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+#ifndef DATABASE_H_0A27B356
+#define DATABASE_H_0A27B356
+
+#include <functional>
+#include <map>
+#include <mutex>
+#include <optional>
+#include <random>
+#include <string>
+#include <vector>
+
+class database {
+ public:
+  // Feel free to throw an exception if the subscriber should be killed.
+  using subscribe_callback_type = std::function<void(const std::string&)>;
+
+  std::string create(std::mt19937& rng);
+
+  void subscribe(const std::string& token, subscribe_callback_type callback);
+  void post(const std::string& token, const std::string& msg);
+
+  void setResult(const std::string& token, const std::string& result);
+  const std::string& getResult(const std::string& token);
+
+  void mark_done(const std::string& token);
+  bool is_done(const std::string& token);
+
+ private:
+  struct request {
+    std::vector<std::optional<subscribe_callback_type>> subscribers;
+    std::string result;
+    bool done = false;
+  };
+
+  std::mutex mutex_;
+
+  std::map<std::string, request> requests_;
+};
+
+#endif /* end of include guard: DATABASE_H_0A27B356 */