data-plane-api/API_OVERVIEW.md

# Envoy v2 APIs for developers

## Goals

This repository contains both the implemented and draft v2 JSON REST and gRPC
[Envoy](https://github.com/envoyproxy/envoy/) APIs.

Version 2 of the Envoy API evolves existing APIs and introduces new APIs to:

* Allow for more advanced load balancing through load and resource utilization reporting to management servers.
* Improve N^2 health check scalability issues by optionally offloading health checking to other Envoy instances.
* Support Envoy deployment in edge, sidecar and middle proxy deployment models via changes to the listener model,
  CDS API, and EDS (formerly called SDS in v1) API.
* Allow streaming updates from the management server on change, instead of polling APIs from Envoy. gRPC APIs will be supported
  alongside JSON REST APIs to provide for this.
* Ensure all Envoy runtime configuration is dynamically discoverable via API
  calls, including listener configuration, certificates and runtime settings, which are today sourced from the filesystem. There
  will still remain a static bootstrap configuration file that will specify items
  unlikely to change during runtime, including the Envoy node identity, xDS
  management server addresses, administration interface and tracing
  configuration.
* Revisit and where appropriate cleanup any v1 technical debt.

## Status

See
[here](https://www.envoyproxy.io/docs/envoy/latest/configuration/overview/v2_overview.html#status)
for the current status of the v2 APIs.

See [here](CONTRIBUTING.md#api-changes) for the v2 API change process.

## Principles

* [Proto3](https://developers.google.com/protocol-buffers/docs/proto3) will be
  used to specify the canonical API. This will provide directly the gRPC API and
  via gRPC-JSON transcoding the JSON REST API. A textual YAML input will be
  supported for filesystem configuration files (e.g. the bootstrap file), in
  addition to JSON, as a syntactic convenience. YAML file contents will be
  internally converted to JSON and then follow the standard JSON-proto3
  conversion during Envoy config ingestion.

* xDS APIs should support eventual consistency. For example, if RDS references a
  cluster that has not yet been supplied by CDS, it should be silently ignored
  and traffic not forwarded until the CDS update occurs. Stronger consistency
  guarantees are possible if the management server is able to sequence the xDS
  APIs carefully (for example by using the ADS API below). By following the
  `[CDS, EDS, LDS, RDS]` sequence for all pertinent resources, it will be
  possible to avoid traffic outages during configuration update.

* The API is primarily intended for machine generation and consumption. It is
  expected that the management server is responsible for mapping higher level
  configuration concepts to API responses. Similarly, static configuration
  fragments may be generated by templating tools, etc. The APIs and tools
  used to generate xDS configuration are beyond the scope of the definitions in
  this repository.

* REST-JSON API equivalents will be provided for the basic singleton xDS
  subscription services CDS/EDS/LDS/RDS/SDS. Advanced APIs such as HDS, ADS and
  EDS multi-dimensional LB will be gRPC only. This avoids having to map
  complicated bidirectional stream semantics onto REST.

* Listeners will be immutable. Any updates to a listener via LDS will require
  the draining of existing connections for the specific bound IP/port. As a
  result, new requests will only be guaranteed to observe the new configuration
  after existing connections have drained or the drain timeout.

* Versioning will be expressed via [proto3 package
  namespaces](https://developers.google.com/protocol-buffers/docs/proto3#packages),
  i.e. `package envoy.api.v2;`.

* Custom components (e.g. filters, resolvers, loggers) will use a reverse DNS naming scheme,
  e.g. `com.google.widget`, `com.lyft.widget`.

## APIs

Unless otherwise stated, the APIs with the same names as v1 APIs have a similar role.

* [Cluster Discovery Service (CDS)](envoy/api/v2/cds.proto).
* [Endpoint Discovery Service (EDS)](envoy/api/v2/eds.proto). This has the same role as SDS in the [v1 API](https://www.envoyproxy.io/docs/envoy/latest/api-v1/cluster_manager/sds),
  the new name better describes what the API does in practice. Advanced global load balancing capable of utilizing N-dimensional upstream metrics is now supported.
* [Health Discovery Service (HDS)](envoy/service/discovery/v2/hds.proto). This new API supports efficient endpoint health discovery by the management server via the Envoy instances it manages. Individual Envoy instances
  will typically receive HDS instructions to health check a subset of all
  endpoints. The health check subset may not be a subset of the Envoy instance's
  EDS endpoints.
* [Listener Discovery Service (LDS)](envoy/api/v2/lds.proto). This new API supports dynamic discovery of the listener configuration (which ports to bind to, TLS details, filter chains, etc.).
* [Metric Service (MS)](envoy/service/metrics/v2/metrics_service.proto). This new API allows Envoy to push (stream) metrics forever for servers to consume.
* [Rate Limit Service (RLS)](envoy/service/ratelimit/v2/rls.proto)
* [Route Discovery Service (RDS)](envoy/api/v2/rds.proto).
* [Secret Discovery Service (SDS)](envoy/service/discovery/v2/sds.proto).

In addition to the above APIs, an aggregation API will be provided to allow for
fine grained control over the sequencing of API updates across discovery
services:

* [Aggregated Discovery Service (ADS)](envoy/api/v2/discovery.proto). See
  the [ADS overview](https://www.envoyproxy.io/docs/envoy/latest/configuration/overview/v2_overview#aggregated-discovery-service).

A protocol description for the xDS APIs is provided [here](https://www.envoyproxy.io/docs/envoy/latest/api-docs/xds_protocol).

## Terminology

Some relevant [existing terminology](https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/terminology.html) is
repeated below and some new v2 terms introduced.

* Cluster: A cluster is a group of logically similar endpoints that Envoy
  connects to. In v2, RDS routes points to clusters, CDS provides cluster configuration and
  Envoy discovers the cluster members via EDS.

* Downstream: A downstream host connects to Envoy, sends requests, and receives responses.

* Endpoint: An endpoint is an upstream host that is a member of one or more clusters. Endpoints are discovered via EDS.

* Listener: A listener is a named network location (e.g., port, unix domain socket, etc.) that can be connected to by downstream clients. Envoy exposes one or more listeners that downstream hosts connect to.

* Locality: A location where an Envoy instance or an endpoint runs. This includes
  region, zone and sub-zone identification.

* Management server: A logical server implementing the v2 Envoy APIs. This is not necessarily a single physical machine since it may be replicated/sharded and API serving for different xDS APIs may be implemented on different physical machines.

* Region: Geographic region where a zone is located.

* Sub-zone: Location within a zone where an Envoy instance or an endpoint runs.
  This allows for multiple load balancing targets within a zone.

* Upstream: An upstream host receives connections and requests from Envoy and returns responses.

* xDS: CDS/EDS/HDS/LDS/RLS/RDS/SDS APIs.

* Zone: Availability Zone (AZ) in AWS, Zone in GCP.