Nomadic transaction management

The distinguishing characteristic for nomadic (mobile) computing is the wireless communication medium. This makes it possible for a fixed computer system to support a mobile user. The general nomadic multidatabase management system (NMDBMS) discussed in this article is a collection of autonomous databases connected to a fixed network. The respective DBMSs retain complete control over their data. Each database may be viewed as an independent site in the network. These databases operate in different environments. Thus, a NMDBMS may be viewed as a multidatabase system that supports mobile users. Users of the independent databases, called internal users, access these databases through their respective DBMSs. The execution of local transactions submitted by these local users is transparent to any global process. Users accessing more than one database, called external users, submit global transactions to the NMDBMS. The NMDBMS is a set of software modules existing on the fixed network that cooperate with each other. Together they project the illusion of a single database to the external user. A global transaction consists of a set of sub-transactions that need to be executed at different sites. The global transaction manager (GTM), a software component of the NMDBMS, manages the executions of the global transactions. Global transactions are allowed only limited access to the individual databases. Consistency and reliability can be achieved by enforcing the ACID (atomicity, consistency, isolation, durability) properties