LogDog: Enable keys-only BigTable queries.

server/logdog/storage/storage.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package storage
 
 import (
         "errors"
         "time"
 
@@ skipping 43 matching lines @@
         Path types.StreamPath
         // Index is the entry's stream index.
         Index types.MessageIndex
 
         // Limit is the maximum number of records to return before stopping iteration.
         // If zero, no maximum limit will be applied.
         //
         // The Storage instance may return fewer records than the supplied Limit as an
         // implementation detail.
         Limit int
+        // KeysOnly, if true, allows (but doesn't require) the Storage instance to
+        // omit entry data in its get callback. For scanning operations, this can be
+        // much cheaper/faster than full data queries.
+        KeysOnly bool
 }
 
 // GetCallback is invoked for each record in the Get request. If it returns
 // false, iteration should stop.
 type GetCallback func(types.MessageIndex, []byte) bool
 
 // Storage is an abstract LogDog storage implementation. Interfaces implementing
 // this may be used to store and retrieve log records by the collection service
 // layer.
 //
@@ skipping 26 matching lines @@
         Get(GetRequest, GetCallback) error
 
         // Tail retrieves the latest log in the stream. If the stream has no logs, it
         // will return ErrDoesNotExist.
         Tail(types.StreamPath) ([]byte, types.MessageIndex, error)
 
         // Config installs the supplied configuration parameters into the storage
         // instance.
         Config(Config) error
 }