Implement a server-side config service interface.

server/config/doc.go

+// Copyright 2015 The LUCI Authors. All rights reserved.

[iannucci, 2017/01/07 20:12:16]

also I wish that rietveld (or gerrit) let you somehow sort stuff
non-alphabetically so that all doc.go went to the top, all _test.go went to the
bottom, etc.

[dnj, 2017/01/10 03:25:58]

Acknowledged.

+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+// Package config contains service implementations for the LUCI configuration
+// service defined in github.com/luci/luci-go/common/config.
+//
+// This defines a service-oriented interface to retrieve configuration service
+// properties and data. The interface is designed to be used with

[iannucci, 2017/01/07 20:12:16]

'service-oriented'? which service? the config service, or the service someone is
implementeting?

can we `s/data/files` too? anything to provide additional specificity to the
discussion at hand.

[dnj, 2017/01/10 03:25:58]

Done. The entire "server/" package has this theme, it seems odd to specifically
have this package define it.

+// service-appropriate caching layers. Config service requests are made with

[iannucci, 2017/01/07 20:12:16]

'service-appropriate'? same questions.

[dnj, 2017/01/10 03:25:58]

Done.

+// authorities, either as the service itself, on behalf of the user (delegation
+// is required), 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 a cachable
+//    format.

[iannucci, 2017/01/07 20:12:16]

an application-specific cachable format

[dnj, 2017/01/10 03:25:58]

Done.

+// 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 user to the underlying configuration data.

[iannucci, 2017/01/07 20:12:16]

s/the user/the application/

since the application probably has its own notion of users, and we probably will
never have the users directly interface with this package

[dnj, 2017/01/10 03:25:58]

Done.

+package config