Virtual Machine Architecture

Before reading this document, be sure to review Istio’s architecture and deployment models. This page builds on those documents to explain how Istio can be extended to support joining virtual machines into the mesh.

Istio’s virtual machine support allows connecting workloads outside of a Kubernetes cluster to the mesh. This enables legacy applications, or applications not suitable to run in a containerized environment, to get all the benefits that Istio provides to applications running inside Kubernetes.

For workloads running on Kubernetes, the Kubernetes platform itself provides various features like service discovery, DNS resolution, and health checks which are often missing in virtual machine environments. Istio enables these features for workloads running on virtual machines, and in addition allows these workloads to utilize Istio functionality such as mutual TLS (mTLS), rich telemetry, and advanced traffic management capabilities.

The following diagram shows the architecture of a mesh with virtual machines:

In this mesh, there is a single network, where pods and virtual machines can communicate directly with each other.

Control plane traffic, including XDS configuration and certificate signing, are sent through a Gateway in the cluster. This ensures that the virtual machines have a stable address to connect to when they are bootstrapping. Pods and virtual machines can communicate directly with each other without requiring any intermediate Gateway.

A service mesh with a single network and virtual machines

A service mesh with a single network and virtual machines

In this mesh, there are multiple networks, where pods and virtual machines are not able to communicate directly with each other.

Control plane traffic, including XDS configuration and certificate signing, are sent through a Gateway in the cluster. Similarly, all communication between pods and virtual machines goes through the gateway, which acts as a bridge between the two networks.

A service mesh with multiple networks and virtual machines

A service mesh with multiple networks and virtual machines

Service association

Istio provides two mechanisms to represent virtual machine workloads:

  • WorkloadGroup represents a logical group of virtual machine workloads that share common properties. This is similar to a Deployment in Kubernetes.
  • WorkloadEntry represents a single instance of a virtual machine workload. This is similar to a Pod in Kubernetes.

Creating these resources (WorkloadGroup and WorkloadEntry) does not result in provisioning of any resources or running any virtual machine workloads. Rather, these resources just reference these workloads and inform Istio how to configure the mesh appropriately.

When adding a virtual machine workload to the mesh, you will need to create a WorkloadGroup that acts as template for each WorkloadEntry instance:

  1. apiVersion: networking.istio.io/v1alpha3
  2. kind: WorkloadGroup
  3. metadata:
  4.   name: product-vm
  5. spec:
  6.   metadata:
  7.     labels:
  8.       app: product
  9.   template:
  10.     serviceAccount: default
  11.   probe:
  12.     httpGet:
  13.       port: 8080

Once a virtual machine has been configured and added to the mesh, a corresponding WorkloadEntry will be automatically created by the Istio control plane. For example:

  1. apiVersion: networking.istio.io/v1beta1
  2. kind: WorkloadEntry
  3. metadata:
  4.   annotations:
  5.     istio.io/autoRegistrationGroup: product-vm
  6.   labels:
  7.     app: product
  8.   name: product-vm-1.2.3.4
  9. spec:
  10.   address: 1.2.3.4
  11.   labels:
  12.     app: product
  13.   serviceAccount: default

This WorkloadEntry resource describes a single instance of a workload, similar to a pod in Kubernetes. When the workload is removed from the mesh, the WorkloadEntry resource will be automatically removed. Additionally, if any probes are configured in the WorkloadGroup resource, the Istio control plane automatically updates the health status of associated WorkloadEntry instances.

In order for consumers to reliably call your workload, it’s recommended to declare a Service association. This allows clients to reach a stable hostname, like product.default.svc.cluster.local, rather than an ephemeral IP addresses. This also enables you to use advanced routing capabilities in Istio via the DestinationRule and VirtualService APIs.

Any Kubernetes service can transparently select workloads across both pods and virtual machines via the selector fields which are matched with pod and WorkloadEntry labels respectively.

For example, a Service named product is composed of a Pod and a WorkloadEntry:

Service Selection

With this configuration, requests to product would be load-balanced across both the pod and virtual machine workload instances.

DNS

Kubernetes provides DNS resolution in pods for Service names allowing pods to easily communicate with one another by stable hostnames.

For virtual machine expansion, Istio provides similar functionality via a DNS Proxy. This feature redirects all DNS queries from the virtual machine workload to the Istio proxy, which maintains a mapping of hostnames to IP addresses.

As a result, workloads running on virtual machines can transparently call Services (similar to pods) without requiring any additional configuration.