2.2 File formats

Office 2000 supports two native file formats: the traditional OLE-based binary format (hereafter, ``OLE archive'') and a new XML-based format. The OLE archives [13,14,15] rely on the OLE Structured Storage Interface (SSI) to provide a unified view of the compound document in a single file. SSI implements an abstraction similar to a file system within a single file. It supports two types of objects: storages and streams. Storages are analogous to directories and contain streams or more storages. Streams are analogous to files and contain the components' data. Office applications vary in the way they use the OLE SSI to store embedded objects. Word and Excel, for instance, use a separate storage for every embedded component, making the component structure of the document visible to the OLE SSI. For example, figure 1 shows the structure of a Word archive with two embedded components. Notice how Word keeps each embedded object in a separate SSI storage. In contrast, PowerPoint compresses embedded object native data and stores it in the main application stream. While this strategy increases document compression, it limits the ability of third-party applications to manipulate components within a PowerPoint document.

  

Figure 1: Word archive. The figure shows two embedded objects, an Excel chart and a PowerPoint slide, each stored in a separate SSI storage.

The new XML format [17] provides a more browser-friendly option for storing Office documents. While an OLE archive appears as a single file, an XML document appears as an entire directory of XML files, approximately one per component, image, or slide. The current implementation of Office supports two forms of XML output: a compact low-fidelity representation that can be read by browsers but cannot be edited by Office tools, and a larger high quality representation that supports editing. In this study we focus on the latter XML representation because it is semantically comparable to the OLE archive.

Aside from the number of files that they use, the two file formats differ mostly in their representation of text and formating information. Images and embedded component native data have similar representation in both formats, with the caveat that component data in the XML-based format is stored in a compressed OLE archive. Moreover, both formats keep in persistent storage two versions of the OLE components they embed. The first one consists of the embedded component's native data, which is used to initialize the component. This data is created and managed by the component itself. The second representation is a cached image of the state of the component the last time it was instantiated. This image, although created by the component, is managed by the container application. This image serves two purposes. First, it allows the document to be rendered quickly, since the code that understands the component's specific type need not be executed until the user wishes to modify the component. Second, the cached image allows the document to be rendered even on systems where some component types are not installed.

There is a significant difference in the way Office supports these two file formats. Office is able to load OLE archives incrementally over a random access file system. In contrast, XML documents must be read in their entirety before control is returned to the user, leading to higher latencies for opening and storing XML-based documents.