Challenge, Easy,  on  Kubernetes

Kubernetes Pod Scheduling: Topology Spread Constraints

Kaloyan Preslavski
by  Kaloyan Preslavski

topologySpreadConstraints gives you fine-grained control over how pods distribute across failure domains. Unlike pod anti-affinity which enforces at most one pod per domain, maxSkew sets the maximum allowed difference in pod count between any two domains.

Topology Spread Constraints - Kubernetes Docs

The cluster is pre-configured with zone labels - cplane-01 and node-01 are in zone-a, node-02 is in zone-b. Run kubectl get nodes --show-labels to confirm before starting.

Task 1 - Node-Level Spread

With topologyKey: kubernetes.io/hostname, each node is a separate domain. The scheduler counts pods per node and enforces that no node has more than maxSkew extra pods compared to any other. With 4 replicas across 3 nodes and maxSkew: 1, the result is a 2-1-1 distribution.

Steps:

  • Create a Deployment named web, image nginx:alpine, 4 replicas, label app: web
  • Add topologySpreadConstraints with maxSkew: 1, topologyKey: kubernetes.io/hostname, whenUnsatisfiable: DoNotSchedule, and a labelSelector matching app: web
Hint: Where topologySpreadConstraints goes

Add topologySpreadConstraints under spec.template.spec. Each entry needs maxSkew, topologyKey, whenUnsatisfiable, and labelSelector.matchLabels pointing to the pod's own labels so the scheduler counts existing pods in each domain.

Task 2 - Zone-Level Spread

Changing topologyKey to topology.kubernetes.io/zone shifts the distribution unit from node to zone. The scheduler counts pods per zone instead of per node - within a zone, multiple pods may land on the same node. With 4 replicas across 2 zones and maxSkew: 1, the result is a 2-2 split.

Steps:

  • Create a Deployment named zone-web, image nginx:alpine, 4 replicas, label app: zone-web
  • Add topologySpreadConstraints with maxSkew: 1, topologyKey: topology.kubernetes.io/zone, whenUnsatisfiable: DoNotSchedule, and a labelSelector matching app: zone-web

This challenge is part of the Kubernetes Pod Scheduling skill path.

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Instead of providing a subpar online editing experience, iximiuz Labs offers a helper CLI tool called labctl, allowing you to use your favorite text editor (or a full-featured IDE) to write content from the comfort of your local machine.

Install labctl CLI

curl -sf https://labs.iximiuz.com/cli/install.sh | sh

This will download and install the latest version of the labctl CLI. You only need to do this once per workstation.

Authorize labctl

labctl auth login

This will open a browser window asking you to authorize labctl to access your account. You need to do it after a fresh install of labctl and repeat it whenever the auth session expires.

Pull challenge content

labctl content pull challenge topology-spread-constraints-a806af86

This will create a local copy of the challenge content in a directory named topology-spread-constraints-a806af86. You only need to do this once per challenge.

Stream your changes

labctl content push -fw challenge topology-spread-constraints-a806af86

Run this command in a separate terminal to continuously upload your changes to the server while editing the challenge in your favorite text editor or IDE.

You can also use labctl to create, list, and delete your content. Learn more about the available commands: labctl content --help

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