Distributed Data Dictionary Management

Distributed Data Dictionary Management

Data Dictionary or Metadata Repository • System catalog Constitutes the data Dictionary. • The classical definition of metadata is “data about data”. • One of the major issues surrounding metadata is whether metadata should be centralized or decentralized.

Centralized Approach • The centralized architecture is the traditional approach to building a metadata repository. It offers efficient access to information, adaptability to additional data stores, scalability to capture additional metadata, and high performance. • Under this approach the system catalog is maintained at one of the participating sites in the distributed database. • This site acts as the central coordinator of the distributed data base management system. • However, like any other centralized architecture, centralized metadata repository is a single point of failure. It requires continuous synchronization with the participants of the data environment, may become a performance bottleneck, and may negatively affect quality of metadata. Indeed, the need to copy information from various applications and data stores into the central repository may compromise data quality if proper data validation procedures are not a part of the data acquisition process. • Moreover people who need to use and change local metadata do not want a central authority controlling their usage of metadata.

Distributed Approach • A distributed architecture avoids the concerns and potential errors of maintaining copies of the source metadata by accessing up-to-date metadata from all systems' metadata repositories in real time. • Distributed metadata repositories offer superior metadata quality since the users see the most current information about the data. • However, since distributed architecture requires real-time availability of all participating systems, a single system failure may potentially bring the metadata repository down. • Also, as source systems configurations change, or as new systems become available, a distributed architecture needs to adapt rapidly to the new environment, and this degree of flexibility may require a temporary shutdown of the repository.

Advantages of Distributed Approach • Each architectural entity has its own metadata • Autonomy of processing • Automatic update as each change is made locally • The technical support for local metadata is built into each product • Local metadata can be changed, on as needed basis with no interference from a centralized authority.

Disadvantages of Distributed Approach • There is a need for exchange of metadata. • There are no central agreed or standard. • Once local metadata has been passed to another location, there is the issue of keeping the metadata up to date .

Distributed Approach

Full Replication Approach

Partial Replication Approach

Full Replication Approach • Under this approach the complete catalog is maintained on all the sites this allows processing locally on all sites in a manner that the system catalog being locally available at each site has a greater degree of Autonomy. • One of the draw back is the storage overhead owing to greater redundancy. • The other draw back is the consistency problem that is how to keep the replicated copies of the system catalog on various sites synchronized.

Partial Replication Approach • Under this approach each site maintains a local catalog where information about the data base objects for which the corresponding site is the birth site is stored. • Additionally it also holds the information of the replicas and each site maintains a set of links to the database objects on the other site. • Whenever any site submits a Query if it can be handled using the local catalog its ok else the links are evaluated. • If the information is not available in the set of links than hunt for the data base object is made and the set of links is accordingly updated.