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Quorum-Based Journaling For Hadoop NameNode High Availability

Given two NameNodes, which both think they are active, each with their own unique epoch numbers, how can we avoid Split Brain Syndrome? The answer is suprisingly simple and elegant: when a NameNode sends any message (or remote procedure call) to a JournalNode, it includes its epoch number as part of the request. Whenever the JournalNode receives such a message, it compares the epoch number against a locally stored value called the promised epoch. If the request is coming from a newer epoch, then it records that new epoch as its promised epoch. If instead the request is coming from an older epoch, then it rejects the request.

This sounds a lot like versioned writes as in MVCC or implementations of optimistic locking. To see the complete details of the implementation check Todd Lipcon’s Qurom-Journal Design paper (PDF).

Original title and link: Quorum-Based Journaling For Hadoop NameNode High Availability (NoSQL database©myNoSQL)

via: http://blog.cloudera.com/blog/2012/10/quorum-based-journaling-in-cdh4-1/