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Recovery Techniques in Mobile Databases Prepared by Ammar Hamamra Introduction Mobile database system in a way is a system where the actual database is stored at one or multiple locations, but the processing units like laptop, PDA, cell phones are mobile. In case of failure, the recovery of the Mobile Database Systems is far more complex than the recovery of the traditional database systems. Factors that affect the recovery: Geographical Movement Disconnection Limited life of batteries The mobile device might also break, fall, gets lost, or stolen Bad connection areas Network issues like the weakness of wireless link Recovery schemes Overview of the traditional recovery schemes as well as one of the new recovery schemes Compares the two, listing the advantages and disadvantages of the schemes My opinion is that the new recovery scheme is more efficient to use in mobile databases Traditional Schemes Two main examples are: lazy scheme The pessimistic scheme The Lazy scheme: The lazy scheme uses the idea of pointers in the recovery process. The logs or the changes made to the data and the recovery information are stored in base stations. The lazy scheme When the mobile node moves from one base station to another, only the pointer is stored in the new base station to refer to the old one. Advantage: there is very low overhead on the network. Disadvantages: The large recovery time it needs More cost and missing deadlines. Pessimistic scheme The whole log information is transferred from one base station to another. Advantage: It is fast Disadvantage: It requires a lot of data transfer Traditional schemes The traditional schemes assume that the mobile unit is going to recover in the same base station that it was crashed in. The traditional schemes do not consider the highly dynamic environments such as the mobile database system. Traditional schemes There are other traditional schemes that fall between the lazy and pessimistic approaches. They have similar advantages and disadvantages. Factors that affect recovery Failure of the host. how much the mobile device moves in a given time. much information it needs to communicate with the host. the size of the message transmitted. message gets bigger, it needs additional power Factors that affect recovery Receiving a message is also different than transmitting a message. The memory issue Because a lot of mobile devices are related to the same base station, the memory of the base station becomes an issue trying to save all the recovery information for all the mobile devices. Recovery time. New scheme States that a good recovery method is a method that concentrates on the management and maintenance of the changes made to the data. Concentrates on the recovery of the mobile unit at the time of handoff. The changes made to data should be managed to help recover the mobile unit when it fails. New scheme The new system groups together a set of base stations and calls that a region. The new system uses the term designated Base Station (DBS) to refer to a station where the mobile node registers itself with at the first time in a particular region. New scheme Changes made to data by a particular mobile device are stored on that DBS. When the mobile unit moves from one region to another, it gets a new DBS. The changes made to data will then be transferred from the old DBS to the new DBS. New scheme The new system also requires that the data should be transferred from the cache of the mobile device itself to its DBS. The new system makes sure that the mobile device has its own identity, the identity of the old base station and the identity of the DBS New scheme The new system basically considers two situations for recovery. The first situation is when the mobile device recovers in the same BS where it crashed. The second situation is the big advantage that the new system has over the traditional ones. New scheme First situation: the mobile device uses the data and changes to data on that base station to recover. Second situation: If the mobile device fails in one base station and then moves to a new base station and try to recover there, then identity of the old base station is not known to the new base station. New scheme The old base station notifies the DBS about the data of the mobile device. The new base station then sends a query to the DBS for information about the mobile device and the data that was on it. Advantages of new scheme Maintaining the data on the previous base station so that there is an updated version incase of recovery. It avoids multiple copies of data by deleting a copy when the data gets transferred to the DBS. the data gets saved at one central place for fast recovery which reduces the recovery time Disadvantage of new scheme a lot of the base stations have the same DBS which means that the data will be stored on one DBS that gets overloaded sometimes. Conclusion the recovery process in mobile databases is more complex than the recovery in traditional databases. Two of the main examples on traditional schemes are the lazy and pessimistic approaches. Conclusion Some of the disadvantages that the traditional schemes have are high cost, large recovery time, or large data transfer. In my opinion and after the comparison, the new scheme is a better scheme. the mobile node can recover instantly in case of failure. Resources Ruchika Dua, Saurabh Bhandari, “Recovery in Mobile Database System”, International Conference on Wireless and Mobile Communications (ICWMC'06), July 2006 Sashidhar Gadiraju, Vijay Kumar, “Recovery in the Mobile Wireless Environment Using Mobile Agents”, IEEE Transactions on Mobile Computing, April 2004 Taesoon Park, Namyoon Woo, Heon Y. Yeom, “An Efficient Recovery Scheme for Mobile Computing Environments”, Eighth International Conference on Parallel and Distributed Systems (ICPADS'01) , June 2001 T. Park, N. Woo, H. Yeom, “Efficient Recovery Information Management Schemes for the Fault Tolerant Mobile Computing Systems”, 20th IEEE Symposium on Reliable Distributed Systems (SRDS'01), October 2001 Questions?
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