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One of the disruptive innovations of recent years, which shifted into focus only in the wake of cloud computing and containers, is undoubtedly the microarchitecture approach to computing (see the "Finally Agile" box). Microarchitecture application developers, however, are faced with the task of operating all of the separate microservices and components and maintaining them as part of their application.

This is where the Istio developers enter the scene. They promise container application developers a central network interface in which the parts of an application talk. The required services (i.e., load balancing, filtering, discovery of existing services, etc.) are provided by Istio. To be able to use all these features, you only have to integrate Istio into your application.

Communication Needed

For the individual parts of a microservice application to work together successfully, communication between components is essential. How can a developer make life easier if they are working on a store application, for example, where a customer navigates through different parts of the program during a purchase and has to be forwarded from one component to the next?

The subject of connecting containers is not trivial per se. Because a container usually runs in its own namespace, it is not so easy to access from the main network. Kubernetes, for example, comes with a proxy server on-board, without which connections from the outside world to applications would simply not be possible. Istio extends an existing Kubernetes setup based on pods by adding APIs and central controls.

What is a Service Mesh?

Istio [1] forms a "service mesh" between the individual components of a microservices application by removing the need for the admin to plan the network connections below the individual components. In the Istio context, service mesh means the totality of all components that are part of a microservice application.

In the previous example from the web store, this could mean all the components involved in the ordering process. The application that provides the website, the one that monitors the queue of incoming orders, the one that generates email for orders, and the one that dispatches the email – there are basically no limits to what you could imagine the mesh doing. What all these components have in common is that they communicate with each other on a network. In the worst case, it involves a huge amount of development overhead that isn't actually necessary.

Clearly developers need to deal with the many aspects of connectivity, starting with the simple question of how the application that dispatches email knows how to reach the application that generates email. In a legacy environment, both functions would be part of the same program – or the dispatcher would simply be connected to the mail generator through a static IP address.

This arrangement is not possible in container applications based on a microservice architecture. After all, the individual parts of the application need to be able to scale seamlessly to reflect the load. When all hell breaks loose on the web store, more email senders and email generators need to be fired up, but they will all have dynamic IP addresses. So how do they discover each other?

Discovering Services

Other solutions to this problem, such as Consul or Etcd, have service directories in which services register to shout, "Here I am!" to other services on the network. If you integrate Consul or Etcd into your application, you have solved the autodiscovery problem, but that's not your only worry.

How is load balancing possible across all instances of a particular microservice? If routing is required, what implements the corresponding rules? Will there be any traffic that you want to stop? If so, how do you do that? All these questions have separate answers: Load balancing is handled by a HAProxy and filtering by nftables; routing rules can be defined manually. However, these services can more easily be provided by Istio.