SERVER-46200 implement basic VectorClock service

1 files changed, 148 insertions, 0 deletions

diff --git a/src/mongo/db/vector_clock.h b/src/mongo/db/vector_clock.h
new file mode 100644
index 00000000000..b7381652e9c
--- /dev/null
+++ b/src/mongo/db/vector_clock.h
@@ -0,0 +1,148 @@
+/**
+ *    Copyright (C) 2020-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 <array>
+
+#include "mongo/db/logical_time.h"
+#include "mongo/db/operation_context.h"
+#include "mongo/db/service_context.h"
+#include "mongo/platform/mutex.h"
+#include "mongo/transport/session.h"
+
+namespace mongo {
+
+class VectorClockMutable;
+
+/**
+ * The VectorClock service provides a collection of cluster-wide logical clocks (including the
+ * clusterTime), that are used to provide causal-consistency to various other services.
+ */
+class VectorClock {
+public:
+    enum class Component : uint8_t {
+        ClusterTime = 0,
+        ConfigTime = 1,
+        _kNumComponents = 2,
+    };
+
+private:
+    template <typename T>
+    class ComponentArray
+        : public std::array<T, static_cast<unsigned long>(Component::_kNumComponents)> {
+    public:
+        const T& operator[](Component component) const {
+            invariant(component != Component::_kNumComponents);
+            return std::array<T, static_cast<unsigned long>(Component::_kNumComponents)>::
+            operator[](static_cast<unsigned long>(component));
+        }
+
+        T& operator[](Component component) {
+            invariant(component != Component::_kNumComponents);
+            return std::array<T, static_cast<unsigned long>(Component::_kNumComponents)>::
+            operator[](static_cast<unsigned long>(component));
+        }
+
+    private:
+        const T& operator[](unsigned long i) const;
+        T& operator[](unsigned long i);
+    };
+
+protected:
+    using LogicalTimeArray = ComponentArray<LogicalTime>;
+
+public:
+    class VectorTime {
+    public:
+        LogicalTime operator[](Component component) const {
+            return _time[component];
+        }
+
+    private:
+        friend class VectorClock;
+
+        explicit VectorTime(LogicalTimeArray time) : _time(time) {}
+
+        const LogicalTimeArray _time;
+    };
+
+    // Decorate ServiceContext with VectorClock* which points to the actual vector clock
+    // implementation.
+    static VectorClock* get(ServiceContext* service);
+    static VectorClock* get(OperationContext* ctx);
+
+    static void registerVectorClockOnServiceContext(ServiceContext* service,
+                                                    VectorClock* vectorClock);
+
+    VectorTime getTime() const;
+
+    // Gossipping
+    void gossipOut(BSONObjBuilder* outMessage,
+                   const transport::Session::TagMask clientSessionTags) const;
+    void gossipIn(const BSONObj& inMessage, const transport::Session::TagMask clientSessionTags);
+
+    bool isEnabled() const;
+    void disable();
+
+protected:
+    VectorClock();
+    virtual ~VectorClock();
+
+    static std::string _componentName(Component component);
+
+    // Internal Gossipping API
+    virtual void _gossipOutInternal(BSONObjBuilder* out) const = 0;
+    virtual void _gossipOutExternal(BSONObjBuilder* out) const = 0;
+    virtual LogicalTimeArray _gossipInInternal(const BSONObj& in) = 0;
+    virtual LogicalTimeArray _gossipInExternal(const BSONObj& in) = 0;
+
+    void _gossipOutComponent(BSONObjBuilder* out, VectorTime time, Component component) const;
+    void _gossipInComponent(const BSONObj& in, LogicalTimeArray* newTime, Component component);
+
+    // Used to atomically advance the time of several components (eg. during gossip-in).
+    void _advanceTime(LogicalTimeArray&& newTime);
+
+    ServiceContext* _service{nullptr};
+
+    // The mutex protects _vectorTime and _isEnabled.
+    //
+    // Note that ConfigTime is advanced under the ReplicationCoordinator mutex, so to avoid
+    // potential deadlocks the ReplicationCoordator mutex should never be acquired whilst the
+    // VectorClock mutex is held.
+    mutable Mutex _mutex = MONGO_MAKE_LATCH("VectorClock::_mutex");
+
+    LogicalTimeArray _vectorTime;
+    bool _isEnabled{true};
+
+private:
+    class GossipFormat;
+};
+
+}  // namespace mongo