Package org.apache.accumulo.server.log

Class WalStateManager

  • public class WalStateManager
extends Object
    This class governs the space in Zookeeper that advertises the status of Write-Ahead Logs in use by tablet servers and the replication machinery.

    The Master needs to know the state of the WALs to mark tablets during recovery. The GC needs to know when a log is no longer needed so it can be removed. The replication mechanism needs to know when a log is closed and can be forwarded to the destination table.

    The state of the WALs is kept in Zookeeper under /accumulo/<instanceid>/wals. For each server, there is a znode formatted like the TServerInstance.toString(): "host:port[sessionid]". Under the server znode, is a node for each log, using the UUID for the log. In each of the WAL znodes, is the current state of the log, and the full path to the log.

    The state [OPEN, CLOSED, UNREFERENCED] is what the tablet server believes to be the state of the file.

    In the event of a recovery, the log is identified as belonging to a dead server. The master will update the tablets assigned to that server with log references. Once all tablets have been reassigned and the log references are removed, the log will be eligible for deletion.

    Even when a log is UNREFERENCED by the tablet server, the replication mechanism may still need the log. The GC will defer log removal until replication is finished with it.