What is Istio?

Cloud platforms provide a wealth of benefits for the organizations that use them. There’s no denying, however, that adopting the cloud can put strains on DevOps teams. Developers must use microservices to architect for portability, meanwhile operators are managing extremely large hybrid and multi-cloud deployments. Istio lets you connect, secure, control, and observe services.

At a high level, Istio helps reduce the complexity of these deployments, and eases the strain on your development teams. It is a completely open source service mesh that layers transparently onto existing distributed applications. It is also a platform, including APIs that let it integrate into any logging platform, or telemetry or policy system. Istio’s diverse feature set lets you successfully, and efficiently, run a distributed microservice architecture, and provides a uniform way to secure, connect, and monitor microservices.

What is a service mesh?

Istio addresses the challenges developers and operators face as monolithic applications transition towards a distributed microservice architecture. To see how, it helps to take a more detailed look at Istio’s service mesh.

The term service mesh is used to describe the network of microservices that make up such applications and the interactions between them. As a service mesh grows in size and complexity, it can become harder to understand and manage. Its requirements can include discovery, load balancing, failure recovery, metrics, and monitoring. A service mesh also often has more complex operational requirements, like A/B testing, canary releases, rate limiting, access control, and end-to-end authentication.

Istio provides behavioral insights and operational control over the service mesh as a whole, offering a complete solution to satisfy the diverse requirements of microservice applications.

Why use Istio?

Istio makes it easy to create a network of deployed services with load balancing, service-to-service authentication, monitoring, and more, without any changes in service code. You add Istio support to services by deploying a special sidecar proxy throughout your environment that intercepts all network communication between microservices, then configure and manage Istio using its control plane functionality, which includes:

  • Automatic load balancing for HTTP, gRPC, WebSocket, and TCP traffic.

  • Fine-grained control of traffic behavior with rich routing rules, retries, failovers, and fault injection.

  • A pluggable policy layer and configuration API supporting access controls, rate limits and quotas.

  • Automatic metrics, logs, and traces for all traffic within a cluster, including cluster ingress and egress.

  • Secure service-to-service communication in a cluster with strong identity-based authentication and authorization.

Istio is designed for extensibility and meets diverse deployment needs.

Core features

Istio provides a number of key capabilities uniformly across a network of services:

Traffic management

Istio’s easy rules configuration and traffic routing lets you control the flow of traffic and API calls between services. Istio simplifies configuration of service-level properties like circuit breakers, timeouts, and retries, and makes it a breeze to set up important tasks like A/B testing, canary rollouts, and staged rollouts with percentage-based traffic splits.

With better visibility into your traffic, and out-of-box failure recovery features, you can catch issues before they cause problems, making calls more reliable, and your network more robust – no matter what conditions you face.

Security

Istio’s security capabilities free developers to focus on security at the application level. Istio provides the underlying secure communication channel, and manages authentication, authorization, and encryption of service communication at scale. With Istio, service communications are secured by default, letting you enforce policies consistently across diverse protocols and runtimes – all with little or no application changes.

While Istio is platform independent, using it with Kubernetes (or infrastructure) network policies, the benefits are even greater, including the ability to secure pod-to-pod or service-to-service communication at the network and application layers.

Observability

Istio’s robust tracing, monitoring, and logging give you deep insights into your service mesh deployment. Gain a real understanding of how service performance impacts things upstream and downstream with Istio’s monitoring features, while its custom dashboards provide visibility into the performance of all your services and let you see how that performance is affecting your other processes.

Istio’s Mixer component is responsible for policy controls and telemetry collection. It provides backend abstraction and intermediation, insulating the rest of Istio from the implementation details of individual infrastructure backends, and giving operators fine-grained control over all interactions between the mesh and infrastructure backends.

All these features let you more effectively set, monitor, and enforce SLOs on services. Of course, the bottom line is that you can detect and fix issues quickly and efficiently.

Platform support

Istio is platform-independent and designed to run in a variety of environments, including those spanning Cloud, on-premise, Kubernetes, Mesos, and more. You can deploy Istio on Kubernetes, or on Nomad with Consul. Istio currently supports:

  • Service deployment on Kubernetes

  • Services registered with Consul

  • Services running on individual virtual machines

Integration and customization

The policy enforcement component of Istio can be extended and customized to integrate with existing solutions for ACLs, logging, monitoring, quotas, auditing, and more.

Architecture

An Istio service mesh is logically split into a data plane and a control plane.

  • The data plane is composed of a set of intelligent proxies (Envoy) deployed as sidecars. These proxies mediate and control all network communication between microservices along with Mixer, a general-purpose policy and telemetry hub.

  • The control plane manages and configures the proxies to route traffic. Additionally, the control plane configures Mixers to enforce policies and collect telemetry.

The following diagram shows the different components that make up each plane:

The overall architecture of an Istio-based application.
Istio Architecture

Envoy

Istio uses an extended version of the Envoy proxy. Envoy is a high-performance proxy developed in C++ to mediate all inbound and outbound traffic for all services in the service mesh. Istio leverages Envoy’s many built-in features, for example:

  • Dynamic service discovery
  • Load balancing
  • TLS termination
  • HTTP/2 and gRPC proxies
  • Circuit breakers
  • Health checks
  • Staged rollouts with %-based traffic split
  • Fault injection
  • Rich metrics

Envoy is deployed as a sidecar to the relevant service in the same Kubernetes pod. This deployment allows Istio to extract a wealth of signals about traffic behavior as attributes. Istio can, in turn, use these attributes in Mixer to enforce policy decisions, and send them to monitoring systems to provide information about the behavior of the entire mesh.

The sidecar proxy model also allows you to add Istio capabilities to an existing deployment with no need to rearchitect or rewrite code. You can read more about why we chose this approach in our Design Goals.

Mixer

Mixer is a platform-independent component. Mixer enforces access control and usage policies across the service mesh, and collects telemetry data from the Envoy proxy and other services. The proxy extracts request level attributes, and sends them to Mixer for evaluation. You can find more information on this attribute extraction and policy evaluation in our Mixer Configuration documentation.

Mixer includes a flexible plugin model. This model enables Istio to interface with a variety of host environments and infrastructure backends. Thus, Istio abstracts the Envoy proxy and Istio-managed services from these details.

Pilot

Pilot provides service discovery for the Envoy sidecars, traffic management capabilities for intelligent routing (e.g., A/B tests, canary deployments, etc.), and resiliency (timeouts, retries, circuit breakers, etc.).

Pilot converts high level routing rules that control traffic behavior into Envoy-specific configurations, and propagates them to the sidecars at runtime. Pilot abstracts platform-specific service discovery mechanisms and synthesizes them into a standard format that any sidecar conforming with the Envoy data plane APIs can consume. This loose coupling allows Istio to run on multiple environments such as Kubernetes, Consul, or Nomad, while maintaining the same operator interface for traffic management.

Citadel

Citadel provides strong service-to-service and end-user authentication with built-in identity and credential management. You can use Citadel to upgrade unencrypted traffic in the service mesh. Using Citadel, operators can enforce policies based on service identity rather than on network controls. Starting from release 0.5, you can use Istio’s authorization feature to control who can access your services.

Galley

Galley validates user authored Istio API configuration on behalf of the other Istio control plane components. Over time, Galley will take over responsibility as the top-level configuration ingestion, processing and distribution component of Istio. It will be responsible for insulating the rest of the Istio components from the details of obtaining user configuration from the underlying platform (e.g. Kubernetes).

Design Goals

A few key design goals informed Istio’s architecture. These goals are essential to making the system capable of dealing with services at scale and with high performance.

  • Maximize Transparency: To adopt Istio, an operator or developer is required to do the minimum amount of work possible to get real value from the system. To this end, Istio can automatically inject itself into all the network paths between services. Istio uses sidecar proxies to capture traffic and, where possible, automatically program the networking layer to route traffic through those proxies without any changes to the deployed application code. In Kubernetes, the proxies are injected into pods and traffic is captured by programming iptables rules. Once the sidecar proxies are injected and traffic routing is programmed, Istio can mediate all traffic. This principle also applies to performance. When applying Istio to a deployment, operators see a minimal increase in resource costs for the functionality being provided. Components and APIs must all be designed with performance and scale in mind.

  • Incrementality: As operators and developers become more dependent on the functionality that Istio provides, the system must grow with their needs. While we continue to add new features, the greatest need is the ability to extend the policy system, to integrate with other sources of policy and control, and to propagate signals about mesh behavior to other systems for analysis. The policy runtime supports a standard extension mechanism for plugging in other services. In addition, it allows for the extension of its vocabulary to allow policies to be enforced based on new signals that the mesh produces.

  • Portability: The ecosystem in which Istio is used varies along many dimensions. Istio must run on any cloud or on-premises environment with minimal effort. The task of porting Istio-based services to new environments must be trivial. Using Istio, you are able to operate a single service deployed into multiple environments. For example, you can deploy on multiple clouds for redundancy.

  • Policy Uniformity: The application of policy to API calls between services provides a great deal of control over mesh behavior. However, it can be equally important to apply policies to resources which are not necessarily expressed at the API level. For example, applying a quota to the amount of CPU consumed by an ML training task is more useful than applying a quota to the call which initiated the work. To this end, Istio maintains the policy system as a distinct service with its own API rather than the policy system being baked into the proxy sidecar, allowing services to directly integrate with it as needed.