Data processing: database and file management or data structures – Database design – Data structure types
Patent
1997-07-03
1999-11-23
Kulik, Paul V.
Data processing: database and file management or data structures
Database design
Data structure types
707201, 3951821, 39518213, G06F 1700
Patent
active
059917717
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to the synchronization of transactions performed on separated disconnectable computers, such as transactions performed on a mobile computer and on a computer network while the mobile computer and the network are disconnected, or transactions performed on separate server computers in a network. More particularly, the present invention relates to the synchronization of transactions when the separate computers are reconnected.
TECHNICAL BACKGROUND OF THE INVENTION
It is often convenient, and sometimes essential, to carry a computer and selected data while traveling. It may also be convenient or essential to access a computer network using a "mobile computer" such as a laptop, palmtop, notebook, or personal digital assistant. However, different types of mobile computing make very different assumptions about the use and availability of computer networks.
Some mobile computers are not ordinarily connected to a computer network. Like their non-traveling "stand-alone" counterparts, such "walk-alone" computers cannot be connected to a network unless significant hardware or software modifications are made to them or to the network.
"Mobile-link" portable computers are typically connected to a computer network and attempt (with varying degrees of success) to maintain that network connection during mobile use through a wireless link. Typical wireless links use radio waves or infrared light as carriers. Mobile-link computers can be used in a walk-alone mode if the network connection is lost. However, mobile-link systems provide few or no automatic facilities to synchronize the mobile-link computer with the network when the connection is re-established.
"Disconnectable" computers include portable computers that operate in either a walk-alone or a mobile-link mode and provide significant automated facilities for synchronizing operations performed on the mobile computer with operations performed on the network. Disconnectable computers need not be portable. For instance, separate server computers in a wide-area network (WAN) or other network that are connected to one another only sporadically or at intervals may be disconnectable computers.
Unfortunately, conventional disconnectable computers still rely routinely on manually directed file copying to select the data that will be used in the field. Moreover, conventional disconnectable computer systems are not easily extended to support a variety of database formats, and they do not properly handle the situation in which changes to the "same" data are made on both the portable computer and on a network computer during disconnected operation.
For instance, the Coda File System ("Coda") is a client-server system that provides limited support for disconnectable operation. To prepare for disconnection, a user may hoard data in a client cache by providing a prioritized list of files. On disconnection, two copies of each cached file exist: the original stored on the server, and a duplicate stored in the disconnected client's cache. The user may alter the duplicate file, making it inconsistent with the server copy. Upon reconnection, this inconsistency may be detected by comparing timestamps.
However, the inconsistency is detected only if an attempt is made to access one of the copies of the file. The Coda system also assumes that the version stored in the client's cache is the correct version, so situations in which both the original and the duplicate were altered are not properly handled. Moreover, the Coda synchronization mechanism is specifically tailored, not merely to file systems, but to a particular file system (a descendant of the Andrew File System). Coda provides no solution to the more general problem of synchronizing transactions in a distributed database that can include objects other than file and directory descriptors.
Some approaches to distributed database replication are not directed to mobile computing per se but do attempt to ensure consistency between widely separated replicas that collectively form the
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Collins Brian J.
Draper Stephen P. W.
Falls Patrick T.
Kulik Paul V.
Novell Inc.
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