This page describes how Istio load balances traffic across instances of a service in a service mesh.
Service registration: Istio assumes the presence of a service registry to keep track of the pods/VMs of a service in the application. It also assumes that new instances of a service are automatically registered with the service registry and unhealthy instances are automatically removed. Platforms such as Kubernetes, Mesos already provide such functionality for container-based applications. A plethora of solutions exist for VM-based applications.
Service Discovery: Pilot consumes information from the service registry and provides a platform-agnostic service discovery interface. Envoy instances in the mesh perform service discovery and dynamically update their load balancing pools accordingly.
As illustrated in the figure above, services in the mesh access each other using their DNS names. All HTTP traffic bound to a service is automatically re-routed through Envoy. Envoy distributes the traffic across instances in the load balancing pool. While Envoy supports several sophisticated load balancing algorithms, Istio currently allows three load balancing modes: round robin, random, and weighted least request.
In addition to load balancing, Envoy periodically checks the health of each instance in the pool. Envoy follows a circuit breaker style pattern to classify instances as unhealthy or healthy based on their failure rates for the health check API call. In other words, when the number of health check failures for a given instance exceeds a pre-specified threshold, it will be ejected from the load balancing pool. Similarly, when the number of health checks that pass exceed a pre-specified threshold, the instance will be added back into the load balancing pool. You can find out more about Envoy’s failure-handling features in Handling Failures.
Services can actively shed load by responding with a HTTP 503 to a health check. In such an event, the service instance will be immediately removed from the caller’s load balancing pool.