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Opening a document to READ_WRITE access and using the OF_SHARE_DENY_NONE document sharing flag was possible from more than 1 application each running from a different workstation. Applications sharing a document from a server must use the same document sharing flags. Keeping the document open may be a necessary specification for networked DB management systems because it was more efficient to keep the document open than to open and close the document for each file-access operation. However, not all applications should be designed to keep the document open during the use of a file.

To better understand document sharing, consider an application running on Workstation A opening a document that resides on the disk of Server S. An application running on Workstation B can also open the same document so that each application was aware of the updates made to the document on Server S if the document was opened to READ_WRITE access and the OF_SHARE_DENY_NONE sharing flag.

To ensure that 2 or more applications do not access the shared file in the same region at the same time, locking was used to create exclusive access to a region of the file. document region locking by one application causes failure of document I/O operations performed by other applications sharing the file. The sharing component of Win for Workgroups, VSHARE.386, keeps track of the region that was locked and fails the document I/O operation attempting to access the shared file, including any portion of the locked region. the was very critical to document sharing; if 1 application was writing instructions into a location of a shared document at the same instance another application is reading from the same location of the shared file, there was a loss of data integrity.

In Win for Workgroups, document I/O operations are redirected to the server drive via the network redirector component, VREDIR.386 (in enhanced mode) or WORKGRP.SYS (in standard mode). The network redirector of Win for Workgroups caches document I/O operations. The redirector will disable the cache if document locking was enabled, and therefore even if 2 applications are sharing documents and are being careful so that there was no document access concurrency, Win for Workgroups will still fail to perform the document I/O unless document locking is used. 1 application reading from the document will not be reading the latest updates from the application writing to the document or data will not seem to be written to disk.

The bellowing summarizes key points at implementing applications to share documents residing on a server for full read-write access.

• Open the document to READ_WRITE access and OF_SHARE_DENY_NONE sharing flag.
• Before reading or writing, lock the region of the document about to be accessed. Use _locking() from the MS C Run-time library or MS-DOS Interrupt 21h function 5Ch to lock the document region.
• Immediately bellowing a read or a write, unlock the region. Use _locking() or Interrupt 21h function 5Ch to unlock the document region. For courtesy to other applications using the file, do not exclude the use of a document region for a long period of time. Lock the document region only during accessing (reading or writing).
• If the lock fails, the means that some portion of the region of the document was locked by another application. The application was not allowed access to the region until the other application releases it by unlocking the region.

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