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gRPC xDS

This directory contains the implementation of gRPC's xDS support. xDS is a set of APIs that allow a gRPC client or server to discover and configure itself dynamically.

See also: gRPC Core overview

Overarching Purpose

The code in this directory provides a gRPC implementation of the xDS APIs. This allows gRPC clients and servers to be configured by a central control plane. xDS is one way that service configurations can be delivered to a gRPC application; see service config for more information.

Core Concepts

  • XdsClient: The XdsClient is the core of the xDS implementation. It is responsible for managing the connection to the xDS server, sending resource requests, and receiving and processing resource updates.
  • Bootstrap File: The XdsClient is configured via a bootstrap file. The bootstrap file is a JSON file that contains the information that the XdsClient needs to connect to the xDS server.
  • xDS Resources: xDS is a set of APIs for discovering and configuring different types of resources. The most important resource types for gRPC are:
    • LDS (Listener Discovery Service): Used to discover the listeners that are running on a gRPC server.
    • RDS (Route Discovery Service): Used to discover the routes that are available for a given listener.
    • CDS (Cluster Discovery Service): Used to discover the clusters that are available to the gRPC client.
    • EDS (Endpoint Discovery Service): Used to discover the endpoints (i.e., the backend servers) that are in a given cluster.

xds_client Directory

The xds_client directory contains the core implementation of the xDS client.

Files

  • xds_client.h, xds_client.cc: These files define the XdsClient class.
  • xds_bootstrap.h, xds_bootstrap.cc: These files define the XdsBootstrap class, which is responsible for reading the xDS bootstrap file and creating the initial XdsClient configuration.
  • xds_api.h, xds_api.cc: These files define the XdsApi class, which provides a high-level API for interacting with the xDS server.
  • lrs_client.h, lrs_client.cc: These files define the LrsClient class, which is used to send load reports to the xDS server.
  • xds_resource_type.h: Defines the interface for a specific xDS resource type. Each resource type (e.g., LDS, RDS, CDS, EDS) has its own implementation of this interface.

Major Classes

  • grpc_core::XdsClient: The core of the xDS client implementation.
  • grpc_core::XdsBootstrap: Responsible for reading the xDS bootstrap file.
  • grpc_core::XdsApi: A high-level API for interacting with the xDS server.
  • grpc_core::LrsClient: A client for sending load reports to the xDS server.

Notes

  • xDS is a complex and powerful feature. A full understanding of xDS requires reading the official Envoy documentation.
  • The gRPC xDS implementation is still under active development.
  • The xDS implementation is a good example of how gRPC can be extended with new features. It is implemented as a set of plugins that are loaded at runtime, and it uses the CoreConfiguration class to register its factories.