Kubernetes is a powerful tool. It can help you manage complex applications and provide you with the ability to easily automate tasks involved in managing them. In this course, we will start exploring Kubernetes from a beginner's standpoint. We will discuss what Kubernetes is and what it does, and we will work with some of the basic functionality of Kubernetes hands-on. We will build a simple Kubernetes cluster. We will also deploy a sample microservice application to the cluster in order to explore how Kubernetes can help easily manage such applications.

A Practical Guide to Kubernetes

# Discovering Services

Services can be discovered through two principal modes:
* Environment variables
* DNS

Every Pod gets environment variables for each of the active Services. They are provided in the same format as what Docker links expect, as well with the simpler Kubernetes-specific syntax.

Let's take a look at the environment variables available in one of the Pods we're running.

POD_NAME=$(kubectl get pod \
    --no-headers \
    -o=custom-columns=NAME:.metadata.name \
    -l type=api,service=go-demo-2 \
    | tail -1)

kubectl exec $POD_NAME env

The **output**, limited to the environment variables related to the `go-demo-2-db` service, is as follows.

GO_DEMO_2_DB_PORT=tcp://10.0.0.250:27017
GO_DEMO_2_DB_PORT_27017_TCP_ADDR=10.0.0.250
GO_DEMO_2_DB_PORT_27017_TCP_PROTO=tcp
GO_DEMO_2_DB_PORT_27017_TCP_PORT=27017
GO_DEMO_2_DB_PORT_27017_TCP=tcp://10.0.0.250:27017
GO_DEMO_2_DB_SERVICE_HOST=10.0.0.250
GO_DEMO_2_DB_SERVICE_PORT=27017

The first five variables are using the Docker format. If you already worked with Docker networking, you should be familiar with them. At least, if you're familiar with the way Swarm (standalone) and Docker Compose operate. Later version of Swarm (Mode) still generate the environment variables but they are mostly abandoned by the users in favour of DNSes.

The last two environment variables are Kubernetes specific and follow the `[SERVICE_NAME]_SERVICE_HOST` and `[SERVICE_NAME]_SERIVCE_PORT` format (service name is upper-cased).

No matter which set of environment variables you choose to use (if any), they all serve the same purpose. They provide a reference we can use to connect to a Service and, therefore to the related Pods.

Things will become more evident when we describe the `go-demo-2-db` Service.

In this lesson, we will go through the discovery process of Services.