K8S Inter-pod Affinity / Anti-affinity

While K8S Node AffinityK8S Node Affinity
Node Affinity is a feature which ensures that K8S PodK8S Pod
Pod is a [[K8S Object]] used to run one or more tightly coupled containers and is rarely used directly, especially in production because of the limitations on the number of properties that can be u... end up running on specific K8S NodeK8S Node
In [[Kubernetes]], a Node is a (physical or virtual) machine in charge of running some workloads. Kubernetes is run entirely on nodes, and everything that happens on Kubernetes cluster, happens on .... We are already able to do this with K8S Node SelectorsK8S Node Selectors
Node Selectors are used to place K8S PodK8S Pod
Pod is a [[K8S Object]] used to run one or more tightly coupled containers and is rarely used directly, especially in production because of the limitations on the number of properties that can be u...s on specific K8S NodeK8S Node
In [[Kubernetes]], a Node is a (physical or virtual) machine in charge of running some workloads. Kubernetes is run entirely on nodes, and everything that happens on Kubernetes cluster, happens on ... by label. For more complex placement options, see [[K8S Node Affinity]].

It's possible to make a pod go to a specific node ba..., but with big limitations.

See also [[... works like an extended K8S Node SelectorsK8S Node Selectors
Node Selectors are used to place K8S PodK8S Pod
Pod is a [[K8S Object]] used to run one or more tightly coupled containers and is rarely used directly, especially in production because of the limitations on the number of properties that can be u...s on specific K8S NodeK8S Node
In [[Kubernetes]], a Node is a (physical or virtual) machine in charge of running some workloads. Kubernetes is run entirely on nodes, and everything that happens on Kubernetes cluster, happens on ... by label. For more complex placement options, see [[K8S Node Affinity]].

It's possible to make a pod go to a specific node ba..., Inter-pod affinity and Anti-affinity focus on labels of the pods rather than labels of the K8S NodeK8S Node
In [[Kubernetes]], a Node is a (physical or virtual) machine in charge of running some workloads. Kubernetes is run entirely on nodes, and everything that happens on Kubernetes cluster, happens on ....

This allows us to dictate which pods can or can't be deployed together on the same node.

Example inter-pod affinity rule might sound like: This pod should run on this part of infrastructure, if this part of infrastructure is already running some pods that meet the specified criteria.

Similarily, anti-affinity rule could be read in the lines of: This pod should not run on this part of infrastructure, if this part of infrastructure is already running one or more pods that meet the specified criteria.

Topology key

When I say This part of infrastructure, it could mean any node or group of nodes, based on a specific label. The rule could set limits:

• on a single node
• on one of nodes in a single cloud provider region / availability zone
• any other label available on your nodes

To determine the part of the infrastructure which will be taken into consideration for these rules, we use topologyKey property. Topology key represents a key of any label that is present on all of your nodes (if some nodes are missing this label it could cause issues).

For example, setting topologyKey to kubernetes.io/hostname will turn the this part of infrastructure into this node in the example above, as each node will have a distinct label for the hostname.

Setting topologyKey to something like topology.kubernetes.io/zone will turn the this part of infrastructure into any node in this zone in the example above, as each node will have a distinct label for the hostname.

Practical example

The full example below demonstrates both the anti affinity (podAntiAffinity key) and the inter-pod affinity (podAffinity key).

First rule says that our K8S Deployment should run on any given node (topologyKey: "kubernetes.io/hostname") only if there are no other K8S PodK8S Pod
Pod is a [[K8S Object]] used to run one or more tightly coupled containers and is rarely used directly, especially in production because of the limitations on the number of properties that can be u...s with label app: web-store already running on it. This stops our deployment's pods to be co-located on a single node (in a way similar to K8S DaemonSet, just with specific number of replicas).

Second rule says that our deployment's pods should run only on nodes where pods with label app: store are already running.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: web-server
spec:
  selector:
    matchLabels:
      app: web-store
  replicas: 3
  template:
    metadata:
      labels:
        app: web-store
    spec:
      affinity:
        podAntiAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
          - labelSelector:
              matchExpressions:
              - key: app
                operator: In
                values:
                - web-store
            topologyKey: "kubernetes.io/hostname"
        podAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
          - labelSelector:
              matchExpressions:
              - key: app
                operator: In
                values:
                - store
            topologyKey: "kubernetes.io/hostname"
      containers:
      - name: web-app
        image: nginx:1.16-alpine

Status: #🌲

References:

• K8S Docs