ABCSG - Distributed Database 1 Data Management Distributed Database Data Replication

2ABCSG - Distributed Database Distributed Database Distributed Database (DDB) - collection of multiple, logically interelated databases distributed over a computer network. Distributed Database Management System (DDBMS) – software system that permits the management of the DDB and makes the distribution transparent to the users. A distributed database

3ABCSG - Distributed Database DDB - Motivation Sharing data – users at one site may be able to access the data residing at other sites. Sharing data – users at one site may be able to access the data residing at other sites. Autonomy – each site is able to retain a degree of control over data that is locally stored. Autonomy – each site is able to retain a degree of control over data that is locally stored. Availability – if one site fails in a distributed system, the remaining sites may be able to continue operating. Availability – if one site fails in a distributed system, the remaining sites may be able to continue operating.

4ABCSG - Distributed Database DDB – Client-Server Architecture Client (query site) – dispatch the SQL queries over to the servers for processing. Client (query site) – dispatch the SQL queries over to the servers for processing. Server (data site) – manages the database and handles user queries that are passed on by the clients. Server (data site) – manages the database and handles user queries that are passed on by the clients.

5ABCSG - Distributed Database DDB – Peer-to-Peer Architecture A peer database management system (PDMS) consist of autonomous peers with mappings to other peers. Each site (peer) has equal funcionality for processing. Queries submitted at a peer are answered with data residing at that peer and by data that is reached along the path.

6ABCSG - Distributed Database DDB - Distributed Data Storage Given a relation r that is to be stored in the DB: Replication – the system maintains several identical replicas (copies) of the relation, and stores each replica at a different site. Replication – the system maintains several identical replicas (copies) of the relation, and stores each replica at a different site. Fragmentation – the system partitions the relation into several fragments, and stores each fragment at a different site. Fragmentation – the system partitions the relation into several fragments, and stores each fragment at a different site. Horizontal – subsets of tuples (rows) from the relation Horizontal – subsets of tuples (rows) from the relation Vertical – subsets of attributes (columns) from the relation Vertical – subsets of attributes (columns) from the relation Transparency – the user of a DB should not be required to know either where the user data are phisically or how the data can be accessed at the specific local site. Transparency – the user of a DB should not be required to know either where the user data are phisically or how the data can be accessed at the specific local site.

7ABCSG - Distributed Database DDB – Advantages & Disadvantages Advantages: Reflects organizational structure Reflects organizational structure Local autonomy Local autonomy Improved availability Improved availability Improved performance Improved performance Economics (hardware cost) Economics (hardware cost) Modularity ModularityDisadvantages: Complexity Complexity Economics (labour cost) Economics (labour cost) Security Security Difficulty to maintain integrity Difficulty to maintain integrity Inexperience Inexperience

8ABCSG - Distributed Database Distributed File Systems vs DDB  No explicit structure and the relationships among data in different files are not managed by the system.  Provide a simple interface to users which allows them to open, read/write (records or bytes), close files.  The user has to know (to some extent) the location of the data.  Organized according to a schema that defines both the structure and the relationships among the data.  Provides high-level, declaritive query capability, transaction management and integrity enforcement.  From a users perspective a DDB is logically a single database.

9ABCSG - Distributed Database Data Mining & Data Warehouse Key: decision support (activities) Data Mining aka Knowledge-Discovery in Databases Data Mining aka Knowledge-Discovery in Databases Process of automatically searching large volumes of data for patterns. Process of automatically searching large volumes of data for patterns. Data Warehouse Data Warehouse Repository of information gathered from multiple sources, stored under a unified schema, at a single site. Repository of information gathered from multiple sources, stored under a unified schema, at a single site. DB that collects, integrates and stores an organization's computer data with the aim of producing accurate and timely management information and supporting data analysis. DB that collects, integrates and stores an organization's computer data with the aim of producing accurate and timely management information and supporting data analysis.

10ABCSG - Distributed Database DDB – Mobile Devices Oracle Database Lite Synchronization Process Component Overview

11ABCSG - Distributed Database DDB – A Case Study Improving communication among the pregnant woman and all the healthcare professionals. Improving communication among the pregnant woman and all the healthcare professionals. Improving support for the pregnant woman (and her partner) in handling information. Improving support for the pregnant woman (and her partner) in handling information. IT Support for Pregnancies

12ABCSG - Distributed Database Discussion How could ABC benefit from the DDB features? Is there a need for relational data?

13ABCSG - Data Replication Data Replication Maintenance of copies of data at multiple computers Provides: High availability – elevated interest for mobile computing and consequently disconnected operation. High availability – elevated interest for mobile computing and consequently disconnected operation. Fault Tolerance – continuous concern for services provided in safety-critical and other important systems. Fault Tolerance – continuous concern for services provided in safety-critical and other important systems.

14ABCSG - Data Replication Data Replication - Motivation Performance enhancement Performance enhancement replication of immutable data replication of immutable data replication of changing data replication of changing data Increased availability Increased availability server failures server failures network partitions and disconnected operations network partitions and disconnected operations Fault tolerance Fault tolerance correctness correctness freshness of data freshness of data timeliness timeliness 1 – probability (all managers failed or unreachable) = 1 – p n

15ABCSG - Data Replication Data Replication - Requirements Transparency – clients should not be aware that multiple physical copies of data exist. Transparency – clients should not be aware that multiple physical copies of data exist. Consistency – operations performed upon replicas should produce results that meet the specification of correcteness (data integrity). Consistency – operations performed upon replicas should produce results that meet the specification of correcteness (data integrity).

16ABCSG - Data Replication Data Replication – System Model 1. Front-End – issues the request to one or more RMs 2. Coordination – RMs coordinate in preparation to execute the request consistently 3. Execution – RMs execute the request 4. Agreement – RMs reach consensus on the effect of the request 5. Response – one or more RMs responds to the FE C – Client FE – Front-End RM – Replica Manager A basic architectural model for the management of replicated data

17ABCSG - ABC Data Strategies ABC – Data Strategies

18ABCSG - ABC Data Strategies ABC – Replication Strategies