LogDog: Add Storage-layer data caching.

logdog/common/storage/caching/cache.go

+// 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 caching
+
+import (
+        "bufio"
+        "crypto/sha256"
+        "encoding/hex"
+        "time"
+
+        "golang.org/x/net/context"
+)
+
+// Item is a single cache item. An item is uniquely identified by its Schema and
+// Key sequence.
+type Item struct {
+        // Schema is the item's schema value. If empty, the item is schemaless.
+        Schema string
+
+        // Type is the item's type identifier. If empty, the item has no type.
+        Type string
+
+        // Key is the item's individual key. It uniquely identifies the Item within
+        // the scope of the Schema and Type.
+        Key string
+
+        // Data is the Item's data. For Put, the contents of this Data will be used to
+        // populate the cache. For Get, this value will be non-nil and populated with
+        // the retrieved data, or nil if the item was not present in the cache.
+        Data []byte
+}
+
+// Cache is a simple cache interface. It is capable of storage and retrieval.
+type Cache interface {
+        // Put caches the supplied Items into the cache. If an Item already exists in
+        // the cache, it will be overridden.
+        //
+        // If exp, the supplied expiration Duration, is >0, the cache should only
+        // store the data if it can expire it after this period of time.
+        //
+        // This method does not return whether or not the caching storage was
+        // successful.
+        Put(c context.Context, exp time.Duration, items ...*Item)
+
+        // Get retrieves a cached entry. If the entry was present, a non-nil value
+        // will be returned, even if the data length is zero. If the entry was not
+        // present, nil will be returned.
+        //
+        // This method does not distinguish between an error and missing data. Either
+        // valid data is returned, or it is not.
+        Get(c context.Context, items ...*Item)
+}
+
+// HashKey composes a hex-encoded SHA256 string from the supplied parts. The
+// local schema version and KeyType are included in the hash.
+func HashKey(parts ...string) string {
+        hash := sha256.New()
+        bio := bufio.NewWriter(hash)
+
+        // Add a full NULL-delimited key sequence segment to our hash.
+        for i, part := range parts {
+                // Write the separator, except for the first key.
+                if i > 0 {
+                        if _, err := bio.WriteRune('\x00'); err != nil {
+                                panic(err)
+                        }
+                }
+
+                if _, err := bio.WriteString(part); err != nil {
+                        panic(err)
+                }
+        }
+        if err := bio.Flush(); err != nil {
+                panic(err)
+        }
+
+        // Return the hex-encoded hash sum.
+        return hex.EncodeToString(hash.Sum(nil))
+}