content.md 3.2 KB
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.
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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 (the zookeeper client port, follower port, election 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
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This will start Zookeeper in replicated mode. Run docker-compose up and wait for it to initialize completely. Run docker-compose ps to figure out exposed ports.
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.
Configuration
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
Environment variables
Variables below are mandatory if you want to run Zookeeper in replicated mode.
ZOO_MY_ID
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 myid file.
ZOO_SERVERS
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 zoo.cfg file.
Where to store data
This image is configured with volumes at /data and /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 effect performance.