Architecture

Emissary is a control plane

Emissary is a specialized control plane for Envoy Proxy. In this architecture, Emissary translates configuration (in the form of Kubernetes Custom Resources) to Envoy configuration. All actual traffic is directly handled by the high-performance Envoy Proxy.

Details

1. The service owner defines configuration in Kubernetes manifests.
2. When the manifest is applied to the cluster, the Kubernetes API notifies Emissary of the change.
3. Emissary parses the change and transforms the configuration into a semantic intermediate representation. Envoy configuration is generated from this IR.
4. The new configuration is passed to Envoy via the gRPC-based Aggregated Discovery Service (ADS) API.
5. Traffic flows through the reconfigured Envoy, without dropping any connections.

Scaling and availability

Emissary relies on Kubernetes for scaling, high availability, and persistence. All Emissary configuration is stored directly in Kubernetes; there is no database. Emissary is packaged as a single container that contains both the control plane and an Envoy Proxy instance. By default, Emissary is deployed as a Kubernetes deployment and can be scaled and managed like any other Kubernetes deployment.

Stateless architecture

By design, Emissary is an entirely stateless architecture. Each individual Emissary instance operates independently of other instances. These Emissary instances rely on Kubernetes to coordinate the configuration between the different Emissary instances. This enables Emissary to sidestep the need to engineer a safe, highly available centralized control plane (and if you don’t think that this is hard, check out Jepsen). By contrast, other control plane architectures rely on a single centralized control plane to manage multiple instances of the data plane. This means that these control plane architectures must engineer resilience and availability into their central control plane.

Envoy Proxy

Emissary closely tracks Envoy Proxy releases. A stable branch of Envoy Proxy is maintained that enables the team to cherry-pick specific fixes into Emissary.