Package org.apache.zookeeper.server

ZooKeeper server theory of operation


Interface Summary
ConnectionMXBean This MBean represents a client connection.
DataTreeMXBean Zookeeper data tree MBean.
RequestProcessor RequestProcessors are chained together to process transactions.
SessionTracker This is the basic interface that ZooKeeperServer uses to track sessions.
ZooKeeperServerMXBean ZooKeeper server MBean.

Class Summary
ConnectionBean Implementation of connection MBean interface.
DatadirCleanupManager This class manages the cleanup of snapshots and corresponding transaction logs by scheduling the auto purge task with the specified 'autopurge.purgeInterval'.
DataNode This class contains the data for a node in the data tree.
DataTree This class maintains the tree data structure.
DataTreeBean This class implements the data tree MBean.
FinalRequestProcessor This Request processor actually applies any transaction associated with a request and services any queries.
NIOServerCnxn This class handles communication with clients using NIO.
ObserverBean ObserverBean
PrepRequestProcessor This request processor is generally at the start of a RequestProcessor change.
PurgeTxnLog this class is used to clean up the snapshot and data log dir's.
Request This is the structure that represents a request moving through a chain of RequestProcessors.
ServerCnxn Interface to a Server connection - represents a connection from a client to the server.
ServerConfig Server configuration storage.
ServerStats Basic Server Statistics
SessionTrackerImpl This is a full featured SessionTracker.
SyncRequestProcessor This RequestProcessor logs requests to disk.
ZKDatabase This class maintains the in memory database of zookeeper server states that includes the sessions, datatree and the committed logs.
ZooKeeperServer This class implements a simple standalone ZooKeeperServer.
ZooKeeperServerBean This class implements the ZooKeeper server MBean interface.
ZooKeeperServerMain This class starts and runs a standalone ZooKeeperServer.
ZooTrace This class encapsulates and centralizes tracing for the ZooKeeper server.

Enum Summary
DatadirCleanupManager.PurgeTaskStatus Status of the dataDir purge task

Exception Summary

Package org.apache.zookeeper.server Description

ZooKeeper server theory of operation

ZooKeeperServer is designed to work in standalone mode and also be extensible so that it can be used to implement the quorum based version of ZooKeeper.

ZooKeeper maintains a order when processing requests:

We will explain the three aspects of ZooKeeperServer: request processing, data structure maintenance, and session tracking.

Request processing

Requests are received by the ServerCnxn. Demarshalling of a request is done by ClientRequestHandler. After a request has been demarshalled, ClientRequestHandler invokes the relevant method in ZooKeeper and marshals the result.

If the request is just a query, it will be processed by ZooKeeper and returned. Otherwise, the request will be validated and a transaction will be generated and logged. This the request will then wait until the request has been logged before continuing processing.

Requests are logged as a group. Transactions are queued up and the SyncThread will process them at predefined intervals. (Currently 20ms) The SyncThread interacts with ZooKeeperServer the txnQueue. Transactions are added to the txnQueue of SyncThread via queueItem. When the transaction has been synced to disk, its callback will be invoked which will cause the request processing to be completed.

Data structure maintenance

ZooKeeper data is stored in-memory. Each znode is stored in a DataNode object. This object is accessed through a hash table that maps paths to DataNodes. DataNodes also organize themselves into a tree. This tree is only used for serializing nodes.

We guarantee that changes to nodes are stored to non-volatile media before responding to a client. We do this quickly by writing changes as a sequence of transactions in a log file. Even though we flush transactions as a group, we need to avoid seeks as much as possible. Also, since the server can fail at any point, we need to be careful of partial records.

We address the above problems by

As the server runs, the log file will grow quite large. To avoid long startup times we periodically take a snapshot of the tree of DataNodes. We cannot take the snapshot synchronously as the data takes a while to write out, so instead we asynchronously write out the tree. This means that we end up with a "corrupt" snapshot of the data tree. More formally if we define T to be the real snapshot of the tree at the time we begin taking the snapshot and l as the sequence of transactions that are applied to the tree between the time the snapshot begins and the time the snapshot completes, we write to disk T+l' where l' is a subset of the transactions in l. While we do not have a way of figuring out which transactions make up l', it doesn't really matter. T+l'+l = T+l since the transactions we log are idempotent (applying the transaction multiple times has the same result as applying the transaction once). So when we restore the snapshot we also play all transactions in the log that occur after the snapshot was begun. We can easily figure out where to start the replay because we start a new logfile when we start a snapshot. Both the snapshot file and log file have a numeric suffix that represent the transaction id that created the respective files.

Session tracking

Rather than tracking sessions exactly, we track them in batches. That are processed at fixed intervals. This is easier to implement than exact session tracking and it is more efficient in terms of performance. It also provides a small grace period for session renewal.

Copyright © 2011 The Apache Software Foundation