SERVER-45816 Write sampling function for command mirroring

1 files changed, 66 insertions, 0 deletions

diff --git a/src/mongo/db/mirroring_sampler.h b/src/mongo/db/mirroring_sampler.h
new file mode 100644
index 00000000000..cdda905692e
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+++ b/src/mongo/db/mirroring_sampler.h
@@ -0,0 +1,66 @@
+/**
+ *    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 <functional>
+#include <memory>
+#include <random>
+#include <vector>
+
+#include "mongo/db/repl/is_master_response.h"
+#include "mongo/util/net/hostandport.h"
+
+namespace mongo {
+
+/**
+ * Populates a random subset of eligible secondaries (using `IsMasterResponse`
+ * and a ratio) for mirroring. An empty subset for eligible secondaries indicates
+ * no mirroring is necessary/possible.
+ */
+class MirroringSampler final {
+public:
+    using RandomFunc = std::function<int()>;
+
+    /**
+     * Sampler function for mirroring commands to eligible secondaries.
+     * The caller must be the primary node of the replica-set.
+     * @arg isMaster  replica-set topology that determines mirroring targets.
+     * @arg ratio  the mirroring ratio (must be between 0 and 1 inclusive).
+     * @arg rnd  the random generator function (default is `std::rand()`).
+     * @arg rndMax  the maximum value that `rnd()` returns (default is `RAND_MAX`).
+     */
+    static std::vector<HostAndPort> getMirroringTargets(
+        std::shared_ptr<const repl::IsMasterResponse> isMaster,
+        const double ratio,
+        RandomFunc rnd = std::rand,
+        const int rndMax = RAND_MAX) noexcept;
+};
+
+}  // namespace mongo