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Characteristics of Distributed databases

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• Introduction
  • Distributed database:

    Distributed database = a database whose data (relations) are stored on multiple autonomous sites

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  • Minimum requirements to be called distributed:

    Nodes (sites) are interconnected by a computer network (not a LAN (local area network)) Each node (site) maintains a database        Data stored at different sites are different The various databases (stored at the sites) are logically related Nodes (computers) at the sites can be heterogeneous (= different types of computers)

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  • Example: bank

    A bank has many branches Each branch keeps a database of accounts at that branch A central branch office keep bank employee records and fee/interest rate information

    Graphically:

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• Logical relations and physical relations
  • Consider the accounts of a bank that are stored in different branches:

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  • Physically speaking:

    There are multiple physical relations stored at different sites

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  • Logically speaking:

    There is only one logical relation that store information of the bank accounts of the bank

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• Tuple storage in distributed database vs. shared-nothing parallel system
  • Recall:

    tuple storage method in shared-nothing parallel system is used to facilitate parallel execution of queries Tuples are hashed and stored at the site with the hash ID

    Result:

    Duplicate tuples will be stored at the same site Many relational algebra operations can be performed locally without using other fragments

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  • Distributed Database:

    Each node/site in a distributed database system is autonomous !!! Tuple storage policy is determined at the site of "creation" Different sites (usually) do not coorperate in tuple storage

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• Improving data availability with distributed system: Data Replication
  • System failures:

    When some part of a system fails, the data stored on the failed part becomes inaccessible Example:

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  • Fault Tolerance:

    Distributed Databases can often tolerate (a limited number of) system (e.g., site) failures The commonly used method to achieve fault tolerance is data replication: Every relation is stored on multiple sites

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  • Note:

    The different copies must be consistent We must use additional replica control protocols to keep the multiple copies of data consistent

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• Advantages of distributed databases
  • Some important advantages of distributed databases:

    Increased availability With multiple copies of the data stored (replicated) at different sites, the distributed database can tolerate some system failures Improved performance: Most operations are local: The operation can be processed locally without communicating with other sites Improved scalability: Distributed databases can easily expand by adding another site (while communication cost will not increase much due to mostly local operations)