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.
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.
Istio provides two mechanisms to represent virtual machine workloads:
WorkloadGrouprepresents a logical group of virtual machine workloads that share common properties. This is similar to a
WorkloadEntryrepresents a single instance of a virtual machine workload. This is similar to a
Creating these resources (
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
Once a virtual machine has been configured and added to the mesh, a corresponding
WorkloadEntry will be automatically created by the Istio control plane.
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
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
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
product is composed of a
Pod and a
With this configuration, requests to
product would be load-balanced across both the pod and virtual machine workload instances.
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.