“IgnoredDuringExecution” means that the pod will still run if labels on a node change and affinity rules are no longer met. There are future plans to offer requiredDuringSchedulingRequiredDuringExecution which will evict pods from nodes as soon as they don’t satisfy the node affinity rule(s).  

Preferred rules mean that if nodes match the rules, they will be chosen first, and only if no preferred nodes are available will non-preferred nodes be chosen. You can prefer instead of require that pods are deployed to us-central1-a by slightly changing the pod spec to use preferredDuringSchedulingIgnoredDuringExecution:  


```
  affinity:
    nodeAffinity:
      preferredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
          - matchExpressions:
            - key: "failure-domain.beta.kubernetes.io/zone"
              operator: In
              values: ["us-central1-a"]
 ```


Node anti-affinity can be achieved by using negative operators. So for instance if we want our pods to avoid us-central1-a we can do this:  



```
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
          - matchExpressions:
            - key: "failure-domain.beta.kubernetes.io/zone"
              operator: NotIn
              values: ["us-central1-a"]
 ```


Valid operators you can use are In, NotIn, Exists, DoesNotExist. Gt, and Lt.  

Additional use cases for this feature are to restrict scheduling based on nodes’ hardware architecture, operating system version, or specialized hardware. Node affinity/anti-affinity is _beta_ in Kubernetes 1.6.   

**Taints and Tolerations**  

A related feature is “[taints and tolerations](/docs/user-guide/node-selection/#taints-and-toleations-beta-feature),” which allows you to mark (“taint”) a node so that no pods can schedule onto it unless a pod explicitly “tolerates” the taint. Marking nodes instead of pods (as in node affinity/anti-affinity) is particularly useful for situations where most pods in the cluster should avoid scheduling onto the node. For example, you might want to mark your master node as schedulable only by Kubernetes system components, or dedicate a set of nodes to a particular group of users, or keep regular pods away from nodes that have special hardware so as to leave room for pods that need the special hardware.  

The kubectl command allows you to set taints on nodes, for example:  



```
kubectl taint nodes node1 key=value:NoSchedule
```


creates a taint that marks the node as unschedulable by any pods that do not have a toleration for taint with key key, value value, and effect NoSchedule. (The other taint effects are PreferNoSchedule, which is the preferred version of NoSchedule, and NoExecute, which means any pods that are running on the node when the taint is applied will be evicted unless they tolerate the taint.) The toleration you would add to a PodSpec to have the corresponding pod tolerate this taint would look like this  



```
  tolerations:
  - key: "key"
    operator: "Equal"
    value: "value"
    effect: "NoSchedule"
 ```



In addition to moving taints and tolerations to _beta_ in Kubernetes 1.6, we have introduced an _alpha_ feature that uses taints and tolerations to allow you to customize how long a pod stays bound to a node when the node experiences a problem like a network partition instead of using the default five minutes. See [this section](/docs/user-guide/node-selection/#per-pod-configurable-eviction-behavior-when-there-are-node-problems-alpha-feature) of the documentation for more details.



**Pod Affinity/Anti-Affinity**



Node affinity/anti-affinity allows you to constrain which nodes a pod can run on based on the nodes’ labels. But what if you want to specify rules about how pods should be placed relative to one another, for example to spread or pack pods within a service or relative to pods in other services? For that you can use [pod affinity/anti-affinity](/docs/user-guide/node-selection/#inter-pod-affinity-and-anti-affinity-beta-feature), which is also _beta_ in Kubernetes 1.6.