As with node affinity/anti-affinity, there is also a preferredDuringSchedulingIgnoredDuringExecution variant.  

Pod affinity/anti-affinity is very flexible. Imagine you have profiled the performance of your services and found that containers from service S1 interfere with containers from service S2 when they share the same node, perhaps due to cache interference effects or saturating the network link. Or maybe due to security concerns you never want containers of S1 and S2 to share a node. To implement these rules, just make two changes to the snippet above -- change podAffinity to podAntiAffinity and change topologyKey to kubernetes.io/hostname.  

**Custom Schedulers**  

If the Kubernetes scheduler’s various features don’t give you enough control over the scheduling of your workloads, you can delegate responsibility for scheduling arbitrary subsets of pods to your own custom scheduler(s) that run(s) alongside, or instead of, the default Kubernetes scheduler. [Multiple schedulers](/docs/admin/multiple-schedulers/) is _beta_ in Kubernetes 1.6.  

Each new pod is normally scheduled by the default scheduler. But if you provide the name of your own custom scheduler, the default scheduler will ignore that Pod and allow your scheduler to schedule the Pod to a node. Let’s look at an example.  

Here we have a Pod where we specify the schedulerName field:  


```
apiVersion: v1
kind: Pod
metadata:
  name: nginx
  labels:
    app: nginx
spec:
  schedulerName: my-scheduler
  containers:
  - name: nginx
    image: nginx:1.10
 ```



If we create this Pod without deploying a custom scheduler, the default scheduler will ignore it and it will remain in a Pending state. So we need a custom scheduler that looks for, and schedules, pods whose schedulerName field is my-scheduler.



A custom scheduler can be written in any language and can be as simple or complex as you need. Here is a very simple example of a custom scheduler written in Bash that assigns a node randomly. Note that you need to run this along with kubectl proxy for it to work.



```
#!/bin/bash

SERVER='localhost:8001'

while true;

do

    for PODNAME in $(kubectl --server $SERVER get pods -o json | jq '.items[] | select(.spec.schedulerName == "my-scheduler") | select(.spec.nodeName == null) | .metadata.name' | tr -d '"')

;

    do

        NODES=($(kubectl --server $SERVER get nodes -o json | jq '.items[].metadata.name' | tr -d '"'))


        NUMNODES=${#NODES[@]}

        CHOSEN=${NODES[$[$RANDOM % $NUMNODES]]}

        curl --header "Content-Type:application/json" --request POST --data '{"apiVersion":"v1", "kind": "Binding", "metadata": {"name": "'$PODNAME'"}, "target": {"apiVersion": "v1", "kind"

: "Node", "name": "'$CHOSEN'"}}' http://$SERVER/api/v1/namespaces/default/pods/$PODNAME/binding/

        echo "Assigned $PODNAME to $CHOSEN"

    done

    sleep 1

done
```  



**Learn more**



The Kubernetes 1.6 [release notes](https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG.md#v160) have more information about these features, including details about how to change your configurations if you are already using the alpha version of one or more of these features (this is required, as the move from alpha to beta is a breaking change for these features).



**Acknowledgements**



The features described here, both in their alpha and beta forms, were a true community effort, involving engineers from Google, Huawei, IBM, Red Hat and more.



**Get Involved**



Share your voice at our weekly [community meeting](https://github.com/kubernetes/community/blob/master/communication.md#weekly-meeting):

- Post questions (or answer questions) on [Stack Overflow](http://stackoverflow.com/questions/tagged/kubernetes)
- Follow us on Twitter [@Kubernetesio](https://twitter.com/kubernetesio) for latest updates
- Connect with the community on [Slack](http://slack.k8s.io/) (room #sig-scheduling)


Many thanks for your contributions.