Distributed Databases Questions

• An increasingly popular option for data distribution as well as for fault tolerance of any database is to store a separate copy of the database at each of two or more sites.
• Asynchronous technology can result in unsatisfactorily slow response time because the distributed DBMS is spending considerable time checking that an update is accurately and completely propagated across the network.
• In a semijoin, only the joining attribute is sent from one site to another, and then only the required rows are returned.
• With failure transparency, all of the actions of a transaction are committed or none of them are committed.
• Data replication is favored where most process requests are read-only and where the data are relatively static.
• With asynchronous technology, if any copy of a data item is updated anywhere on the network, the same update is immediately applied to all other copies or it is aborted.
• The semijoin approach saves network traffic.
• Replication may use either synchronous or asynchronous distributed database technologies, although asynchronous technologies are more typical in a replicated environment.
• The cost to perform a snapshot refresh may depend on whether the snapshot is simple or complex.
• Each site (or node) in a distributed system is subject to the same types of failure as in a centralized system.