LogDog: Add Storage-layer data caching.

logdog/common/storage/bigtable/cache.go

Index: logdog/common/storage/bigtable/cache.go
diff --git a/logdog/common/storage/bigtable/cache.go b/logdog/common/storage/bigtable/cache.go
new file mode 100644
index 0000000000000000000000000000000000000000..64bad7dc9108cc1030e0e1c43c1227aa189472a9
--- /dev/null
+++ b/logdog/common/storage/bigtable/cache.go
@@ -0,0 +1,68 @@
+// Copyright 2016 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 bigtable
+
+import (
+	"bytes"
+	"encoding/binary"
+	"time"
+
+	"github.com/luci/luci-go/common/config"
+	log "github.com/luci/luci-go/common/logging"
+	"github.com/luci/luci-go/logdog/common/storage/caching"
+	"github.com/luci/luci-go/logdog/common/types"
+
+	"golang.org/x/net/context"
+)
+
+// cacheSchema represents the cache schema used by this version of the tail
+// cache. If the underlying data format changes, this value must also be
+// updated.
+const cacheSchema = "v1"
+
+// lastTailIndexCacheDuration is the amount of time that the last tail index
+// should be cached.
+const lastTailIndexCacheDuration = 1 * time.Hour
+
+// getLastTailIndex will return the cached last tail index of a given stream.
+//
+// If there was an error, or if the item was not cached, 0 (first index) will be
+// returned.
+func getLastTailIndex(c context.Context, cache caching.Cache, project config.ProjectName, path types.StreamPath) int64 {
+	itm := mkLastTailItem(project, path)
+	cache.Get(c, itm)
+	if itm.Data == nil {
+		return 0
+	}
+
+	v, err := binary.ReadVarint(bytes.NewReader(itm.Data))
+	if err != nil {
+		log.Fields{
+			log.ErrorKey: err,
+			"project":    project,
+			"path":       path,
+		}.Warningf(c, "Could not decode last tail cache.")
+		return 0
+	}
+
+	return v
+}
+
+func putLastTailIndex(c context.Context, cache caching.Cache, project config.ProjectName, path types.StreamPath, v int64) {
+	buf := make([]byte, binary.MaxVarintLen64)
+	buf = buf[:binary.PutVarint(buf, v)]
+
+	itm := mkLastTailItem(project, path)
+	itm.Data = buf
+	cache.Put(c, lastTailIndexCacheDuration, itm)
+}
+
+func mkLastTailItem(project config.ProjectName, path types.StreamPath) *caching.Item {
+	return &caching.Item{
+		Schema: cacheSchema,
+		Type:   "bt_tail_idx",
+		Key:    caching.HashKey(string(project), string(path)),
+	}
+}