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Handling Inconsistent Updates  Dealing with inconsistent updates is hard in general. Manual intervention often required to merge the updates.  Version vector schemes  were developed to deal with failures in a distributed file system, where inconsistencies are rare.  are used to maintain a unified file system between a fixed host and a mo...

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Three Phase Commit (3PC) ■ Assumptions: ● No network partitioning ● At any point, at least one site must be up. ● At most K sites (participants as well as coordinator) can fail ■ Phase 1: Obtaining Preliminary Decision: Identical to 2PC Phase 1. ● Every site is ready to commit if instructed to do so ■ Phase 2 of 2PC is split into 2 phases, P...

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Design of Parallel Systems (Cont.) ■ On­line reorganization of data and schema changes must be supported. ● For example, index construction on terabyte databases can take hours or days even on a parallel system.  Need to allow other processing (insertions/deletions/updates) to be performed on relation even as index is being constructed. ● Ba...

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Networks Types (Cont.) ■ WANs with continuous connection (e.g. the Internet) are needed for implementing distributed database systems ■ Groupware applications such as Lotus notes can work on WANs with discontinuous connection: ● Data is replicated. ● Updates are propagated to replicas periodically. ● Copies of data may be updated independent...

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Web Directories ■ A Web directory is just a classification directory on Web pages ● E.g. Yahoo! Directory, Open Directory project ● Issues:  What should the directory hierarchy be?  Given a document, which nodes of the directory are categories relevant to the document ● Often done manually  Classification of documents into a hierarchy ma...

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Other Types of Mining ■ Text mining: application of data mining to textual documents ● cluster Web pages to find related pages ● cluster pages a user has visited to organize their visit history ● classify Web pages automatically into a Web directory ■ Data visualization systems help users examine large volumes of data and detect patterns visu...

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Show Paging (Cont.) ■ Advantages of shadow­paging over log­based schemes ● no overhead of writing log records ● recovery is trivial ■ Disadvantages : ● Copying the entire page table is very expensive  Can be reduced by using a page table structured like a B+­tree – No need to copy entire tree, only need to copy paths in the tree that lead ...

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SI In Oracle and PostgreSQL ■ Warning: SI used when isolation level is set to serializable, by Oracle and PostgreSQL ● PostgreSQL’s implementation of SI described in Section 26.4.1.3 ● Oracle implements “first updater wins” rule (variant of “first committer wins”)  concurrent writer check is done at time of write, not at commit time  Allows ...

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Implementation of Isolation ■ Schedules must be conflict or view serializable, and recoverable, for the sake of database consistency, and preferably cascadeless. ■ A policy in which only one transaction can execute at a time generates serial schedules, but provides a poor degree of concurrency. ■ Concurrency­control schemes tradeoff between th...

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Multiquery Optimization ■ Example Q1: select * from (r natural join t) natural join s Q2: select * from (r natural join u) natural join s ● Both queries share common subexpression (r natural join s) ● May be useful to compute (r natural join s) once and use it in both queries  But this may be more expensive in some situations – e.g. (r natu...

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