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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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VII Other Topics
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23 Advanced Data Types and New Applications
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The McGraw Hill Companies, 2001
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User time is a highly valuable commodity in many business applications Connection time is the unit by which monetary charges are assigned in some cellular systems Number of bytes, or packets, transferred is the unit by which charges are computed in some digital cellular systems Time-of-day-based charges vary, depending on whether communication occurs during peak or off-peak periods Energy is limited Often, battery power is a scarce resource whose use must be optimized A basic principle of radio communication is that it requires less energy to receive than to transmit radio signals Thus, transmission and reception of data impose different power demands on the mobile host
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2353 Broadcast Data
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It is often desirable for frequently requested data to be broadcast in a continuous cycle by mobile support stations, rather than transmitted to mobile hosts on demand A typical application of such broadcast data is stock-market price information There are two reasons for using broadcast data First, the mobile host avoids the energy cost for transmitting data requests Second, the broadcast data can be received by a large number of mobile hosts at once, at no extra cost Thus, the available transmission bandwidth is utilized more effectively A mobile host can then receive data as they are transmitted, rather than consuming energy by transmitting a request The mobile host may have local nonvolatile storage available to cache the broadcast data for possible later use Given a query, the mobile host may optimize energy costs by determining whether it can process that query with only cached data If the cached data are insuf cient, there are two options: Wait for the data to be broadcast, or transmit a request for data To make this decision, the mobile host must know when the relevant data will be broadcast Broadcast data may be transmitted according to a xed schedule or a changeable schedule In the former case, the mobile host uses the known xed schedule to determine when the relevant data will be transmitted In the latter case, the broadcast schedule must itself be broadcast at a well-known radio frequency and at well-known time intervals In effect, the broadcast medium can be modeled as a disk with a high latency Requests for data can be thought of as being serviced when the requested data are broadcast The transmission schedules behave like indices on the disk The bibliographical notes list recent research papers in the area of broadcast data management
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2354 Disconnectivity and Consistency
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Since wireless communication may be paid for on the basis of connection time, there is an incentive for certain mobile hosts to be disconnected for substantial periods Mobile computers without wireless connectivity are disconnected most of the time
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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
VII Other Topics
23 Advanced Data Types and New Applications
The McGraw Hill Companies, 2001
23
Advanced Data Types and New Applications
when they are being used, except periodically when they are connected to their host computers, either physically or through a computer network During these periods of disconnection, the mobile host may remain in operation The user of the mobile host may issue queries and updates on data that reside or are cached locally This situation creates several problems, in particular: Recoverability: Updates entered on a disconnected machine may be lost if the mobile host experiences a catastrophic failure Since the mobile host represents a single point of failure, stable storage cannot be simulated well Consistency: Locally cached data may become out of date, but the mobile host cannot discover this situation until it is reconnected Likewise, updates occurring in the mobile host cannot be propagated until reconnection occurs We explored the consistency problem in 19, where we discussed network partitioning, and we elaborate on it here In wired distributed systems, partitioning is considered to be a failure mode; in mobile computing, partitioning via disconnection is part of the normal mode of operation It is therefore necessary to allow data access to proceed despite partitioning, even at the risk of some loss of consistency For data updated by only the mobile host, it is a simple matter to propagate the updates when the mobile host reconnects However, if the mobile host caches readonly copies of data that may be updated by other computers, the cached data may become inconsistent When the mobile host is connected, it can be sent invalidation reports that inform it of out-of-date cache entries However, when the mobile host is disconnected, it may miss an invalidation report A simple solution to this problem is to invalidate the entire cache on reconnection, but such an extreme solution is highly costly Several caching schemes are cited in the bibliographical notes If updates can occur at both the mobile host and elsewhere, detecting con icting updates is more dif cult Version-numbering-based schemes allow updates of shared les from disconnected hosts These schemes do not guarantee that the updates will be consistent Rather, they guarantee that, if two hosts independently update the same version of a document, the clash will be detected eventually, when the hosts exchange information either directly or through a common host The version-vector scheme detects inconsistencies when copies of a document are independently updated This scheme allows copies of a document to be stored at multiple hosts Although we use the term document, the scheme can be applied to any other data items, such as tuples of a relation The basic idea is for each host i to store, with its copy of each document d, a version vector that is, a set of version numbers {Vd,i [j]}, with one entry for each other host j on which the document could potentially be updated When a host i updates a document d, it increments the version number Vd,i [i] by one Whenever two hosts i and j connect with each other, they exchange updated documents, so that both obtain new versions of the documents However, before exchanging documents, the hosts have to discover whether the copies are consistent: 1 If the version vectors are the same on both hosts that is, for each k, Vd,i [k] = Vd,j [k] then the copies of document d are identical
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