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+// Copyright (C) 2018-2019,2021 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#ifndef SERVER_SELECTOR_H
+#define SERVER_SELECTOR_H
+
+#include <cc/server_tag.h>
+#include <set>
+#include <string>
+
+namespace isc {
+namespace db {
+
+/// @brief Server selector for associating objects in a database with
+/// specific servers.
+///
+/// Configuration information stored in the configuration backends can be
+/// associated with selected servers, all servers or no particular server.
+/// For example: a particular subnet definition in the database may be
+/// associated with one server or can be shared by multiple servers.
+/// In the latter case, a subnet may be associated with a subset of
+/// servers or all servers. An administrator may also add the
+/// configuration data into the database and do not associate this data
+/// with any particular server.
+///
+/// When fetching the configuration data from a database or when storing
+/// data in the database there is a need to specify which servers this
+/// data is associated with. The @c ServerSelector class represents
+/// such associations.
+///
+/// It includes three modes of selection: UNASSIGNED, ALL and SUBSET and
+/// several factory functions making associations described above.
+///
+/// The @c ServerSelector class should be used in objects derived from
+/// @c BaseConfigBackendPool and in objects derived from
+/// @c BaseConfigBackend to indicate which servers the specific calls
+/// exposed by these objects refer to.
+///
+/// @todo Add server selector for selecting only those configuration
+/// elements that are associated with all servers (and not with any
+/// particular server tags). Translating this to SQL queries it would
+/// probably be an empty or non-existing server tag.
+class ServerSelector {
+public:
+
+    /// @brief Type of the server selection.
+    enum class Type {
+        UNASSIGNED,
+        ALL,
+        SUBSET,
+        ANY
+    };
+
+    /// @brief Factory returning "unassigned" server selector.
+    static ServerSelector UNASSIGNED() {
+        ServerSelector selector(Type::UNASSIGNED);
+        return (selector);
+    }
+
+    /// @brief Factory returning "all servers" selector.
+    static ServerSelector ALL() {
+        ServerSelector selector(Type::ALL);
+        return (selector);
+    }
+
+    /// @brief Factory returning selector of one server.
+    ///
+    /// @param server_tag tag of the single server to be selected.
+    static ServerSelector ONE(const std::string& server_tag) {
+        ServerSelector selector((data::ServerTag(server_tag)));
+        return (selector);
+    }
+
+    /// @brief Factory returning "multiple servers" selector.
+    ///
+    /// @param server_tags set of server tags to be selected.
+    /// @throw InvalidOperation if no server tags provided.
+    static ServerSelector MULTIPLE(const std::set<std::string>& server_tags);
+
+    /// @brief Factory returning "any server" selector.
+    static ServerSelector ANY() {
+        ServerSelector selector(Type::ANY);
+        return (selector);
+    }
+
+    /// @brief Returns type of the selector.
+    Type getType() const {
+        return (type_);
+    }
+
+    /// @brief Returns tags associated with the selector.
+    ///
+    /// @return server tags for multiple selections and for one server,
+    /// empty set for all servers and and unassigned.
+    std::set<data::ServerTag> getTags() const {
+        return (tags_);
+    }
+
+    /// @brief Convenience method checking if the server selector has no tags.
+    ///
+    /// @return true when the server selector has no tags, false otherwise.
+    bool hasNoTags() const {
+        return (tags_.empty());
+    }
+
+    /// @brief Convenience method checking if the server selector is "unassigned".
+    ///
+    /// @return true if the selector is "unassigned", false otherwise.
+    bool amUnassigned() const {
+        return (getType() == Type::UNASSIGNED);
+    }
+
+    /// @brief Convenience method checking if the server selector is "all".
+    ///
+    /// @return true if the selector is "all", false otherwise.
+    bool amAll() const {
+        return (getType() == Type::ALL);
+    }
+
+    /// @brief Convenience method checking if the server selector is "any".
+    ///
+    /// @return true if the selector is "any", false otherwise.
+    bool amAny() const {
+        return (getType() == Type::ANY);
+    }
+
+    /// @brief Convenience method checking if the server selector has multiple tags.
+    ///
+    /// @return true if it has multiple tags, false otherwise.
+    bool hasMultipleTags() const {
+        return (tags_.size() > 1);
+    }
+
+private:
+
+    /// @brief Constructor used for "unassigned" and "all" selection types.
+    ///
+    /// @param type selector type.
+    explicit ServerSelector(const Type& type);
+
+    /// @brief Constructor used for selecting a single server.
+    ///
+    /// @param server_tag tag of the server to be selected.
+    explicit ServerSelector(const data::ServerTag& server_tag);
+
+    /// @brief Constructor used for selecting multiple servers.
+    ///
+    /// @param server_tags set of server tags.
+    explicit ServerSelector(const std::set<data::ServerTag>& server_tags);
+
+    /// @brief Selection type used.
+    Type type_;
+
+    /// @brief Holds tags of explicitly selected servers.
+    std::set<data::ServerTag> tags_;
+};
+
+
+} // end of namespace isc::db
+} // end of namespace isc
+
+#endif