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Replication in Distributed Database
Replication in distributed databases enhances data availability by allowing data to be stored at multiple sites, with three main types: full, no, and partial replication. Full replication offers high availability and performance but can slow down updates, while no replication ensures all data is disjoint and non-redundant. Partial replication allows for flexibility in which fragments are copied based on performance and availability needs, but finding the optimal allocation remains a complex challenge.
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Discusses replication in distributed databases to improve data availability through full, partial, or no replication.
Details full replication, where entire databases are copied at all sites, enhancing availability but complicating updates.
Explains no replication, where database fragments are stored at one site only, preventing redundancy.
Describes partial replication, where some fragments are replicated, useful for mobile workers who sync with servers.
Explains the process of data distribution in replication, balancing performance, availability and transaction types.
Replication in Distributed Database
- 1. Replication in DDB Full, no, partial
- 2. Replication isuseful in improving the availability of data by coping data at multiple sites. Either a relation or a fragment can be replicated at one or more sites. Fully redundant databases are those in which every site contains a copy of the entire database. Depending on the availability and redundancy factor there are three types of replications: Full replication. No replication. Partial replication.
- 3. Full Replication The most extreme case is replication of the whole database at every site in the distributed system. This can improve availability remarkably because the system can continue to operate as long as at least one site is up. It also improves performance for retrieval of global queries as the result can be obtained locally at any client . Disadvantage: Slows the update process as a single update must be performed at different databases to keep the copies consistent.
- 4. No replication The other extreme from full replication involves having no replication—that is, each fragment is stored at exactly one site. In this case, all fragments must be disjoint, except for the repetition of primary keys among vertical (or mixed) fragments. This is also called nonredundant allocation.
- 5. Partial Replication Here some fragments of the database may be replicated whereas others may not. The number of copies of each fragment can range from one up to the total number of sites in the distributed system. For example: mobile workers—sales forces, financial planners, Carry partially replicated databases on their laptops and synchronize periodically with the server databases. A description of the replication of fragments is sometimes called a replication schema.
- 6. Each fragment—oreach copy of a fragment—must be assigned to a particular site in the distributed system. This process is called data distribution (or data allocation). The choice of sites and the degree of replication depend on the performance and availability goals of the system and on the types and frequencies of transactions submitted at each site. For example, if high availability is required, transactions can be submitted at any site, and most transactions are retrieval only, a fully replicated database is a good choice. However, if certain transactions that access particular parts of the database are mostly submitted at a particular site, the corresponding set of fragments can be allocated at that site only. Data that is accessed at multiple sites can be replicated at those sites. If many updates are performed, it may be useful to limit replication. Finding an optimal or even a good solution to distributed data allocation is a complex optimization problem.