Why do we need ZooKeeper
The first question I had to ask myself was, why do we need ZooKeeper? After all I could store/publish the host/port etc. to a Redis server, a database or just a plain centrally located web service (which for all I care could store the information to a simple file).
ZooKeeper has been designed specifically for handling configuration and name registry functionality in a distributed and clustered environment, hence comes with more advanced features to ensure consistency of data along with capabilities to handle cluster management of the servers.
ZooKeeper in Docker
I’m using ZooKeeper within docker.
To run a ZooKeeper instance within a Docker container, simply use
docker run --name zookeeper1 --restart always -d zookeeper
My first instance is named zookeeper1. This command will run a server instance of ZooKeeper.
We may also need to attach to the service with a client, we can run the following command
docker run -it --rm --link zookeeper1:zookeeper zookeeper zkCli.sh -server zookeeper
Ensure the name of the Docker instance matches the name you assigned to the server.
Client commands
- create We can create a path, /root/sub format, to our data, for example create /services “hello”. Note: it seems that to create child nodes we cannot just type create /services/hello-service “hello”, we need to first create the root node then the child node.
- ls We can list nodes by typing ls / or using root and/child nodes, for example ls /services/hello-service. If nodes exist below the listed node the output will be [node-name], so for example ls /services will result in [hello-service]. When no child nodes exists we’ll get [].
- get We can get any data stored at the end of a node, so for example get /services/hello-service will display the data stored in the node along with data such as the date/time the data was stored, it’s size etc.
- rmr We can recursively remove nodes (i.e. remove a node and all it’s children) using rmr. For example rmr /services.
- delete We can delete an end node using delete but this will not work on a node which has children (i.e. it works only on empty nodes).
- connect We can connect the client to a running instance of a ZooKeeper server using connect, i.e. connect 172.17.0.2:2181
- quit Exists the client.
There are other client commands, but these are probably the main one’s from the client run help to see a full list of commands.
Some command have a [watch] options, which can be enabled by either supplying 1 or true as the last argument to the relevant commands.
Useful References