Implement a server-side config service interface.

luci_config/server/cfgclient/doc.go

Index: luci_config/server/cfgclient/doc.go
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+// Copyright 2015 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+// Package cfgclient contains service implementations for the LUCI configuration
+// service defined in github.com/luci/luci-go/common/config.
+//
+// This defines an interface to the LUCI configuration service properties and
+// files. The interface is designed to be used by services which handle user
+// data, and has the ability to operate on behalf of authorities, either the
+// service itself (privileged), on behalf of the user (delegation), or
+// anonymously.
+//
+// This package also offers the concept of resolution, where a configuration
+// value is transformed into a more versatile application format prior to being
+// cached and/or returned. Resolution allows configuration data consumers to
+// handle configuration data as native Go types instead of raw configuration
+// service data.
+//
+// Configuration requests pass through the following layers:
+// 1) A Backend, which is the configured configuration authority.
+// 2) Cache resolution, which optionally transforms the data into an
+//    application-specific cachable format.
+// 3) A cache layer, which caches the data.
+// 4) Value resolution, which transforms the cached data format from (2) into
+//    a Go value.
+// 5) The Go value is retuned to the user.
+//
+// Layers (2) and (4) are managed by the Resolver type, which is associated by
+// the application with the underlying configuration data.
+package cfgclient