Add module for transactions cache (#440)

* Add transactions cache, write tests.

Add a transactions module in dendrite/common. This is needed for idempotent APIs.

Signed-off-by: Anant Prakash <anantprakashjsr@gmail.com>

* Use cycling double map instead, improve code logic, remove unneeded test

Signed-off-by: Anant Prakash <anantprakashjsr@gmail.com>

* Update code comments

Signed-off-by: Anant Prakash <anantprakashjsr@gmail.com>

* Use two constructors for default and custom cleanupPeriod
Add code comments

Signed-off-by: Anant Prakash <anantprakashjsr@gmail.com>

src/github.com/matrix-org/dendrite/common/transactions/transactions.go

@@ -0,0 +1,88 @@
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package transactions
+
+import (
+	"sync"
+	"time"
+
+	"github.com/matrix-org/util"
+)
+
+// DefaultCleanupPeriod represents the default time duration after which cacheCleanService runs.
+const DefaultCleanupPeriod time.Duration = 30 * time.Minute
+
+type txnsMap map[string]*util.JSONResponse
+
+// Cache represents a temporary store for response entries.
+// Entries are evicted after a certain period, defined by cleanupPeriod.
+// This works by keeping two maps of entries, and cycling the maps after the cleanupPeriod.
+type Cache struct {
+	sync.RWMutex
+	txnsMaps      [2]txnsMap
+	cleanupPeriod time.Duration
+}
+
+// New is a wrapper which calls NewWithCleanupPeriod with DefaultCleanupPeriod as argument.
+func New() *Cache {
+	return NewWithCleanupPeriod(DefaultCleanupPeriod)
+}
+
+// NewWithCleanupPeriod creates a new Cache object, starts cacheCleanService.
+// Takes cleanupPeriod as argument.
+// Returns a reference to newly created Cache.
+func NewWithCleanupPeriod(cleanupPeriod time.Duration) *Cache {
+	t := Cache{txnsMaps: [2]txnsMap{make(txnsMap), make(txnsMap)}}
+	t.cleanupPeriod = cleanupPeriod
+
+	// Start clean service as the Cache is created
+	go cacheCleanService(&t)
+	return &t
+}
+
+// FetchTransaction looks up an entry for txnID in Cache.
+// Looks in both the txnMaps.
+// Returns (JSON response, true) if txnID is found, else the returned bool is false.
+func (t *Cache) FetchTransaction(txnID string) (*util.JSONResponse, bool) {
+	t.RLock()
+	defer t.RUnlock()
+	for _, txns := range t.txnsMaps {
+		res, ok := txns[txnID]
+		if ok {
+			return res, true
+		}
+	}
+	return nil, false
+}
+
+// AddTransaction adds an entry for txnID in Cache for later access.
+// Adds to the front txnMap.
+func (t *Cache) AddTransaction(txnID string, res *util.JSONResponse) {
+	t.Lock()
+	defer t.Unlock()
+
+	t.txnsMaps[0][txnID] = res
+}
+
+// cacheCleanService is responsible for cleaning up entries after cleanupPeriod.
+// It guarantees that an entry will be present in cache for at least cleanupPeriod & at most 2 * cleanupPeriod.
+// This cycles the txnMaps forward, i.e. back map is assigned the front and front is assigned an empty map.
+func cacheCleanService(t *Cache) {
+	ticker := time.Tick(t.cleanupPeriod)
+	for range ticker {
+		t.Lock()
+		t.txnsMaps[1] = t.txnsMaps[0]
+		t.txnsMaps[0] = make(txnsMap)
+		t.Unlock()
+	}
+}

src/github.com/matrix-org/dendrite/common/transactions/transactions_test.go

@@ -0,0 +1,50 @@
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package transactions
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/matrix-org/util"
+)
+
+type fakeType struct {
+	ID string `json:"ID"`
+}
+
+var (
+	fakeTxnID    = "aRandomTxnID"
+	fakeResponse = &util.JSONResponse{Code: http.StatusOK, JSON: fakeType{ID: "0"}}
+)
+
+// TestCache creates a New Cache and tests AddTransaction & FetchTransaction
+func TestCache(t *testing.T) {
+	fakeTxnCache := New()
+	fakeTxnCache.AddTransaction(fakeTxnID, fakeResponse)
+
+	// Add entries for noise.
+	for i := 1; i <= 100; i++ {
+		fakeTxnCache.AddTransaction(
+			fakeTxnID+string(i),
+			&util.JSONResponse{Code: http.StatusOK, JSON: fakeType{ID: string(i)}},
+		)
+	}
+
+	testResponse, ok := fakeTxnCache.FetchTransaction(fakeTxnID)
+	if !ok {
+		t.Error("Failed to retrieve entry for txnID: ", fakeTxnID)
+	} else if testResponse.JSON != fakeResponse.JSON {
+		t.Error("Fetched response incorrect. Expected: ", fakeResponse.JSON, " got: ", testResponse.JSON)
+	}
+}