dashboard: cache todo responses

R=golang-dev, dsymonds, adg
CC=golang-dev
https://golang.org/cl/5500057

misc/dashboard/app/build/cache.go

+// Copyright 2011 The Go 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 build
+
+import (
+	"appengine"
+	"appengine/memcache"
+	"json"
+	"os"
+)
+
+const (
+	todoCacheKey    = "build-todo"
+	todoCacheExpiry = 3600 // 1 hour in seconds
+	uiCacheKey      = "build-ui"
+	uiCacheExpiry   = 10 * 60 // 10 minutes in seconds
+)
+
+// invalidateCache deletes the build cache records from memcache.
+// This function should be called whenever the datastore changes.
+func invalidateCache(c appengine.Context) {
+	keys := []string{uiCacheKey, todoCacheKey}
+	errs := memcache.DeleteMulti(c, keys)
+	for i, err := range errs {
+		if err != nil && err != memcache.ErrCacheMiss {
+			c.Errorf("memcache.Delete(%q): %v", keys[i], err)
+		}
+	}
+}
+
+// cachedTodo gets the specified todo cache entry (if it exists) from the
+// shared todo cache.
+func cachedTodo(c appengine.Context, todoKey string) (todo *Todo, hit bool) {
+	t, _ := todoCache(c)
+	if t == nil {
+		return nil, false
+	}
+	todos := unmarshalTodo(c, t)
+	if todos == nil {
+		return nil, false
+	}
+	todo, hit = todos[todoKey]
+	return
+}
+
+// cacheTodo puts the provided todo cache entry into the shared todo cache.
+// The todo cache is a JSON-encoded map[string]*Todo, where the key is todoKey.
+func cacheTodo(c appengine.Context, todoKey string, todo *Todo) {
+	// Get the todo cache record (or create a new one).
+	newItem := false
+	t, miss := todoCache(c)
+	if miss {
+		newItem = true
+		t = &memcache.Item{
+			Key:   todoCacheKey,
+			Value: []byte("{}"), // default is an empty JSON object
+		}
+	}
+	if t == nil {
+		return
+	}
+
+	// Unmarshal the JSON value.
+	todos := unmarshalTodo(c, t)
+	if todos == nil {
+		return
+	}
+
+	// Update the map.
+	todos[todoKey] = todo
+
+	// Marshal the updated JSON value.
+	var err os.Error
+	t.Value, err = json.Marshal(todos)
+	if err != nil {
+		// This shouldn't happen.
+		c.Criticalf("marshal todo cache: %v", err)
+		return
+	}
+
+	// Set a new expiry.
+	t.Expiration = todoCacheExpiry
+
+	// Update the cache record (or Set it, if new).
+	if newItem {
+		err = memcache.Set(c, t)
+	} else {
+		err = memcache.CompareAndSwap(c, t)
+	}
+	if err == memcache.ErrCASConflict || err == memcache.ErrNotStored {
+		// No big deal if it didn't work; it should next time.
+		c.Warningf("didn't update todo cache: %v", err)
+	} else if err != nil {
+		c.Errorf("update todo cache: %v", err)
+	}
+}
+
+// todoCache gets the todo cache record from memcache (if it exists).
+func todoCache(c appengine.Context) (item *memcache.Item, miss bool) {
+	t, err := memcache.Get(c, todoCacheKey)
+	if err == memcache.ErrCacheMiss {
+		return nil, true
+	} else if err != nil {
+		c.Errorf("get todo cache: %v", err)
+		return nil, false
+	}
+	return t, false
+}
+
+// unmarshalTodo decodes the given item's memcache value into a map.
+func unmarshalTodo(c appengine.Context, t *memcache.Item) map[string]*Todo {
+	todos := make(map[string]*Todo)
+	if err := json.Unmarshal(t.Value, &todos); err != nil {
+		// This shouldn't happen.
+		c.Criticalf("unmarshal todo cache: %v", err)
+		// Kill the bad record.
+		if err := memcache.Delete(c, todoCacheKey); err != nil {
+			c.Errorf("delete todo cache: %v", err)
+		}
+		return nil
+	}
+	return todos
+}

misc/dashboard/app/build/handler.go

@@ -7,7 +7,6 @@ package build
 import (
 	"appengine"
 	"appengine/datastore"
-	"appengine/memcache"
 	"crypto/hmac"
 	"fmt"
 	"http"
@@ -147,8 +146,18 @@ type Todo struct {
 // todoHandler returns the next action to be performed by a builder.
 // It expects "builder" and "kind" query parameters and returns a *Todo value.
 // Multiple "kind" parameters may be specified.
-func todoHandler(r *http.Request) (todo interface{}, err os.Error) {
+func todoHandler(r *http.Request) (interface{}, os.Error) {
 	c := appengine.NewContext(r)
+
+	todoKey := r.Form.Encode()
+	if t, hit := cachedTodo(c, todoKey); hit {
+		c.Debugf("cache hit")
+		return t, nil
+	}
+	c.Debugf("cache miss")
+
+	var todo *Todo
+	var err os.Error
 	builder := r.FormValue("builder")
 	for _, kind := range r.Form["kind"] {
 		var data interface{}
@@ -156,17 +165,19 @@ func todoHandler(r *http.Request) (todo interface{}, err os.Error) {
 		case "build-go-commit":
 			data, err = buildTodo(c, builder, "", "")
 		case "build-package":
-			data, err = buildTodo(
-				c, builder,
-				r.FormValue("packagePath"),
-				r.FormValue("goHash"),
-			)
+			packagePath := r.FormValue("packagePath")
+			goHash := r.FormValue("goHash")
+			data, err = buildTodo(c, builder, packagePath, goHash)
 		}
 		if data != nil || err != nil {
-			return &Todo{Kind: kind, Data: data}, err
+			todo = &Todo{Kind: kind, Data: data}
+			break
 		}
 	}
-	return nil, nil
+	if err == nil {
+		cacheTodo(c, todoKey, todo)
+	}
+	return todo, err
 }
 
 // buildTodo returns the next Commit to be built (or nil if none available).
@@ -379,11 +390,3 @@ func logErr(w http.ResponseWriter, r *http.Request, err os.Error) {
 	w.WriteHeader(http.StatusInternalServerError)
 	fmt.Fprint(w, "Error: ", err)
 }
-
-// invalidateCache deletes the ui cache record from memcache.
-func invalidateCache(c appengine.Context) {
-	err := memcache.Delete(c, uiCacheKey)
-	if err != nil && err != memcache.ErrCacheMiss {
-		c.Errorf("memcache.Delete(%q): %v", uiCacheKey, err)
-	}
-}

misc/dashboard/app/build/ui.go

@@ -22,11 +22,6 @@ import (
 	"template"
 )
 
-const (
-	uiCacheKey    = "build-ui"
-	uiCacheExpiry = 10 * 60 // 10 minutes in seconds
-)
-
 func init() {
 	http.HandleFunc("/", uiHandler)
 	html.Escape(uiTemplate)