Implemented Refresh-Ahead caching for Redis

This updates the AdapterCacheRedis instance to be able to handle updating
itself when reading from the cache. For this to work we need to pass a
`fetchData` function to the `get` method.

In the case of a cache miss, we will read the data via the `fetchData`
function, and store it in the cache, before returning the value to the caller.

When coupled with a `refreshAheadFactor` config, we will go a step further and
implement the "Refresh Ahead" caching strategy. What this means is that we will
refresh the contents of the cache in the background, this happens on a cache
read and only once the data in the cache has only a certain percentage of the
TTL left, which is set as a decimal value between 0 and 1.

e.g.

ttl = 100s
refreshAheadFactor = 0.2;

Any read from the cache that happens _after_ 80s will do a background refresh

ghost/adapter-cache-redis/lib/AdapterCacheRedis.js

@@ -17,6 +17,7 @@ class AdapterCacheRedis extends BaseCacheAdapter {
      * @param {Object} [config.clusterConfig] - redis cluster config used in case no cache instance provided
      * @param {Object} [config.storeConfig] - extra redis client config used in case no cache instance provided
      * @param {Number} [config.ttl] - default cached value Time To Live (expiration) in *seconds*
+     * @param {Number} [config.refreshAheadFactor] - 0-1 number to use to determine how old (as a percentage of ttl) an entry should be before refreshing it
      * @param {String} [config.keyPrefix] - prefix to use when building a unique cache key, e.g.: 'some_id:image-sizes:'
      * @param {Boolean} [config.reuseConnection] - specifies if the redis store/connection should be reused within the process
      */
@@ -57,6 +58,9 @@ class AdapterCacheRedis extends BaseCacheAdapter {
             });
         }
 
+        this.ttl = config.ttl;
+        this.refreshAheadFactor = config.refreshAheadFactor || 0;
+        this.currentlyExecutingBackgroundRefreshes = new Set();
         this.keyPrefix = config.keyPrefix;
         this._keysPattern = config.keyPrefix ? `${config.keyPrefix}*` : '';
         this.redisClient = this.cache.store.getClient();
@@ -134,11 +138,67 @@ class AdapterCacheRedis extends BaseCacheAdapter {
 
     /**
      *
-     * @param {String} key
+     * @param {String} internalKey
      */
-    async get(key) {
+    async shouldRefresh(internalKey) {
+        if (this.refreshAheadFactor === 0) {
+            debug(`shouldRefresh ${internalKey}: false - refreshAheadFactor = 0`);
+            return false;
+        }
+        if (this.refreshAheadFactor === 1) {
+            debug(`shouldRefresh ${internalKey}: true - refreshAheadFactor = 1`);
+            return true;
+        }
         try {
-            return await this.cache.get(this._buildKey(key));
+            const ttlRemainingForEntry = await this.cache.ttl(internalKey);
+            const shouldRefresh = ttlRemainingForEntry < this.refreshAheadFactor * this.ttl;
+            debug(`shouldRefresh ${internalKey}: ${shouldRefresh} - TTL remaining = ${ttlRemainingForEntry}`);
+            return shouldRefresh;
+        } catch (err) {
+            logging.error(err);
+            return false;
+        }
+    }
+
+    /**
+     *
+     * @param {string} key
+     * @param {() => Promise<any>} [fetchData] An optional function to fetch the data, which will be used in the case of a cache MISS or a background refresh
+     */
+    async get(key, fetchData) {
+        const internalKey = this._buildKey(key);
+        try {
+            const result = await this.cache.get(internalKey);
+            debug(`get ${internalKey}: Cache ${result ? 'HIT' : 'MISS'}`);
+            if (!fetchData) {
+                return result;
+            }
+            if (result) {
+                const shouldRefresh = await this.shouldRefresh(internalKey);
+                const isRefreshing = this.currentlyExecutingBackgroundRefreshes.has(internalKey);
+                if (isRefreshing) {
+                    debug(`Background refresh already happening for ${internalKey}`);
+                }
+                if (!isRefreshing && shouldRefresh) {
+                    debug(`Doing background refresh for ${internalKey}`);
+                    this.currentlyExecutingBackgroundRefreshes.add(internalKey);
+                    fetchData().then(async (data) => {
+                        await this.set(key, data); // We don't use `internalKey` here because `set` handles it
+                        this.currentlyExecutingBackgroundRefreshes.delete(internalKey);
+                    }).catch((error) => {
+                        this.currentlyExecutingBackgroundRefreshes.delete(internalKey);
+                        logging.error({
+                            message: 'There was an error refreshing cache data in the background',
+                            error: error
+                        });
+                    });
+                }
+                return result;
+            } else {
+                const data = await fetchData();
+                await this.set(key, data); // We don't use `internalKey` here because `set` handles it
+                return data;
+            }
         } catch (err) {
             logging.error(err);
         }
@@ -150,9 +210,10 @@ class AdapterCacheRedis extends BaseCacheAdapter {
      * @param {*} value
      */
     async set(key, value) {
-        debug('set', key);
+        const internalKey = this._buildKey(key);
+        debug('set', internalKey);
         try {
-            return await this.cache.set(this._buildKey(key), value);
+            return await this.cache.set(internalKey, value);
         } catch (err) {
             logging.error(err);
         }

ghost/adapter-cache-redis/test/adapter-cache-redis.test.js

@@ -59,6 +59,127 @@ describe('Adapter Cache Redis', function () {
 
             assert.equal(value, 'value from cache');
         });
+        it('can update the cache in the case of a cache miss', async function () {
+            const KEY = 'update-cache-on-miss';
+            let cachedValue = null;
+            const redisCacheInstanceStub = {
+                get: function (key) {
+                    assert(key === KEY);
+                    return cachedValue;
+                },
+                set: function (key, value) {
+                    assert(key === KEY);
+                    cachedValue = value;
+                },
+                store: {
+                    getClient: sinon.stub().returns({
+                        on: sinon.stub()
+                    })
+                }
+            };
+            const cache = new RedisCache({
+                cache: redisCacheInstanceStub
+            });
+
+            const fetchData = sinon.stub().resolves('Da Value');
+
+            checkFirstRead: {
+                const value = await cache.get(KEY, fetchData);
+                assert.equal(fetchData.callCount, 1);
+                assert.equal(value, 'Da Value');
+                break checkFirstRead;
+            }
+
+            checkSecondRead: {
+                const value = await cache.get(KEY, fetchData);
+                assert.equal(fetchData.callCount, 1);
+                assert.equal(value, 'Da Value');
+                break checkSecondRead;
+            }
+        });
+
+        it('Can do a background update of the cache', async function () {
+            const KEY = 'update-cache-in-background';
+            let cachedValue = null;
+            let remainingTTL = 100;
+
+            const redisCacheInstanceStub = {
+                get: function (key) {
+                    assert(key === KEY);
+                    return cachedValue;
+                },
+                set: function (key, value) {
+                    assert(key === KEY);
+                    cachedValue = value;
+                },
+                ttl: function (key) {
+                    assert(key === KEY);
+                    return remainingTTL;
+                },
+                store: {
+                    getClient: sinon.stub().returns({
+                        on: sinon.stub()
+                    })
+                }
+            };
+            const cache = new RedisCache({
+                cache: redisCacheInstanceStub,
+                ttl: 100,
+                refreshAheadFactor: 0.2
+            });
+
+            const fetchData = sinon.stub();
+            fetchData.onFirstCall().resolves('First Value');
+            fetchData.onSecondCall().resolves('Second Value');
+
+            checkFirstRead: {
+                const value = await cache.get(KEY, fetchData);
+                assert.equal(fetchData.callCount, 1);
+                assert.equal(value, 'First Value');
+                break checkFirstRead;
+            }
+
+            // We simulate having been in the cache for 15 seconds
+            remainingTTL = 85;
+
+            checkSecondRead: {
+                const value = await cache.get(KEY, fetchData);
+                assert.equal(fetchData.callCount, 1);
+                assert.equal(value, 'First Value');
+                break checkSecondRead;
+            }
+
+            // We simulate having been in the cache for 30 seconds
+            remainingTTL = 70;
+
+            checkThirdRead: {
+                const value = await cache.get(KEY, fetchData);
+                assert.equal(fetchData.callCount, 1);
+                assert.equal(value, 'First Value');
+                break checkThirdRead;
+            }
+
+            // We simulate having been in the cache for 85 seconds
+            // This should trigger a background refresh
+            remainingTTL = 15;
+
+            checkFourthRead: {
+                const value = await cache.get(KEY, fetchData);
+                assert.equal(fetchData.callCount, 2);
+                assert.equal(value, 'First Value');
+                break checkFourthRead;
+            }
+
+            // We reset the TTL to 100 for the most recent write
+            remainingTTL = 100;
+
+            checkFifthRead: {
+                const value = await cache.get(KEY, fetchData);
+                assert.equal(fetchData.callCount, 2);
+                assert.equal(value, 'Second Value');
+                break checkFifthRead;
+            }
+        });
     });
 
     describe('set', function () {

ghost/api-framework/lib/pipeline.js

@@ -238,8 +238,7 @@ const pipeline = (apiController, apiUtils, apiType) => {
             const cacheKey = stringify(cacheKeyData);
 
             if (apiImpl.cache) {
-                const response = await apiImpl.cache.get(cacheKey);
+                const response = await apiImpl.cache.get(cacheKey, getResponse);
-
                 if (response) {
                     return Promise.resolve(response);
                 }