LevelDB

LevelDB is Google's open source on-disk key-value storage library that provides an ordered mapping from string keys to binary values. In many applications where only key based accesses are needed, it tends to be a faster alternative than databases. LevelDB was written in C++ with a C calling API included. This module provides a Julia interface to LevelDB using Julia's ccall mechanism.

Install LevelDB

You can build LevelDB from its source code at https://github.com/google/leveldb. Please install the final dynamic library into a system directory such as /usr/lib or make sure libleveldb.so is in one of your LD_LIBRARY_PATH directories. If libleveldb.so is not installed, Julia will try to download and build it automatically.

Run Testing Code

(v1.1) pkg> test LevelDB

This will exercise batched and non-batched writes and reads for string and float array values.

Create/Open/Close a LevelDB database

julia> db = LevelDB.DB(file_path; create_if_missing = false, error_if_exists = false)

Here file_path is the full path to a directory that hosts a LevelDB database. create_if_missing is a boolean flag when true the database will be created if it does not exist. error_if_exists is a boolean flag when true an error will be thrown if the database already exists. The return value is a database object for passing to read/write calls.

julia> close(db)

Close a database, db is the object returned from a LevelDB.DB call. A directory can only be opened by a single LevelDB.DB at a time.

Read and Write Operations

julia> db[key] = value

key and value are Array{UInt8}.

julia> db[key]

Return value is an Array{UInt8}, one can use the reinterpret function to cast it into the right array type (see test code).

julia> delete!(db, key)

Delete a key from db.

Batched Write

LevelDB supports grouping a number of put operations into a write batch, the batch will either succeed as a whole or fail altogether, behaving like an atomic update.

julia> db[keys] = values

keys and values must behave like iterators returning Array{UInt8}. Creates a write batch internally which is then commited to db.

Iterate

julia> for (key, value) in db
           #do something with the key value pair
       end

Iterate over all key => value pairs in a LevelDB.DB.

julia> for (key, value) in LevelDB.RangeView(db, key1, key2)
           #do something with the key value pair
       end

Iterate over a range between key1 and key2 (inclusive)

Authors

• Jerry Zhenlei Cai ( jpenguin at gmail dot com )
• Guido Kraemer

additional contributions by

• @huwenshuo
• @tmlbl