COM Structured Storage

COM Structured Storage (variously also known as COM structured storage or OLE structured storage) is a technology developed by Microsoft as part of its Windows operating system for storing hierarchical data within a single file. Strictly speaking, the term structured storage refers to a set of COM interfaces that a conforming implementation must provide, and not to a specific implementation, nor to a specific file format (in fact, a structured storage implementation need not store its data in a file at all). In addition to providing a hierarchical structure for data, structured storage may also provide a limited form of transactional support for data access. Microsoft provides an implementation that supports transactions, as well as one that does not (called simple-mode storage, the latter implementation is limited in other ways as well, although it performs better).

Structured storage is widely used in Microsoft Office applications, although newer releases (starting with Office 2007) use the XML-based Office Open XML by default. It is also an important part of both COM and the related Object Linking and Embedding (OLE) technologies. Other notable applications of structured storage include SQL Server, the Windows shell, and many third-party CAD programs.

Structured storage addresses some inherent difficulties of storing multiple data objects within a single file. One difficulty arises when an object persisted in the file changes in size due to an update. If the application that is reading/writing the file expects the objects in the file to remain in a certain order, everything following that object's representation in the file may need to be shifted backward to make room if the object grows, or forward to fill in the space left over if the object shrinks. If the file is large, this could be a costly operation. Of course, there are many possible solutions to this difficulty, but often the application programmer does not want to deal with low level details such as binary file formats.

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