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Learn how to effectively manage your Docker Swarm cluster by ensuring that each Zookeeper instance runs on separate nodes using `placement constraints`.
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How to Limit Container Running on Different Node using Docker Stack Deploy

If you're managing a multi-node setup in Docker Swarm, you might encounter the need to run specific services on designated nodes. This challenge is particularly prevalent when deploying cluster services like Zookeeper. In this post, we'll explore how to ensure that each Zookeeper instance runs on a different node in your Docker Swarm setup.

The Problem

You have a three-node Docker Swarm configuration, all set as manager nodes, and you want to deploy a Zookeeper cluster using Docker stack. Your goal is to ensure that each Zookeeper instance is allocated to different nodes. However, you've noticed that occasionally one node ends up starting multiple Zookeeper containers. This leads to concerns about reliability and optimal resource usage.

The Solution

To achieve the desired setup where each Zookeeper service runs on a distinct node, you need to make use of Docker's placement constraints. Placement constraints allow you to specify which nodes a particular service can run on, thereby avoiding the situation where a single node ends up hosting multiple instances of the same service.

Steps to Implement Placement Constraints

Modify Your Docker Compose File: You will need to add a deploy section to each Zookeeper service declaration in your Docker Compose file. Here’s how you can modify the docker-compose.yml for each Zookeeper instance:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Configuration

Placement Constraints: The deploy section with placement constraints tells Docker Swarm to schedule each Zookeeper instance on a specific node identified by its hostname. This ensures that each service is deployed on a different node, thus limiting multiple container instances from being scheduled onto a single node.

Port Configuration: The ports for each Zookeeper service remain the same, as they do not change based on which node they run on. This ensures that each instance of Zookeeper can communicate with one another as expected.

Environment Variables: For Zookeeper to function correctly, the ZOO_SERVERS configuration must reference the other nodes correctly. Ensure you are using the right server IDs and hostnames.

Conclusion

By utilizing Docker Swarm's placement constraints, you can effectively manage your service deployments to ensure that each Zookeeper instance operates on separate nodes. This approach enhances your cluster's reliability and performance.

Now, you're ready to deploy your Zookeeper cluster with confidence! Happy Dockering!