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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_client_config 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_client_config.
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
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