As part of my Master studies I got the task to present the ZooKeeper service. And what makes a presentation more interesting and demonstrates the functionality? A small running demonstration!
That it's why I decided to implement simple lock mechanism which is described on the ZooKeeper website
Attention: This code is written mainly to demonstrate the API of ZooKeeper and to show the mechanism of a simple lock service. The code has been kept simple and should not be used for any other purpose.
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A running groovy installation (can be easily installed with sdkman)
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A running ZooKeeper service (e.g. within docker)
docker run --name some-zookeeper --restart always -p 2181:2181 -d zookeeper:3.5 -
Optional: The ZooKeeper CLI to see what's going on
docker run -it --rm --link some-zookeeper:zookeeper zookeeper:3.5 zkCli.sh -server zookeeper
Execute the grovvy script
groovy lock-example.groovy
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Three clients (Client A, B and C) are trying to lock the same Resource. Because the use the Sequential flag ZooKeeper is appending a counter to the created nodes. The client with the lowest node (Client A) can acquire the lock, all the others have to wait and are therefore watching their predecessor node.
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After client A releases the lock (deletes the node), client B is notified and is holding now the lock.
Observation: Only the next client in the row gets notified. All other clients are not waking up and are therefore not sending requests to ZooKeeper - the herd effect is avoided!