Job with Pod-to-Pod Communication [page]deterministic
In this example, you will run a Job in [Indexed completion mode](/blog/2021/04/19/introducing-indexed-jobs/) configured such that the pods created by the Job can communicate with each other using pod hostnames rather than pod IP addresses.
Pods within a Job might need to communicate among themselves. The user workload running in each pod could query the Kubernetes API server to learn the IPs of the other Pods, but it's much simpler to rely on Kubernetes' built-in DNS resolution.
Jobs in Indexed completion mode automatically set the pods' hostname to be in the format of `${jobName}-${completionIndex}`. You can use this format to deterministically build pod hostnames and enable pod communication *without* needing to create a client connection to the Kubernetes control plane to obtain pod hostnames/IPs via API requests.
This configuration is useful for use cases where pod networking is required but you don't want to depend on a network connection with the Kubernetes API server.
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You should already be familiar with the basic use of [Job](/docs/concepts/workloads/controllers/job/).
> Note: If you are using minikube or a similar tool, you may need to take [extra steps](https://minikube.sigs.k8s.io/docs/handbook/addons/ingress-dns/) to ensure you have DNS.
## Starting a Job with pod-to-pod communication
To enable pod-to-pod communication using pod hostnames in a Job, you must do the following:
1. Set up a [headless Service](/docs/concepts/services-networking/service/#headless-services) with a valid label selector for the pods created by your Job. The headless service must be in the same namespace as the Job. One easy way to do this is to use the `job-name: <your-job-name>` selector, since the `job-name` label will be automatically added by Kubernetes. This configuration will trigger the DNS system to create records of the hostnames of the pods running your Job.
1. Configure the headless service as subdomain service for the Job pods by including the following value in your Job template spec:
```yaml subdomain: <headless-svc-name> ```
### Example
Below is a working example of a Job with pod-to-pod communication via pod hostnames enabled. The Job is completed only after all pods successfully ping each other using hostnames.
> Note: In the Bash script executed on each pod in the example below, the pod hostnames can be prefixed by the namespace as well if the pod needs to be reached from outside the namespace.
```yaml apiVersion: v1 kind: Service metadata: name: headless-svc spec: clusterIP: None # clusterIP must be None to create a headless service selector: job-name: example-job # must match Job name --- apiVersion: batch/v1 kind: Job metadata: name: example-job spec: completions: 3 parallelism: 3 completionMode: Indexed template: spec: subdomain: headless-svc # has to match Service name restartPolicy: Never containers: - name: example-workload image: bash:latest command: - bash - -c - | for i in 0 1 2 do gotStatus="-1" wantStatus="0" while [ $gotStatus -ne $wantStatus ] do ping -c 1 example-job-${i}.headless-svc > /dev/null 2>&1 gotStatus=$? if [ $gotStatus -ne $wantStatus ]; then echo "Failed to ping pod example-job-${i}.headless-svc, retrying in 1 second..." sleep 1 fi done echo "Successfully pinged pod: example-job-${i}.headless-svc" done ```
After applying the example above, reach each other over the network using: `<pod-hostname>.<headless-service-name>`. You should see output similar to the following:
```shell kubectl logs example-job-0-qws42 ```
``` Failed to ping pod …(trimmed)