OSI/FTAM Programming Guide

ISO FTAM Concepts
HP NonStop OSI/FTAM Programming Guide—528612-001
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Virtual Filestore
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Unconstrained service class
If none of the proposed service classes is acceptable to the responder, the
responder can reject the association. The responder also checks the list of
proposed functional units and removes from the list any functional units that it does
not support and any functional units not allowed by the selected service class. The
resulting list of functional units, along with the selected service class, is returned to
the initiator in the F-INITIALIZE response primitive.
3. The initiator checks the returned functional-unit list and limits subsequent protocol
to that supported by the negotiated functional units.
Virtual Filestore
Because file storage systems differ widely among computer manufacturers, OSI
defines a common model of the file storage system for describing files and their
attributes. This model is called the virtual filestore (VFS). Each FTAM implementation
maps this common virtual filestore to the real file storage system that supports it. This
mapping allows your application to access files on other systems.
The virtual filestore defines a structural model for all files on the file storage system, a
set of file-access actions that can be performed on those files, and a set of attributes
that describe each file type. These aspects of the VFS are described below.
Hierarchical File Model
OSI uses a general hierarchical file model to describe files. It defines document types,
which limit the contents of the file (for example, by specifying allowable data types). It
also uses constraint sets to define possible actions and other characteristics of file
access: for instance, whether a file must be read sequentially or can be read randomly,
and whether node descriptors (such as record numbers in a structured file) are
transferred along with the data.
To illustrate the wide range of possibilities for file structures, this subsection describes
the hierarchical file model in detail. However, remember that the typical FTAM
implementation is not as complex as this model implies. In the NonStop FTAM
implementation, for example, most of the supported document types consist of only a
root and a single node; only the NonStop FTAM-2 document type supports a root with
multiple nodes.
The hierarchical file model uses the tree structure shown in Figure 2-5 on page 2-23, in
which each node consists of an optional node name and zero or one data units (DU).
The number of levels in the hierarchy is unlimited. The model is generic enough that it
applies to a wide range of files from simple unstructured files, to flat files, to
hierarchical files.