We usually run interesting services and applications inside our cluster. And as we’ll see throughout the book, Istio allows us to solve some difficult challenges in service-to-service communication. It is this intra-service communication where Istio shines (within a cluster or across clusters).
Before services communicate with each other, something must trigger the interactions. For example, an end user purchasing an item, a client querying our API, and so on. What each of these triggers have in common is that they originate outside of the cluster. This raises the question: how do we get traffic from the outside of the cluster and into it (see figure 4.1)? In this chapter, we will answer the question by opening an entry point for clients that live outside the cluster to connect securely to services running inside the cluster.