zookeeperEstimated reading time: 9 minutes
Apache ZooKeeper is an open-source server which enables highly reliable distributed coordination.
GitHub repo: https://github.com/31z4/zookeeper-docker
Supported tags and respective
Where to file issues:
the Docker Community
What is Apache Zookeeper?
Apache ZooKeeper is a software project of the Apache Software Foundation, providing an open source distributed configuration service, synchronization service, and naming registry for large distributed systems. ZooKeeper was a sub-project of Hadoop but is now a top-level project in its own right.
How to use this image
Start a Zookeeper server instance
$ docker run --name some-zookeeper --restart always -d zookeeper
This image includes
EXPOSE 2181 2888 3888 8080 (the zookeeper client port, follower port, election port, AdminServer port respectively), so standard container linking will make it automatically available to the linked containers. Since the Zookeeper “fails fast” it’s better to always restart it.
Connect to Zookeeper from an application in another Docker container
$ docker run --name some-app --link some-zookeeper:zookeeper -d application-that-uses-zookeeper
Connect to Zookeeper from the Zookeeper command line client
$ docker run -it --rm --link some-zookeeper:zookeeper zookeeper zkCli.sh -server zookeeper
docker stack deploy or
version: '3.1' services: zoo1: image: zookeeper restart: always hostname: zoo1 ports: - 2181:2181 environment: ZOO_MY_ID: 1 ZOO_SERVERS: server.1=0.0.0.0:2888:3888;2181 server.2=zoo2:2888:3888;2181 server.3=zoo3:2888:3888;2181 zoo2: image: zookeeper restart: always hostname: zoo2 ports: - 2182:2181 environment: ZOO_MY_ID: 2 ZOO_SERVERS: server.1=zoo1:2888:3888;2181 server.2=0.0.0.0:2888:3888;2181 server.3=zoo3:2888:3888;2181 zoo3: image: zookeeper restart: always hostname: zoo3 ports: - 2183:2181 environment: ZOO_MY_ID: 3 ZOO_SERVERS: server.1=zoo1:2888:3888;2181 server.2=zoo2:2888:3888;2181 server.3=0.0.0.0:2888:3888;2181
This will start Zookeeper 3.5 in replicated mode. Please note, that Zookeeper 3.4 has slightly different
ZOO_SERVERS format. Run
docker stack deploy -c stack.yml zookeeper (or
docker-compose -f stack.yml up) and wait for it to initialize completely. Ports
2181-2183 will be exposed.
Please be aware that setting up multiple servers on a single machine will not create any redundancy. If something were to happen which caused the machine to die, all of the zookeeper servers would be offline. Full redundancy requires that each server have its own machine. It must be a completely separate physical server. Multiple virtual machines on the same physical host are still vulnerable to the complete failure of that host.
Consider using Docker Swarm when running Zookeeper in replicated mode.
Zookeeper configuration is located in
/conf. One way to change it is mounting your config file as a volume:
$ docker run --name some-zookeeper --restart always -d -v $(pwd)/zoo.cfg:/conf/zoo.cfg zookeeper
ZooKeeper recommended defaults are used if
zoo.cfg file is not provided. They can be overridden using the following environment variables.
$ docker run -e "ZOO_INIT_LIMIT=10" --name some-zookeeper --restart always -d 31z4/zookeeper
The length of a single tick, which is the basic time unit used by ZooKeeper, as measured in milliseconds. It is used to regulate heartbeats, and timeouts. For example, the minimum session timeout will be two ticks
Amount of time, in ticks (see tickTime), to allow followers to connect and sync to a leader. Increased this value as needed, if the amount of data managed by ZooKeeper is large.
Amount of time, in ticks (see tickTime), to allow followers to sync with ZooKeeper. If followers fall too far behind a leader, they will be dropped.
Limits the number of concurrent connections (at the socket level) that a single client, identified by IP address, may make to a single member of the ZooKeeper ensemble.
Prior to 3.5.0, one could run ZooKeeper in Standalone mode or in a Distributed mode. These are separate implementation stacks, and switching between them during run time is not possible. By default (for backward compatibility) standaloneEnabled is set to true. The consequence of using this default is that if started with a single server the ensemble will not be allowed to grow, and if started with more than one server it will not be allowed to shrink to contain fewer than two participants.
New in 3.5.0: The AdminServer is an embedded Jetty server that provides an HTTP interface to the four letter word commands. By default, the server is started on port 8080, and commands are issued by going to the URL “/commands/[command name]”, e.g., http://localhost:8080/commands/stat.
The time interval in hours for which the purge task has to be triggered. Set to a positive integer (1 and above) to enable the auto purging. Defaults to 0.
When enabled, ZooKeeper auto purge feature retains the autopurge.snapRetainCount most recent snapshots and the corresponding transaction logs in the dataDir and dataLogDir respectively and deletes the rest. Defaults to 3. Minimum value is 3.
A list of comma separated Four Letter Words commands that user wants to use. A valid Four Letter Words command must be put in this list else ZooKeeper server will not enable the command. By default the whitelist only contains “srvr” command which zkServer.sh uses. The rest of four letter word commands are disabled by default.
Environment variables below are mandatory if you want to run Zookeeper in replicated mode.
The id must be unique within the ensemble and should have a value between 1 and 255. Do note that this variable will not have any effect if you start the container with a
/data directory that already contains the
This variable allows you to specify a list of machines of the Zookeeper ensemble. Each entry has the form of
server.id=host:port:port. Entries are separated with space. Do note that this variable will not have any effect if you start the container with a
/conf directory that already contains the
In 3.5, the syntax of this has changed. Servers should be specified as such:
server.id=<address1>:<port1>:<port2>[:role];[<client port address>:]<client port> Zookeeper Dynamic Reconfiguration
Where to store data
This image is configured with volumes at
/datalog to hold the Zookeeper in-memory database snapshots and the transaction log of updates to the database, respectively.
Be careful where you put the transaction log. A dedicated transaction log device is key to consistent good performance. Putting the log on a busy device will adversely affect performance.
How to configure logging
By default, ZooKeeper redirects stdout/stderr outputs to the console. You can redirect to a file located in
/logs by passing environment variable
ZOO_LOG4J_PROP as follows:
$ docker run --name some-zookeeper --restart always -e ZOO_LOG4J_PROP="INFO,ROLLINGFILE" zookeeper
This will write logs to
/logs/zookeeper.log. Check ZooKeeper Logging for more details.
This image is configured with a volume at
/logs for your convenience.
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As with all Docker images, these likely also contain other software which may be under other licenses (such as Bash, etc from the base distribution, along with any direct or indirect dependencies of the primary software being contained).
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As for any pre-built image usage, it is the image user’s responsibility to ensure that any use of this image complies with any relevant licenses for all software contained within.library, sample, zookeeper