In May Google open-sourced a BigTable-inspired key-value database library called LevelDB under a BSD license. It was created by Jeff Dean and Sanjay Ghemawat of the BigTable project at Google. It’s available for Unix based systems, Mac OS X, Windows, and Android.
Although it first appeared in Google Code months ago, a blog post from Google earlier this week made the project more widely known.
LevelDB is not a database server like other other key-value stores like Redis or Membase. Instead, it would most likely be used as an embedded database for other applications, much the way SQLite or Berkley DB are used. The technical advantage to using LevelDB instead of other key-value stores is its support for ordered data. Also, its BSD license is more liberal than the GPLSleepycat license of Berkley DB.
According to the announcement:
For example, LevelDB may be used by a web browser to store a cache of recently accessed web pages, or by an operating system to store the list of installed packages and package dependencies, or by an application to store user preference settings. We designed LevelDB to also be useful as a building block for higher-level storage systems. Upcoming versions of the Chrome browser include an implementation of the IndexedDB HTML5 API that is built on top of LevelDB. Google’s Bigtable manages millions of tablets where the contents of a particular tablet are represented by a precursor to LevelDB.
LevelDB isn’t limited to just being used as an embedded database, however. Basho is already exploring the possibility of using LevelDB with Riak as an alternative to Bitcask or InnoDB. The company conducted some benchmarks, which you can find in this blog post.
Google also released its own set of benchmarks here.
According to the project site the key features are:
- Keys and values are arbitrary byte arrays.
- Data is stored sorted by key.
- Callers can provide a custom comparison function to override the sort order.
- The basic operations are Put(key,value), Get(key), Delete(key).
- Multiple changes can be made in one atomic batch.
- Users can create a transient snapshot to get a consistent view of data.
- Forward and backward iteration is supported over the data.
- Data is automatically compressed using the Snappy compression library.
- External activity (file system operations etc.) is relayed through a virtual interface so users can customize the operating system interactions.
- Detailed documentation about how to use the library is included with the source code.
And the limitations are:
- This is not a SQL database. It does not have a relational data model, it does not support SQL queries, and it has no support for indexes.
- Only a single process (possibly multi-threaded) can access a particular database at a time.
- There is no client-server support builtin to the library. An application that needs such support will have to wrap their own server around the library.
