Hadoop Journalnodes metadata

In an HA deployment, edits are logged to a separate set of daemons called JournalNodes. A JournalNode’s metadata directory is configured by setting dfs.journalnode.edits.dir. The JournalNode contains a VERSION file, multiple edits__ files and an edits_inprogress_, just like the NameNode. The JournalNode does not have fsimage files or seen_txid. In addition, it contains several other files relevant to the HA implementation. These files help prevent a split-brain scenario, in which multiple NameNodes could think they are active and all try to write edits.

committed-txid

Tracks last transaction ID committed by a NameNode.

last-promised-epoch

Contains the “epoch,” which is a monotonically increasing number. When a new NameNode, starts as active, it increments the epoch and presents it in calls to the JournalNode. This scheme is the NameNode’s way of claiming that it is active and requests from another NameNode, presenting a lower epoch, must be ignored.

last-writer-epoch

Contains the epoch number associated with the writer who last actually wrote a transaction.

paxos

Specifies the directory that temporary files used in the implementation of the Paxos distributed consensus protocol. This directory often appears as empty.