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Cassandra 1.2 gears up for 'fat servers' with update

Cassandra 1.2 comes with a new approach to managing data on nodes

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The Apache Software Foundation's Cassandra NoSQL distributed database has been updated, in response to changes in corporate hardware buying habits, to better use larger servers through the introduction of virtual nodes and configurable policies for disk failure.

Cassandra 1.2 also features the ability to perform atomic batch operations, and comes with a new version of the Cassandra Query Language, CQL3.

Traditionally, "Cassandra's sweet spot has been in scaling out across a lot of relatively lightweight machines," said Jonathan Ellis, Apache project chair of Cassandra and a co-founder of DataStax, which offers commercial support for the software.

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Recently however, more organisations have been buying "denser" servers with more memory and hard drive space because such servers now offer the best price-performance value, Ellis said. So much of the work to update Cassandra has been around better supporting such machines.

First developed internally at Facebook and released to the public in 2008, Cassandra was designed to store massive data sets across multiple servers. Adobe, Cisco, Disney, eBay, IBM, Netflix, Reddit, Spotify, Twitter and Williams-Sonoma all use the technology.

vnodes

The new version of the software features the ability to create virtual nodes (vnodes), designed to streamline the recovery process should an individual server in a Cassandra cluster fail. Vnodes should also improve performance in general.

Vnodes was one of the chief features of Amazon's Dynamo distributed data store, which the developers of Cassandra used as the model for Cassandra. The developers initially opted for a simpler architecture of assigning one node to each server.

The new virtual node technology should simplify the process of managing clusters, particularly when adding and rebuilding individual nodes. With vnodes, each server can hold multiple nodes.

Because individual vnodes don't have to occupy the entire space of a server, multiple nodes can be created on each server, which then can be re-created more quickly from replicas on other servers should the drive fail, compared to the time it would take to copy the replicated data from a single server.

Smaller nodes spread across a greater number of servers also better balances the workload among all the machines in a cluster.

"Each virtual node is managed by one Java process per machine, so we're not adding a lot of operating system processes. We're just virtualising out that storage responsibility," Ellis said.

Atomic batching

Another new feature, Atomic batching, should help organisations that require transactional integrity across business processes, such as an online merchant that needs to make sure orders are captured even when a component such as a hard drive fails right in the middle of a transaction. Previously, developers would have to build processes, such as retry mechanisms, into their code to guarantee transactional integrity.

The CQL language, used to query and fetch data from Cassandra, has also been significantly updated. The updated version of the language, CQL3, offers a number of new calls that simplify the way applications work with data in Cassandra. Now, system information can be queried. Also the language recognises collections as a query type. If, for instance, a single entry in a data store contains multiple email addresses, the language provides the syntax to retrieve all these email addresses.

The version of CQL will help "people wrap their minds around how to model applications of Cassandra. This meets a lot of common challenges," Ellis said.

Other new features included in Apache Cassandra version 1.2 include request tracing - a debugging tool -- and configurable policies for disk failure. When a disk fails, an administrator can choose to stop operations entirely or continue operations on a best-effort basis. Various performance improvements and bug fixes have also been made.

Cassandra is available as open source under the Apache License version 2. Created in 1999, the Apache Software Foundation oversees about 150 volunteer-driven open source projects.



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