How to select a specific pod for a service in Kubernetes
The feature you're looking for is called StatefulSets, which just launched to beta with Kubernetes 1.5 (note that it was previously available in alpha under a different name, PetSets).
In a StatefulSet, each replica has a unique name that is persisted across restarts. In your example, these would be instance-1, instance-2, instance-3. Since the instance names are persisted (even if the pod is recreated on another node), you don't need a service-per-instance.
The documentation has more details:
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Hotstepper13
Updated on June 04, 2022Comments
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Hotstepper13 over 1 year
I have a kubernetes cluster of 3 hosts where each Host has a unique id label. On this cluster, there is a software that has 3 instances (replicas).
Each replica requires to talk to all other replicas. In addition, there is a service that contains all pods so that this application is permanently available.
So I have:
Instance1 (with labels run: theTool,instanceid: 1) Instance2 (with labels run: theTool,instanceid: 2) Instance3 (with labels run: theTool,instanceid: 3)
and
Service1 (selecting pods with label instanceid=1) Service2 (selecting pods with label instanceid=2) Service3 (selecting pods with label instanceid=3) Service (selecting pods with label run=theTool)
This approach works but have I cannot scale or use the rolling-update feature.
I would like to define a deployment with 3 replicas, where each replicate gets a unique generic label (for instance the replica-id like 1/3, 2/3 and so on).
Within the services, I could use the selector to fetch this label which will exist even after an update.
Another solution might be to select the pod/deployment, depending on the host where it is running on. I could use a DaemonSet or just a pod/deployment with affinity to ensure that each host has an exact one replica of my deployment.
But I didn't know how to select a pod based on a host label where it runs on.
Using the hostname is not an option as hostnames will change in different environments.
I have searched the docs but didn't find anything matching this use case. Hopefully, someone here has an idea how to solve this.
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Hotstepper13 almost 7 yearsI successfully created a StatefulSet but after that i discovered that it is currently not possible to resolve the ip for a pod by name.
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Hotstepper13 almost 7 yearsThe problem with this approach is that I need to know the IPs or resolveable dns names of the other pods while starting in order for them to sync. They need to be aware of each other.
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Hotstepper13 almost 7 yearsOk, found the issue. The service MUST be headless. Then it is possible to resolve the pody by using <petname>.<service>.<namespace>.svc.<cluster_domain>