add a new key-value pair
4> ok = Handle:put(<<"b">>, <<"2">>).

%% search a non-existing key <<"c">>
5> none = Handle:get(<<"c">>).

%% retrieve the value for key <<"b">>
6> {ok, <<"2">>} = Handle:get(<<"b">>).

%% retrieve the value for key <<"a">>
7> {ok, <<"1">>} = Handle:get(<<"a">>).

%% delete key <<"b">>
8> ok = Handle:del(<<"b">>).

%% search a non-existing key <<"b">>
9> none = Handle:get(<<"b">>).

%% delete a non-existing key <<"z">>
10> none = Handle:del(<<"z">>).

%% ensure key <<"a">>'s value is still <<"1">>
11> {ok, <<"1">>} = Handle:get(<<"a">>).
%% update the value for key <<"a">>
12> ok = Handle:update(<<"a">>, <<"7">>).

%% check the new value for key <<"a">>
13> {ok, <<"7">>} = Handle:get(<<"a">>).

%% delete all key-value pairs in the database
14> ok = Handle:drop().

%% try to retrieve key <<"a">> value
15> none = Handle:get(<<"a">>).

%% close the database
16> ok = Handle:close().

...

17> q().  

####Note: The code below creates a new database with 80GB MapSize, avoid fsync after each commit (for max speed) and use the experimental MDB_FIXEDMAP.

{ok, Handle} = emdb:open("/tmp/emdb2", 85899345920, ?MDB_NOSYNC bor ?MDB_FIXEDMAP).

Performance

For maximum speed, this library use only binaries for both keys and values.

See the impressive microbench against:

• Google's LevelDB
• SQLite
• Kyoto TreeDB
• BerkeleyDB

MDB performs better on 64-bit arch.

Supported OSes

Should work on 32/64-bit architectures:

• Linux
• OSX
• FreeBSD
• Windows

TODO

• Unit tests
• PropEr testing
• Bulk "writing"

Volunteers are always welcome!

Status

Work in progress. Don't use it in production!

LICENSE

EMDB is Copyright (C) 2012 by Aleph Archives, and released under the OpenLDAP License.