Misconceptions about real-time databases
Misconceptions About Real-Time Databases atabases have become an integral part of many computer systems-ranging from complex systems that control air traffic, plant operations, and stock market transactions to general-purpose computing systems that run audio and video applications. Increasingly, computer systems-even general-purpose systems-are requiring real-time support, so it's not surprising to hear more about real-time databases. Unfortunately, there are many misconceptions about the
... ime aspects of databases. Ironically, the state of confusion that exists today about real-time databases parallels the confusion that existed a decade ago surrounding the differences between real-time and general-purpose computing. 1 We believe that a careful definition of real-time databases will help dispel these misconceptions and will encourage research efforts similar to those that have advanced real-time systems over this past decade. REAL-TIME DATABASES: SOME DEFINITIONS We must first note that a system using real-time data, such as sensor data, does not in itself constitute a realtime database system. Because a real-time database is by definition a database system, it has queries, schemas, transactions, commit protocols, concurrency control support, and storage management. In a real-time database system, timing constraints are associated with transactions, and data are valid for specific time intervals. 2, 3 The transaction timing constraints can be completion deadlines, start times, periodic invocations, and so on. It is not necessary that every transaction have a timing constraint, only that some do. In addition to transaction timing requirements, data has time semantics as well. Data such as sensor data, stock market prices, and locations of moving objects all have semantics indicating that the recorded values are valid only for a certain time interval. A real-time database makes this validity interval explicit as part of its database schema. We can define transaction correctness as a transaction meeting its timing constraints and using data that is absolutely and relatively timing-consistent. Absolute time consistency means that individual data items used by a transaction are still temporally valid and reflect the true state of the world to an acceptable degree of accuracy. Relative time consistency means that multiple data items used by a transaction are updated (sensed) within a specified time interval of each other. For example, if a transaction uses temperature and pressure data to make a decision regarding a chemical process, these two data values must correlate closely in time or the computation will likely make no sense. Cybersquare Some database users think "real-time" databases just need to be fast and that conventional databases are adequate for real-time applications. Real-time database designers don't agree.