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gRPC Name Resolution

Overview

gRPC supports DNS as the default name-system. A number of alternative name-systems are used in various deployments. We support an API that is general enough to support a range of name-systems and the corresponding syntax for names. The gRPC client library in various languages will provide a plugin mechanism so resolvers for different name-systems can be plugged in.

Detailed Design

Name Syntax

A fully qualified, self contained name used for gRPC channel construction uses the syntax:

scheme://authority/endpoint_name

Here, scheme indicates the name-system to be used. Currently, we support the following schemes:

  • dns

  • ipv4 (IPv4 address)

  • ipv6 (IPv6 address)

  • unix (path to unix domain socket -- unix systems only)

In the future, additional schemes such as etcd could be added.

The authority indicates some scheme-specific bootstrap information, e.g., for DNS, the authority may include the IP[:port] of the DNS server to use. Often, a DNS name may be used as the authority, since the ability to resolve DNS names is already built into all gRPC client libraries.

Finally, the endpoint_name indicates a concrete name to be looked up in a given name-system identified by the scheme and the authority. The syntax of the endpoint name is dictated by the scheme in use.

Resolver Plugins

The gRPC client library will use the specified scheme to pick the right resolver plugin and pass it the fully qualified name string.

Resolvers should be able to contact the authority and get a resolution that they return back to the gRPC client library. The returned contents include:

  • A list of resolved addresses, each of which has three attributes:
    • The address itself, including both IP address and port.
    • A boolean indicating whether the address is a backend address (i.e., the address to use to contact the server directly) or a balancer address (for cases where external load balancing is in use).
    • The name of the balancer, if the address is a balancer address. This will be used to perform peer authorization.
  • A service config.

The plugin API allows the resolvers to continuously watch an endpoint and return updated resolutions as needed.