1 Distributed Databases BUAD/American University Distributed Databases

2 BUAD/American University Definitions Distributed Database: A single logical database that is spread physically across computers in multiple locations (possibly global) that are connected by a data communications link. Decentralized Database: A collection of independent databases on non-networked computers. (possibly global)

3 Distributed Databases BUAD/American University Reasons for Distributed Database Local business units want control over data. Consolidate data across local databases for integrated decision making. Reduce telecommunications costs. Reduce the risk of telecommunications failures.

4 Distributed Databases BUAD/American University Distributed Database Options Homogeneous - Same DBMS at each node. Heterogeneous - Different DBMSs at different nodes. Systems - Supports some or all of the functionality of one logical database.

5 Distributed Databases BUAD/American University Homogeneous, Non-Autonomous Database Data is distributed across all the nodes. Same DBMS at each node. All data is managed by the distributed DBMS (no exclusively local data.) All access is through one, global schema. The global schema is the union of all the local schema.

6 Distributed Databases BUAD/American University Focus on The Following Heterogeneous Environment Data distributed across all the nodes. Different DBMSs may be used at each node. Local access is done using the local DBMS and schema. Remote access is done using the global schema.

7 Distributed Databases BUAD/American University Objectives and Trade-offs Location Transparency - User does not have to know the location of the data. Local Autonomy - Local site can operate with its database when central site is down. Synchronous Distributed Database - All copies of the same data are always identical. Asynchronous Distributed Database - Some data inconsistency is tolerated.

8 Distributed Databases BUAD/American University Advantages of Distributed Database Increased reliability and availability. Local control over data. Modular growth. Lower communication costs. Faster response for certain queries.

9 Distributed Databases BUAD/American University Disadvantages of Distributed Database Software cost and complexity. Processing overhead. Data integrity exposure. Slower response for certain queries.

10 Distributed Databases BUAD/American University Options for Distributing a Database Data replication. Horizontal partitioning. Vertical partitioning. Combinations of the above.

11 Distributed Databases BUAD/American University Data Replication Advantages - –Reliability. –Fast response. –May avoid complicated distributed transaction integrity routines (if replicated data is refreshed at scheduled intervals.) –De-couples nodes (transactions proceed even if some nodes are down.) –Reduced network traffic at prime time (if updates can be delayed.)

12 Distributed Databases BUAD/American University Data Replication Disadvantages - –Additional requirements for storage space. –Additional time for update operations. –Complexity and cost of updating. –Integrity exposure of getting incorrect data if replicated data is not updated simultaneously. Therefore, better when used for non-volatile data.

13 Distributed Databases BUAD/American University Types of Data Replication Snapshot Replication - –Changes are periodically sent to a master site which sends an updated snapshot out to the other sites. Near Real-Time Replication - –Broadcast update orders without requiring confirmation. Pull Replication - –Each site controls when it wants updates.

14 Distributed Databases BUAD/American University Issues in Data Replication Use Data timeliness. Useful if DBMS cannot reference data from more than one node. Batched updates can cause performance problems. Updates complicated with heterogeneous DBMSs or database design. Telecommunications speeds may limit mass updates.

15 Distributed Databases BUAD/American University Horizontal Partitioning Different records of a file at different sites. Advantages - –Data stored close to where it is used. –Local access optimization. –Security. Disadvantages –Accessing data across partitions. –No data replication.

16 Distributed Databases BUAD/American University Vertical Partitioning Different columns of a file at different sites. Advantages and disadvantages are the same as for horizontal partitioning except that combining data across partitions is more difficult because it requires joins.

17 Distributed Databases BUAD/American University Five Distributed Database Organizations ¬Centralized database, distributed access. ­Replication with periodic snapshot update. ®Replication with near real-time synchronization of updates. ¯Partitioned, one logical database. °Partitioned, independent, non-integrated segments.

18 Distributed Databases BUAD/American University Factors in Choice of Distributed Strategy Funding, autonomy, security. Site data referencing patterns. Growth and expansion needs. Technological capabilities. Costs of managing complex technologies. Need for reliable service.

19 Distributed Databases BUAD/American University Requirements for a Distributed DBMS Ability to locate data with a distributed data dictionary. Determine the location from which to retrieve data and the location at which to process each part of a distributed query. Heterogeneous DBMS translation. Security, concurrency, query optimization, failure recovery. Consistency of replicated data.