Kubernetes: How to connect one pod to another on an arbitrary port - with or without services?

Question

We are currently transitioning our apps to Kubernetes and I have two apps, appP and appH, that I need to communicate with each other over a port unknown at start up time.

Unlike most of our apps, we don't have a set port for them will to communicate over. Before Kubernetes, third party app (out of my control) would tell appP to start processing an item, itemA, identified with a unique id and it would also tell appH to handle the processed data produced by appP.

To coordinate communications between appP and appH, appH would generate a port based on the unique id and publish the host and port info to connect on to an intermediate app (IA). appP, once done with it's processing queries IA for the connection information based on the unique id and sends it over.

Now we have to adapt this to kubernetes. Each app runs in its own deployment, as does the IA. So how can I setup appH to accept the connection over a port without being able to specify it in the service definition?

Note: I've seen some posts say that pods should be able to communicate to any other pods in the cluster regardless of specifying the ports in the service definition but I can't seem to find a ton of confirming information on this and I don't have a ton of time on our cluster where it is free to bang my head against.

Would it would just fine as is regardless? My biggest worry is the ip resolution. Currently appH grabs its ip based on the host it's running on (using boost). Not sure how this resolves within a container.

If not, my next thought would be if I could setup a headless service with selector for appH in order to allow for ip resolution. What I am unsure of then is if I could have appP connect to <appH_Service>:<arbitrary_port>?

Would the service even have to be headless in this scenario? I mostly say headless w/ selector because I saw in one specific post that it is the only one you don't need a port in the spec for it. Also because I am unsure if the connection would go through unless it was the actual pod's ip it was connecting with, rather than the services.

Any info or clarification is appreciated. For the most part, I can't really change the architecture of these apps right now, I just have to get them talking to each other as is and haven't found a ton of clear information on this type of case.

Note: We use helm and coredns if anyone is curious.

Answer 1

The Kubernetes networking model is as follows: a Pod is a group of containers that share a single network identity (a cluster IP). Any port exposed by a container is thus automatically exposed on the Pod. The model demands that each Pods can communicate with other Pods.

This means that your current design can work without modifications.

What Services bring to the table is that you can bring a stable network identity to a group of Pods that is otherwise very volatile. It does not apply to your appP/appH coupling, I think.