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diff --git a/src/mongo/db/session_killer.h b/src/mongo/db/session_killer.h
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+/**
+ * Copyright (C) 2017 MongoDB Inc.
+ *
+ * This program is free software: you can redistribute it and/or  modify
+ * it under the terms of the GNU Affero General Public License, version 3,
+ * as published by the Free Software Foundation.
+ *
+ * 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
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * 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 GNU Affero General 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 <boost/optional.hpp>
+#include <memory>
+#include <random>
+#include <vector>
+
+#include "mongo/base/status_with.h"
+#include "mongo/db/kill_sessions.h"
+#include "mongo/stdx/condition_variable.h"
+#include "mongo/stdx/functional.h"
+#include "mongo/stdx/mutex.h"
+#include "mongo/stdx/thread.h"
+#include "mongo/stdx/unordered_set.h"
+#include "mongo/util/net/hostandport.h"
+
+namespace mongo {
+
+/**
+ * The SessionKiller enforces a single thread for session killing for any given ServiceContext.
+ *
+ * The killer owns a background thread which actually does the work, and callers batch their kills
+ * together each round, before the killer starts its dispatching, after which they batch up for the
+ * next round.
+ *
+ * The KillFunc's kill function is passed in to its constructor, and parameterizes its behavior
+ * depending on context (mongod/mongos).
+ */
+class SessionKiller {
+public:
+    /**
+     * The Result of a call is either:
+     *
+     * Status::OK(), empty vector - we killed everything
+     * Status::OK(), filled vector - we killed something.  HostAndPort is filled with nodes we
+     *                               failed to kill on.
+     *
+     * !Status::OK() - The kill function itself failed.  I.e. we may have killed nothing.
+     *
+     * This contract has a helper in kill_sessions_common.h which adapts results for command
+     * implementations. (killSessionsCmdHelper)
+     */
+    using Result = StatusWith<std::vector<HostAndPort>>;
+    using UniformRandomBitGenerator = std::minstd_rand;
+
+    class Matcher {
+    public:
+        Matcher(KillAllSessionsByPatternSet&& patterns);
+
+        const KillAllSessionsByPatternSet& getPatterns() const;
+
+        const KillAllSessionsByPattern* match(const LogicalSessionId& lsid) const;
+
+    private:
+        KillAllSessionsByPatternSet _patterns;
+        LogicalSessionIdMap<const KillAllSessionsByPattern*> _lsids;
+        stdx::unordered_map<SHA256Block, const KillAllSessionsByPattern*, SHA256Block::Hash> _uids;
+        const KillAllSessionsByPattern* _killAll = nullptr;
+    };
+
+    /**
+     * A process specific kill function (we have a different impl in mongos versus mongod).
+     */
+    using KillFunc =
+        stdx::function<Result(OperationContext*, const Matcher&, UniformRandomBitGenerator* urbg)>;
+
+    /**
+     * The killer lives as a decoration on the service context.
+     */
+    static SessionKiller* get(ServiceContext* service);
+    static SessionKiller* get(OperationContext* opCtx);
+
+    /**
+     * This method binds the SessionKiller to the ServiceContext.
+     */
+    static void set(ServiceContext* ctx, std::shared_ptr<SessionKiller> sk);
+
+    explicit SessionKiller(ServiceContext* sc, KillFunc killer);
+    ~SessionKiller();
+
+    /**
+     * This is the api for killSessions commands to invoke the killer.  It blocks until the kill is
+     * finished, or until it fails (times out on all nodes in mongos).
+     */
+    std::shared_ptr<Result> kill(OperationContext* opCtx,
+                                 const KillAllSessionsByPatternSet& toKill);
+
+private:
+    /**
+     * This struct is a helper to default fill in the result, which is otherwise a little error
+     * prone.
+     */
+    struct ReapResult {
+        ReapResult();
+
+        std::shared_ptr<boost::optional<Result>> result;
+    };
+
+    void _periodicKill(OperationContext* opCtx, stdx::unique_lock<stdx::mutex>& lk);
+
+    KillFunc _killFunc;
+
+    stdx::thread _thread;
+
+    stdx::mutex _mutex;
+    stdx::condition_variable _callerCV;
+    stdx::condition_variable _killerCV;
+
+    UniformRandomBitGenerator _urbg;
+
+    ReapResult _reapResults;
+    KillAllSessionsByPatternSet _nextToReap;
+
+    bool _inShutdown = false;
+};
+
+}  // namespace mongo