Initial checkin of gRPC to third_party/

third_party/grpc/src/core/client_config/README.md

+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_subchannel_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