Apache > ZooKeeper

ZooKeeper Administrator's Guide

A Guide to Deployment and Administration


This section contains information about deploying Zookeeper and covers these topics:

The first two sections assume you are interested in installing ZooKeeper in a production environment such as a datacenter. The final section covers situations in which you are setting up ZooKeeper on a limited basis - for evaluation, testing, or development - but not in a production environment.

System Requirements

Supported Platforms

ZooKeeper consists of multiple components. Some components are supported broadly, and other components are supported only on a smaller set of platforms.

The following matrix describes the level of support committed for running each component on different operating system platforms.

Support Matrix
Operating System Client Server Native Client Contrib
GNU/Linux Development and Production Development and Production Development and Production Development and Production
Solaris Development and Production Development and Production Not Supported Not Supported
FreeBSD Development and Production Development and Production Not Supported Not Supported
Windows Development and Production Development and Production Not Supported Not Supported
Mac OS X Development Only Development Only Not Supported Not Supported

For any operating system not explicitly mentioned as supported in the matrix, components may or may not work. The ZooKeeper community will fix obvious bugs that are reported for other platforms, but there is no full support.

Required Software

ZooKeeper runs in Java, release 1.7 or greater (JDK 7 or greater, FreeBSD support requires openjdk7). It runs as an ensemble of ZooKeeper servers. Three ZooKeeper servers is the minimum recommended size for an ensemble, and we also recommend that they run on separate machines. At Yahoo!, ZooKeeper is usually deployed on dedicated RHEL boxes, with dual-core processors, 2GB of RAM, and 80GB IDE hard drives.

Clustered (Multi-Server) Setup

For reliable ZooKeeper service, you should deploy ZooKeeper in a cluster known as an ensemble. As long as a majority of the ensemble are up, the service will be available. Because Zookeeper requires a majority, it is best to use an odd number of machines. For example, with four machines ZooKeeper can only handle the failure of a single machine; if two machines fail, the remaining two machines do not constitute a majority. However, with five machines ZooKeeper can handle the failure of two machines.


As mentioned in the ZooKeeper Getting Started Guide , a minimum of three servers are required for a fault tolerant clustered setup, and it is strongly recommended that you have an odd number of servers.

Usually three servers is more than enough for a production install, but for maximum reliability during maintenance, you may wish to install five servers. With three servers, if you perform maintenance on one of them, you are vulnerable to a failure on one of the other two servers during that maintenance. If you have five of them running, you can take one down for maintenance, and know that you're still OK if one of the other four suddenly fails.

Your redundancy considerations should include all aspects of your environment. If you have three ZooKeeper servers, but their network cables are all plugged into the same network switch, then the failure of that switch will take down your entire ensemble.

Here are the steps to setting a server that will be part of an ensemble. These steps should be performed on every host in the ensemble:

  1. Install the Java JDK. You can use the native packaging system for your system, or download the JDK from: http://java.sun.com/javase/downloads/index.jsp

  2. Set the Java heap size. This is very important to avoid swapping, which will seriously degrade ZooKeeper performance. To determine the correct value, use load tests, and make sure you are well below the usage limit that would cause you to swap. Be conservative - use a maximum heap size of 3GB for a 4GB machine.

  3. Install the ZooKeeper Server Package. It can be downloaded from: http://zookeeper.apache.org/releases.html

  4. Create a configuration file. This file can be called anything. Use the following settings as a starting point:


    You can find the meanings of these and other configuration settings in the section Configuration Parameters. A word though about a few here: Every machine that is part of the ZooKeeper ensemble should know about every other machine in the ensemble. You accomplish this with the series of lines of the form server.id=host:port:port. The parameters host and port are straightforward. You attribute the server id to each machine by creating a file named myid, one for each server, which resides in that server's data directory, as specified by the configuration file parameter dataDir.

  5. The myid file consists of a single line containing only the text of that machine's id. So myid of server 1 would contain the text "1" and nothing else. The id must be unique within the ensemble and should have a value between 1 and 255. IMPORTANT: if you enable extended features such as TTL Nodes (see below) the id must be between 1 and 254 due to internal limitations.

  6. If your configuration file is set up, you can start a ZooKeeper server:

    $ java -cp zookeeper.jar:lib/slf4j-api-1.7.5.jar:lib/slf4j-log4j12-1.7.5.jar:lib/log4j-1.2.17.jar:conf \\
    org.apache.zookeeper.server.quorum.QuorumPeerMain zoo.cfg

QuorumPeerMain starts a ZooKeeper server, JMX management beans are also registered which allows management through a JMX management console. The ZooKeeper JMX document contains details on managing ZooKeeper with JMX. See the script bin/zkServer.sh, which is included in the release, for an example of starting server instances.

  1. Test your deployment by connecting to the hosts: In Java, you can run the following command to execute simple operations:

    $ bin/zkCli.sh -server

Single Server and Developer Setup

If you want to setup ZooKeeper for development purposes, you will probably want to setup a single server instance of ZooKeeper, and then install either the Java or C client-side libraries and bindings on your development machine.

The steps to setting up a single server instance are the similar to the above, except the configuration file is simpler. You can find the complete instructions in the Installing and Running ZooKeeper in Single Server Mode section of the ZooKeeper Getting Started Guide.

For information on installing the client side libraries, refer to the Bindings section of the ZooKeeper Programmer's Guide.


This section contains information about running and maintaining ZooKeeper and covers these topics:

Designing a ZooKeeper Deployment

The reliability of ZooKeeper rests on two basic assumptions.

  1. Only a minority of servers in a deployment will fail. Failure in this context means a machine crash, or some error in the network that partitions a server off from the majority.
  2. Deployed machines operate correctly. To operate correctly means to execute code correctly, to have clocks that work properly, and to have storage and network components that perform consistently.

The sections below contain considerations for ZooKeeper administrators to maximize the probability for these assumptions to hold true. Some of these are cross-machines considerations, and others are things you should consider for each and every machine in your deployment.

Cross Machine Requirements

For the ZooKeeper service to be active, there must be a majority of non-failing machines that can communicate with each other. To create a deployment that can tolerate the failure of F machines, you should count on deploying 2xF+1 machines. Thus, a deployment that consists of three machines can handle one failure, and a deployment of five machines can handle two failures. Note that a deployment of six machines can only handle two failures since three machines is not a majority. For this reason, ZooKeeper deployments are usually made up of an odd number of machines.

To achieve the highest probability of tolerating a failure you should try to make machine failures independent. For example, if most of the machines share the same switch, failure of that switch could cause a correlated failure and bring down the service. The same holds true of shared power circuits, cooling systems, etc.

Single Machine Requirements

If ZooKeeper has to contend with other applications for access to resources like storage media, CPU, network, or memory, its performance will suffer markedly. ZooKeeper has strong durability guarantees, which means it uses storage media to log changes before the operation responsible for the change is allowed to complete. You should be aware of this dependency then, and take great care if you want to ensure that ZooKeeper operations aren’t held up by your media. Here are some things you can do to minimize that sort of degradation:


Things to Consider: ZooKeeper Strengths and Limitations



Little long term maintenance is required for a ZooKeeper cluster however you must be aware of the following:

Ongoing Data Directory Cleanup

The ZooKeeper Data Directory contains files which are a persistent copy of the znodes stored by a particular serving ensemble. These are the snapshot and transactional log files. As changes are made to the znodes these changes are appended to a transaction log. Occasionally, when a log grows large, a snapshot of the current state of all znodes will be written to the filesystem and a new transaction log file is created for future transactions. During snapshotting, ZooKeeper may continue appending incoming transactions to the old log file. Therefore, some transactions which are newer than a snapshot may be found in the last transaction log preceding the snapshot.

A ZooKeeper server will not remove old snapshots and log files when using the default configuration (see autopurge below), this is the responsibility of the operator. Every serving environment is different and therefore the requirements of managing these files may differ from install to install (backup for example).

The PurgeTxnLog utility implements a simple retention policy that administrators can use. The API docs contains details on calling conventions (arguments, etc...).

In the following example the last count snapshots and their corresponding logs are retained and the others are deleted. The value of should typically be greater than 3 (although not required, this provides 3 backups in the unlikely event a recent log has become corrupted). This can be run as a cron job on the ZooKeeper server machines to clean up the logs daily.

java -cp zookeeper.jar:lib/slf4j-api-1.7.5.jar:lib/slf4j-log4j12-1.7.5.jar:lib/log4j-1.2.17.jar:conf org.apache.zookeeper.server.PurgeTxnLog <dataDir> <snapDir> -n <count>

Automatic purging of the snapshots and corresponding transaction logs was introduced in version 3.4.0 and can be enabled via the following configuration parameters autopurge.snapRetainCount and autopurge.purgeInterval. For more on this, see Advanced Configuration below.

Debug Log Cleanup (log4j)

See the section on logging in this document. It is expected that you will setup a rolling file appender using the in-built log4j feature. The sample configuration file in the release tar's conf/log4j.properties provides an example of this.


You will want to have a supervisory process that manages each of your ZooKeeper server processes (JVM). The ZK server is designed to be "fail fast" meaning that it will shutdown (process exit) if an error occurs that it cannot recover from. As a ZooKeeper serving cluster is highly reliable, this means that while the server may go down the cluster as a whole is still active and serving requests. Additionally, as the cluster is "self healing" the failed server once restarted will automatically rejoin the ensemble w/o any manual interaction.

Having a supervisory process such as daemontools or SMF (other options for supervisory process are also available, it's up to you which one you would like to use, these are just two examples) managing your ZooKeeper server ensures that if the process does exit abnormally it will automatically be restarted and will quickly rejoin the cluster.


The ZooKeeper service can be monitored in one of two primary ways; 1) the command port through the use of 4 letter words and 2) JMX. See the appropriate section for your environment/requirements.


ZooKeeper uses log4j version 1.2 as its logging infrastructure. The ZooKeeper default log4j.properties file resides in the conf directory. Log4j requires that log4j.properties either be in the working directory (the directory from which ZooKeeper is run) or be accessible from the classpath.

For more information, see Log4j Default Initialization Procedure of the log4j manual.


Configuration Parameters

ZooKeeper's behavior is governed by the ZooKeeper configuration file. This file is designed so that the exact same file can be used by all the servers that make up a ZooKeeper server assuming the disk layouts are the same. If servers use different configuration files, care must be taken to ensure that the list of servers in all of the different configuration files match.

Minimum Configuration

Here are the minimum configuration keywords that must be defined in the configuration file:

Advanced Configuration

The configuration settings in the section are optional. You can use them to further fine tune the behaviour of your ZooKeeper servers. Some can also be set using Java system properties, generally of the form zookeeper.keyword. The exact system property, when available, is noted below.

Cluster Options

The options in this section are designed for use with an ensemble of servers -- that is, when deploying clusters of servers.

If you really need enable all four letter word commands by default, you can use the asterisk option so you don't have to include every command one by one in the list. As an example, this will enable all four letter word commands:


Authentication and Authorization Options

The options in this section allow control over encryption/authentication/authorization performed by the service.

$ perl -e "print 'stmk', pack('q>', 0b0011111010)" | nc localhost 2181

Experimental Options/Features

New features that are currently considered experimental.

Unsafe Options

The following options can be useful, but be careful when you use them. The risk of each is explained along with the explanation of what the variable does.

Communication using the Netty framework

Netty is an NIO based client/server communication framework, it simplifies (over NIO being used directly) many of the complexities of network level communication for java applications. Additionally the Netty framework has built in support for encryption (SSL) and authentication (certificates). These are optional features and can be turned on or off individually.

In versions 3.5+, a ZooKeeper server can use Netty instead of NIO (default option) by setting the environment variable zookeeper.serverCnxnFactory to org.apache.zookeeper.server.NettyServerCnxnFactory; for the client, set zookeeper.clientCnxnSocket to org.apache.zookeeper.ClientCnxnSocketNetty.

TBD - tuning options for netty - currently there are none that are netty specific but we should add some. Esp around max bound on the number of reader worker threads netty creates.

TBD - how to manage encryption

TBD - how to manage certificates

ZooKeeper Commands: The Four Letter Words

ZooKeeper responds to a small set of commands. Each command is composed of four letters. You issue the commands to ZooKeeper via telnet or nc, at the client port.

Three of the more interesting commands: "stat" gives some general information about the server and connected clients, while "srvr" and "cons" give extended details on server and connections respectively.

$ echo mntr | nc localhost 2185
              zk_version  3.4.0
              zk_avg_latency  0
              zk_max_latency  0
              zk_min_latency  0
              zk_packets_received 70
              zk_packets_sent 69
              zk_outstanding_requests 0
              zk_server_state leader
              zk_znode_count   4
              zk_watch_count  0
              zk_ephemerals_count 0
              zk_approximate_data_size    27
              zk_followers    4                   - only exposed by the Leader
              zk_synced_followers 4               - only exposed by the Leader
              zk_pending_syncs    0               - only exposed by the Leader
              zk_open_file_descriptor_count 23    - only available on Unix platforms
              zk_max_file_descriptor_count 1024   - only available on Unix platforms
              zk_fsync_threshold_exceed_count 0

The output is compatible with java properties format and the content may change over time (new keys added). Your scripts should expect changes. ATTENTION: Some of the keys are platform specific and some of the keys are only exported by the Leader. The output contains multiple lines with the following format:

key \t value

Here's an example of the ruok command:

$ echo ruok | nc 5111

Data File Management

ZooKeeper stores its data in a data directory and its transaction log in a transaction log directory. By default these two directories are the same. The server can (and should) be configured to store the transaction log files in a separate directory than the data files. Throughput increases and latency decreases when transaction logs reside on a dedicated log devices.

The Data Directory

This directory has two or three files in it:

Each ZooKeeper server has a unique id. This id is used in two places: the myid file and the configuration file. The myid file identifies the server that corresponds to the given data directory. The configuration file lists the contact information for each server identified by its server id. When a ZooKeeper server instance starts, it reads its id from the myid file and then, using that id, reads from the configuration file, looking up the port on which it should listen.

The snapshot files stored in the data directory are fuzzy snapshots in the sense that during the time the ZooKeeper server is taking the snapshot, updates are occurring to the data tree. The suffix of the snapshot file names is the zxid, the ZooKeeper transaction id, of the last committed transaction at the start of the snapshot. Thus, the snapshot includes a subset of the updates to the data tree that occurred while the snapshot was in process. The snapshot, then, may not correspond to any data tree that actually existed, and for this reason we refer to it as a fuzzy snapshot. Still, ZooKeeper can recover using this snapshot because it takes advantage of the idempotent nature of its updates. By replaying the transaction log against fuzzy snapshots ZooKeeper gets the state of the system at the end of the log.

The Log Directory

The Log Directory contains the ZooKeeper transaction logs. Before any update takes place, ZooKeeper ensures that the transaction that represents the update is written to non-volatile storage. A new log file is started when the number of transactions written to the current log file reaches a (variable) threshold. The threshold is computed using the same parameter which influences the frequency of snapshotting (see snapCount above). The log file's suffix is the first zxid written to that log.

File Management

The format of snapshot and log files does not change between standalone ZooKeeper servers and different configurations of replicated ZooKeeper servers. Therefore, you can pull these files from a running replicated ZooKeeper server to a development machine with a stand-alone ZooKeeper server for trouble shooting.

Using older log and snapshot files, you can look at the previous state of ZooKeeper servers and even restore that state. The LogFormatter class allows an administrator to look at the transactions in a log.

The ZooKeeper server creates snapshot and log files, but never deletes them. The retention policy of the data and log files is implemented outside of the ZooKeeper server. The server itself only needs the latest complete fuzzy snapshot, all log files following it, and the last log file preceding it. The latter requirement is necessary to include updates which happened after this snapshot was started but went into the existing log file at that time. This is possible because snapshotting and rolling over of logs proceed somewhat independently in ZooKeeper. See the maintenance section in this document for more details on setting a retention policy and maintenance of ZooKeeper storage.


The data stored in these files is not encrypted. In the case of storing sensitive data in ZooKeeper, necessary measures need to be taken to prevent unauthorized access. Such measures are external to ZooKeeper (e.g., control access to the files) and depend on the individual settings in which it is being deployed.

Recovery - TxnLogToolkit

TxnLogToolkit is a command line tool shipped with ZooKeeper which is capable of recovering transaction log entries with broken CRC.

Running it without any command line parameters or with the -h,--help argument, it outputs the following help page:

$ bin/zkTxnLogToolkit.sh
usage: TxnLogToolkit [-dhrv] txn_log_file_name
-d,--dump      Dump mode. Dump all entries of the log file. (this is the default)
-h,--help      Print help message
-r,--recover   Recovery mode. Re-calculate CRC for broken entries.
-v,--verbose   Be verbose in recovery mode: print all entries, not just fixed ones.
-y,--yes       Non-interactive mode: repair all CRC errors without asking

The default behaviour is safe: it dumps the entries of the given transaction log file to the screen: (same as using -d,--dump parameter)

$ bin/zkTxnLogToolkit.sh log.100000001
ZooKeeper Transactional Log File with dbid 0 txnlog format version 2
4/5/18 2:15:58 PM CEST session 0x16295bafcc40000 cxid 0x0 zxid 0x100000001 createSession 30000
CRC ERROR - 4/5/18 2:16:05 PM CEST session 0x16295bafcc40000 cxid 0x1 zxid 0x100000002 closeSession null
4/5/18 2:16:05 PM CEST session 0x16295bafcc40000 cxid 0x1 zxid 0x100000002 closeSession null
4/5/18 2:16:12 PM CEST session 0x26295bafcc90000 cxid 0x0 zxid 0x100000003 createSession 30000
4/5/18 2:17:34 PM CEST session 0x26295bafcc90000 cxid 0x0 zxid 0x200000001 closeSession null
4/5/18 2:17:34 PM CEST session 0x16295bd23720000 cxid 0x0 zxid 0x200000002 createSession 30000
4/5/18 2:18:02 PM CEST session 0x16295bd23720000 cxid 0x2 zxid 0x200000003 create '/andor,#626262,v{s{31,s{'world,'anyone}}},F,1
EOF reached after 6 txns.

There's a CRC error in the 2nd entry of the above transaction log file. In dump mode, the toolkit only prints this information to the screen without touching the original file. In recovery mode (-r,--recover flag) the original file still remains untouched and all transactions will be copied over to a new txn log file with ".fixed" suffix. It recalculates CRC values and copies the calculated value, if it doesn't match the original txn entry. By default, the tool works interactively: it asks for confirmation whenever CRC error encountered.

$ bin/zkTxnLogToolkit.sh -r log.100000001
ZooKeeper Transactional Log File with dbid 0 txnlog format version 2
CRC ERROR - 4/5/18 2:16:05 PM CEST session 0x16295bafcc40000 cxid 0x1 zxid 0x100000002 closeSession null
Would you like to fix it (Yes/No/Abort) ?

Answering Yes means the newly calculated CRC value will be outputted to the new file. No means that the original CRC value will be copied over. Abort will abort the entire operation and exits. (In this case the ".fixed" will not be deleted and left in a half-complete state: contains only entries which have already been processed or only the header if the operation was aborted at the first entry.)

$ bin/zkTxnLogToolkit.sh -r log.100000001
ZooKeeper Transactional Log File with dbid 0 txnlog format version 2
CRC ERROR - 4/5/18 2:16:05 PM CEST session 0x16295bafcc40000 cxid 0x1 zxid 0x100000002 closeSession null
Would you like to fix it (Yes/No/Abort) ? y
EOF reached after 6 txns.
Recovery file log.100000001.fixed has been written with 1 fixed CRC error(s)

The default behaviour of recovery is to be silent: only entries with CRC error get printed to the screen. One can turn on verbose mode with the -v,--verbose parameter to see all records. Interactive mode can be turned off with the -y,--yes parameter. In this case all CRC errors will be fixed in the new transaction file.

Things to Avoid

Here are some common problems you can avoid by configuring ZooKeeper correctly:

Best Practices

For best results, take note of the following list of good Zookeeper practices:

For multi-tenant installations see the section detailing ZooKeeper "chroot" support, this can be very useful when deploying many applications/services interfacing to a single ZooKeeper cluster.