Data processing: database and file management or data structures – Database design – Data structure types
Patent
1996-11-12
1999-03-16
Black, Thomas G.
Data processing: database and file management or data structures
Database design
Data structure types
707 9, G06F 1730
Patent
active
058843086
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to data storage and more particularly to the storage of data files which may be subjected to updating at more than one location.
RELATED ART
In any organisation which has data processing capability at different sites, conflict can arise if common data is differently modified at one site compared to another.
A data file at site "A" may be transferred to site "B" and subsequent amendments at each site result in two completely different end files. This may result in different decisions being taken at the two sites based on what is believed to be the same data.
One way to avoid such conflict is for one site, say site "A" to be in absolute control of the data file, to be responsible for all amendments and data input to that file.
This complicates procedure. For example, if new data is to be added from site "B", the data must first be keyed at site "B", transferred to site "A" and checked and entered at site "A" before further processing can take place.
Alternatively, site "B" may have direct input capability to data stores at site "A" but this could result in users at both sites updating a data file at the same time resulting in an unrecognisable file output for either party. Further, such an arrangement requires the use of wide area network technology and high speed data links, which may add significant cost penalties. Incompatability of the processing systems at the two sites can also be a problem.
Making data file copies from a master library at "run-time" is one way to ensure that the latest available data file is available. However, if more than one copy is in use, being updated and potentially returned to the master library only amendments at one site may be recorded.
Such conflicts may have serious effects on multi-sited businesses, particularly if data entry on one site (perhaps sales entry) leads into activity on another site (such as purchasing or scheduling).
In published European patent application number 398496 a fileserver system is disclosed in which shared data files are accessible by a number of client machines. At any one time on-line access to a specific shared data file is controlled by the fileserver such that if a first client machine has write access no other machine has read or write access to that file. Further, the system disclosed in EP398496 prevents write access to data file if any client machine currently has read access. Thus, if any client wishes to update a data file, the fileserver must revoke all current read accesses (and receive an acknowledgement of revocation of the read token from each current client "reader") prior to permitting issue of a read/write token.
Such a system while efficacious for on-line access to a single fileserver site is not suitable for use where files may be required in a number of different locations or where data transfer may take a considerable time. Further, where data transfer between nodes is by way of virtual mail messages delay in acknowledgement of a read revocation may inhibit data update at the read/write requesting node.
SUMMARY OF THE INVENTION
According to the present invention there is provided a computer system having a plurality of nodes (A-G) at different locations, each node comprising one or more processor units (1) data storage (2) of the computer system containing common data files for which users at different locations have update access, each such common data file having a single respective data token which permits update access to the data file only at the node currently holding the respective data token, characterised in that the data storage comprises a respective data store (2) at each node (A-G), each data store (2) storing a respective copy of at least some common data files and for each common data file for which the system holds a plurality of copies one of the data stores is designated a master library, the master library retaining a record of the node currently holding the respective data token.
Preferably, data tokens are passed between nodes of the syst
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Black Thomas G.
British Telecommunications public limited company
Coby Frantz
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