Client Configuration Support for GRPC
=====================================

This library provides high level configuration machinery to construct client
channels and load balance between them.

Each grpc_channel is created with a grpc_resolver. It is the resolver's duty
to resolve a name into configuration data for the channel. Such configuration
data might include:

- a list of (ip, port) addresses to connect to
- a load balancing policy to decide which server to send a request to
- a set of filters to mutate outgoing requests (say, by adding metadata)

The resolver provides this data as a stream of grpc_resolver_result objects to
the channel. We represent configuration as a stream so that it can be changed
by the resolver during execution, by reacting to external events (such as a
new configuration file being pushed to some store).


Load Balancing
--------------

Load balancing configuration is provided by a grpc_lb_policy object, stored as
part of grpc_resolver_result.

The primary job of the load balancing policies is to pick a target server given only the
initial metadata for a request. It does this by providing a grpc_subchannel
object to the owning channel.


Sub-Channels
------------

A sub-channel provides a connection to a server for a client channel. It has a
connectivity state like a regular channel, and so can be connected or
disconnected. This connectivity state can be used to inform load balancing
decisions (for example, by avoiding disconnected backends).

Configured sub-channels are fully setup to participate in the grpc data plane.
Their behavior is specified by a set of grpc channel filters defined at their
construction. To customize this behavior, resolvers build
grpc_client_channel_factory objects, which use the decorator pattern to customize
construction arguments for concrete grpc_subchannel instances.


Naming for GRPC
===============

Names in GRPC are represented by a URI (as defined in
[RFC 3986](https://tools.ietf.org/html/rfc3986)).

The following schemes are currently supported:

dns:///host:port - dns schemes are currently supported so long as authority is
                   empty (authority based dns resolution is expected in a future
                   release)

unix:path        - the unix scheme is used to create and connect to unix domain
                   sockets - the authority must be empty, and the path
                   represents the absolute or relative path to the desired
                   socket

ipv4:host:port   - a pre-resolved ipv4 dotted decimal address/port combination

ipv6:[host]:port - a pre-resolved ipv6 address/port combination