[#28,!20] Added server selector to the config_backend.

Also, modfied config backend pool functions to support variable number
of parameters.

Author: msiodelski

src/lib/config_backend/base_config_backend.h

@@ -9,11 +9,118 @@
 
 #include <boost/shared_ptr.hpp>
 #include <cstdint>
+#include <set>
 #include <string>
 
 namespace isc {
 namespace cb {
 
+/// @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 patrticular server.
+///
+/// When fetching the configuration data from a databse 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.
+class ServerSelector {
+public:
+
+    /// @brief Type of the server selection.
+    enum class Type {
+        UNASSIGNED,
+        ALL,
+        SUBSET
+    };
+
+    /// @brief Factory returning "unassigned" server selector.
+    static ServerSelector& UNASSIGNED() {
+        static ServerSelector selector(Type::UNASSIGNED);
+        return (selector);
+    }
+
+    /// @brief Factory returning "all servers" selector.
+    static ServerSelector& ALL() {
+        static 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) {
+        static ServerSelector selector(server_tag);
+        return (selector);
+    }
+
+    /// @brief Factory returning "multiple servers" selector.
+    ///
+    /// @param server_tags set of server tags to be selected.
+    static ServerSelector& MULTIPLE(const std::set<std::string>& server_tags) {
+        static ServerSelector selector(server_tags);
+        return (selector);
+    }
+
+    /// @brief Returns type of the selector.
+    Type getType() const {
+        return (type_);
+    }
+
+    /// @brief Returns tags associated with the selector.
+    ///
+    /// @return server tags for mutliple selections and for one server,
+    /// empty set for all servers and and unassigned.
+    std::set<std::string> getTags() const {
+        return (tags_);
+    }
+
+private:
+
+    /// @brief Constructor used for "unassigned" and "all" slection types.
+    ///
+    /// @param type selector type.
+    explicit ServerSelector(const Type& type)
+        : type_(type), tags_() {
+    }
+
+    /// @brief Constructor used for selecting a single server.
+    ///
+    /// @param server_tag tag of the server to be selected.
+    explicit ServerSelector(const std::string& server_tag)
+        : type_(Type::SUBSET), tags_() {
+        tags_.insert(server_tag);
+    }
+
+    /// @brief Constructor used for selecting multiple servers.
+    ///
+    /// @param server_tags set of server tags.
+    explicit ServerSelector(const std::set<std::string>& server_tags)
+        : type_(Type::SUBSET), tags_(server_tags) {
+    }
+
+    /// @brief Selection type used.
+    Type type_;
+
+    /// @brief Holds tags of explicitly selected servers.
+    std::set<std::string> tags_;
+};
+
 /// @brief Base class for server specific configuration backends.
 class BaseConfigBackend {
 public: