 interfaces to use on-memory hash database, on-memory tree
 database, hash database, B+ tree database, fixed-length database, and table database with the
 same API.  Before operations to store or retrieve records, it is necessary to connect the
 abstract database object to the concrete one.  The method `open' is used to open a concrete
 database and the method `close' is used to close the database.  To avoid data missing or
 corruption, it is important to close every database instance when it is no longer i