TRANSFER Programming Manual

Interobject Relationships
TRANSFER Objects
40970 Tandem Computers Incorporated 3–9
An alternate external object that contains item data that is of interest to the local
node only and that is not transported to other nodes. Your application can attach
this object to either a TRANSFER item or a primary external object, creating what
is known as an alternate attachment. Alternate external objects allow you to avoid
making unnecessary copies of files or shipping files to nodes that do not require
them. They are used chiefly for attachments that add nothing new to the item or
primary external object, or that are reproducible from it. You can use them, for
instance, to attach a translated document to a primary external object that contains
the original.
For any particular item, the TRANSFER delivery system allows you to specify up to
9999 primary and alternate attachments.
When an external object is attached for the first time, it is assigned a unique ID. This
ID is similar to an item ID and identifies the object uniquely.
Interobject
Relationships
The use of network-unique IDs for TRANSFER objects provides for important logical
relationships among them and efficient use of disk storage space. When a user process
sends a package to a local correspondent, the TRANSFER delivery system does not
make a copy of the package; instead, the TRANSFER delivery system makes the
INBOX folder of that correspondent point to the package.
TRANSFER objects are stored in the following files:
The profiles of all correspondents defined at a particular node are stored in the
Profile file.
The contents of folders are described in the Folder file and the Inverted Folder file.
The names assigned to instances of items in folders are also maintained in these
files.
Records that describe packages and items, called item descriptors, are stored in the
Item Descriptor file. The relationship between an item (possibly a package) and its
component items is specified in the item descriptor.
The data records for the items are stored in the Item Data file.
External objects are referenced through the Item Descriptor file and the Extobjs
file.
Attachment relationships between items (such as parent-component relationships)
are maintained in the Inverted Attachment file.
A depot itself is simply a network-unique ID that appears in records of various
kinds, distributed over several TRANSFER files.
When an item is saved in multiple folders or even at multiple depots, pointers are also
used to avoid duplication of data. For example, an item could be saved in three
folders:
In the sender's depot, in a folder named MEMOS
In the recipient's depot, in the INBOX folder