Storages and Streams

COM provides a set of services collectively called structured storage. The purpose of these services is to reduce the performance penalties and overhead associated with storing separate objects in a flat file. Instead, COM stores the separate objects in a single, structured file consisting of two main elements: storage objects and stream objects. Together, they function like a file system within a file.

A storage object is analogous to a file system directory. Just as a directory can contain other directories and files, a storage object can contain other storage objects and stream objects. Also like a directory, a storage object tracks the locations and sizes of the storage objects and stream objects nested beneath it.

A stream object is analogous to the traditional notion of a file. Like a file, a stream contains data stored as a consecutive sequence of bytes.

A COM compound file consists of a root storage object containing at least one stream object representing its native data along with one or more storage objects corresponding to its linked and embedded objects. The root storage object maps to a filename in whatever file system it happens to reside in. Each of the objects inside the document also is represented by a storage object containing one or more stream objects, and perhaps also containing one or more storage objects. In this way, a document can consist of an unlimited number of nested objects.

Structured storage solves the performance problem because whenever a new object is added to a compound file, or an existing object increases in size, the file does not have to be totally rewritten to storage. Instead, the new data is written to the next available location in permanent storage, and the storage object updates the table of pointers it maintains to track the locations of its storage objects and stream objects. At the same time, structured storage enables end users to interact and manage a compound file as if it were a single file rather than a nested hierarchy of separate objects.

Structured storage also provides additional benefits:

• Incremental access. If a user needs access to an object within a compound file, the user can load and save only that object, rather than the entire file.
• Multiple use. More than one end user or application can concurrently read and write information in the same compound file.
• Transaction processing. Users can read or write to COM compound files in transacted mode, where changes made to the file are buffered and can subsequently either be committed to the file or reversed.
• Low memory saves. Structured storage provides facilities for saving files in low memory situations.