4. gdbm::ResourceDB

A gdbm::ResourceDB class is a protected subclass of the gdbm class. gdbm::ResourceDB hides the raw record-based GDBM interface, and replaces it with a slightly higher-level version. Although the keys are still arbitrary strings, instead of storing an arbitrary chunk of bytes, gdbm::ResourceDB stores a set of Dependency objects.

The gdbm::ResourceDB object stores zero or more Dependency objects in the underlying GDBM record, associated with a particular key, and retrieves them upon demand.

Well, not exactly. The gdbm::ResourceDB object does not actually store new records in the underlying GDBM file. The gdbm::ResourceDB object saves new records in an internal buffer. Subsequent requests for a record with the same key returns the saved Dependency objects.


An attempt to store an empty list of Dependency objects for a given key gets interpreted as a request to delete the underlying GDBM record. It follows that a request to retrieve the Dependency list of a nonexistent key returns an empty list.

The gdbm::ResourceDB object's commit method receives a FileInstall object, that actually records the updated records. Later, when the FileInstall gets commited, the GDBM file gets actually updated with the new content.


After storing the accumulated list of changed records in the FileInstall object, the internal buffer gets cleared and the underlying GDBM file gets closed.

This is because the file is presently opened in read-only mode (the updated records are not written to a file, they get saved in a buffer), and the same GDBM file is going to be opened in read-write mode very soon. This means that the read-only file descriptor must be closed (together with its corresponding lock file), so that the file can be reopened in read-write mode (after everything gets commited, if the same gdbm::ResourceDB object is accessed, it will automatically reopen the updated file).