SERVER-67996 implement collection statistics read through cache

1 files changed, 89 insertions, 0 deletions

diff --git a/src/mongo/db/query/ce/stats_cache.h b/src/mongo/db/query/ce/stats_cache.h
new file mode 100644
index 00000000000..31bf6802b5c
--- /dev/null
+++ b/src/mongo/db/query/ce/stats_cache.h
@@ -0,0 +1,89 @@
+/**
+ *    Copyright (C) 2022-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+#include "mongo/base/string_data.h"
+#include "mongo/db/namespace_string.h"
+#include "mongo/db/query/ce/collection_statistics.h"
+#include "mongo/db/query/ce/stats_cache_loader.h"
+#include "mongo/util/concurrency/thread_pool.h"
+#include "mongo/util/read_through_cache.h"
+
+namespace mongo {
+
+using namespace mongo::ce;
+
+using StatsCacheType = ReadThroughCache<NamespaceString, CollectionStatistics>;
+using StatsCacheValueHandle = StatsCacheType::ValueHandle;
+
+/**
+ * Collectoin statistics read through cache. It reads from the persitent storage but never wrties to
+ * it. Stored on the service context.
+ */
+class StatsCache : public StatsCacheType {
+public:
+    /**
+     * Stores the cache on the specified service context. May only be called once for the lifetime
+     * of the service context.
+     */
+    static void set(ServiceContext* serviceContext, std::unique_ptr<StatsCache> cache);
+    static void clearForTests(ServiceContext* serviceContext);
+
+    static StatsCache& get(ServiceContext* serviceContext);
+    static StatsCache& get(OperationContext* opCtx);
+
+    /**
+     * The constructor provides the Service context under which this cache has been instantiated,
+     * and a Thread pool to be used for invoking the blocking 'lookup' calls. The size is the number
+     * of entries the underlying LRU cache will hold.
+     */
+    StatsCache(ServiceContext* service, ThreadPoolInterface& threadPool, int size);
+
+    /**
+     *  Returns statsCacheLoader currently used for testing only.
+     */
+    StatsCacheLoader& getStatsCacheLoader() {
+        return _statsCacheLoader;
+    }
+
+private:
+    /**
+     * Reads collection stats from the underlying storage if its not found in the in memory cache.
+     */
+    LookupResult _lookupStats(OperationContext* opCtx,
+                              const NamespaceString& nss,
+                              const ValueHandle& stats);
+
+    Mutex _mutex = MONGO_MAKE_LATCH("StatsCache::_mutex");
+
+    StatsCacheLoader _statsCacheLoader;
+};
+
+}  // namespace mongo