1. The ZooKeeper Data Model

1.1 ZNodes

Znodes maintain a stat structure:

The Stat structure for each znode in ZooKeeper is made up of the following fields:

  • czxid

    The zxid of the change that caused this znode to be created.

  • mzxid

    The zxid of the change that last modified this znode.

  • ctime

    The time in milliseconds from epoch when this znode was created.

  • mtime

    The time in milliseconds from epoch when this znode was last modified.

  • version

    The number of changes to the data of this znode.

  • cversion

    The number of changes to the children of this znode.

  • aversion

    The number of changes to the ACL of this znode.

  • ephemeralOwner

    The session id of the owner of this znode if the znode is an ephemeral node. If it is not an ephemeral node, it will be zero.

  • dataLength

    The length of the data field of this znode.

  • numChildren

    The number of children of this znode.

须知:

ZooKeeper was not designed to be a general database or large object store. Instead, it manages coordination data. This data can come in the form of configuration, status information, rendezvous, etc. A common property of the various forms of coordination data is that they are relatively small: measured in kilobytes. The ZooKeeper client and the server implementations have sanity checks to ensure that znodes have less than 1M of data, but the data should be much less than that on average. Operating on relatively large data sizes will cause some operations to take much more time than others and will affect the latencies of some operations because of the extra time needed to move more data over the network and onto storage media. If large data storage is needed, the usually pattern of dealing with such data is to store it on a bulk storage system, such as NFS or HDFS, and store pointers to the storage locations in ZooKeeper.

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