From where appeared these variables in my kubernetes container?

Q: What created this variables?

   A: discovery-service (more at the end)

Q: Could I somehow isolate container from them?

   A: If you want to disable that, you can set enableServiceLinks: false on your PodSpec

Q: Or are they set intentionally by kubernetes, and serve some purpose I don't know about?

   A: No, they are there just there to give options besides DNS and names, kubernetes does not use them

Q: How should I name my configuration variables to avoid naming collisions?

   A: Either you use enableServiceLinks: false or use a naming pattern that does not conflict with the pattern described on the docs, usually I prefer use _SVC_PORT when I need to do something like you

Q: Should I use that variables istead of using services names as hostnames?

   A: From docs: "You can (and almost always should) set up a DNS service for your Kubernetes cluster",

Q: There is following documentation, but it only explains variable TEST_SERVICE_SERVICE_PORT and TEST_SERVICE_SERVICE_HOST. What TEST_SERVICE_PORT and others mean then? What adds TEST_SERVICE_SERVICE_PORT_GRPC_API?

   A: You have a named port called grpc-api, in that case it is using the named instead protocol + port number. Note: I could not find any references on the docs for that, so I had dig into code

From docs discovery-service

When a Pod is run on a Node, the kubelet adds a set of environment variables for each active Service. ... simpler {SVCNAME}_SERVICE_HOST and {SVCNAME}_SERVICE_PORT variables, where the Service name is upper-cased and dashes are converted to underscores...

For example, the Service "redis-master" which exposes TCP port 6379 and has been allocated cluster IP address 10.0.0.11, produces the following environment variables:

REDIS_MASTER_SERVICE_PORT=6379 REDIS_MASTER_PORT=tcp://10.0.0.11:6379REDIS_MASTER_PORT_6379_TCP=tcp://10.0.0.11:6379REDIS_MASTER_PORT_6379_TCP_PROTO=tcpREDIS_MASTER_PORT_6379_TCP_PORT=6379REDIS_MASTER_PORT_6379_TCP_ADDR=10.0.0.11 

From k8s api PodSpec/EnableServiceLinks:

EnableServiceLinks indicates whether information about services should be injected into pod's environment variables, matching the syntax of Docker links. Optional: Defaults to true.

These environment variables help with service discovery. You can disable them by setting

$ kubectl explain deployment.spec.template.spec.enableServiceLinksKIND:     DeploymentVERSION:  extensions/v1beta1FIELD:    enableServiceLinks <boolean>DESCRIPTION:     EnableServiceLinks indicates whether information about services should be     injected into pod's environment variables, matching the syntax of Docker     links. Optional: Defaults to true.