Simple server software, simple browsers, and a common set of IPs were all it seemed to take to make it work. With the first Web sites and the first Web browser, it became evident that the way we were to interact with information was rapidly changing. Web sites sprang up on standard servers that ran standard software. If the Internet brought a quiet and relatively slow revolution, the World Wide Web brought an explosive revolution. The consequences? Increased operating costs and insecurity were pervasive. More so, much infrastructure appeared to grow organically and was less planned than a garden of weeds. Again, we saw erosion in security as these conveniences made life simpler for all, including those who delighted in exploiting poor software and poor implementations. Where we were once limited to interacting with computers via direct-connected card readers and terminals, we experienced a great untethering, first via primitive modems, later with the Internet, and more recently with pervasive high-bandwidth networking and wireless. With client/server came advances in more user-friendly interfaces. Now the PC was connected by a more general purpose local area network or wide area network that had other uses as well. But this quickly changed as the power of the underlying PC client proved to make some local computation important for overall performance and increased functionality. Similar to transaction processing systems, client/server began with the commodity PC client simply performing input/output and the server ran the custom software. Initially, the client had no local storage and was connected to the server via a dedicated communications link. Airline reservation systems took this model and pushed connected clients to the far corners of the Earth. In this model, a single server performed computation and data storage while simpler client machines served for input and output. Transaction processing systems arose to meet the need for interaction by increasing numbers of people with a single database. Vic (J.R.) Winkler, in Securing the Cloud, 2011 Networking, the Internet, and the Web However, in this case, real-time applications generally make direct use of low-level synchronization primitives for mutual exclusion, rather than relying on a general-purpose synchronization mechanism that is hidden behind the transaction abstraction. When processing real-time inputs to shared data, the notion of serializability is as relevant as it is to TP. No special mechanisms, such as locking, are needed. Since the processing of two different inputs does not affect each other, even if they’re processed concurrently, they’ll behave like a serial execution. In most real-time applications, processing of input messages involves no access to shared data. Real-time systems are generally not concerned with serializability. Thus, the fault-tolerance requirements between the two types of systems are rather different. If there is a failure, it can stop collecting input, run a recovery procedure, and then resume processing input. By contrast, a TP environment can generally stop accepting input for a short time or can buffer the input for awhile. But the system certainly can’t stop operating to go back to fix things up like a TP system would do-the data keeps coming in and the system must do its best to continue processing it. It’s not good if the system misses some of the data coming in. To see why, consider the example of a system that collects input from a monitoring satellite. If they lose some of that input, they ignore the loss and keep on running. They simply process the input as quickly as they can. Real-time systems generally don’t need or use special mechanisms for atomicity and durability. Real-time systems usually emphasize gathering input rather than processing it, whereas TP systems generally do both.ĭue to the variety of real-world processes they control, real-time systems generally have to deal with more specialized devices than TP, such as laboratory equipment, factory shop floor equipment, or sensors and control systems in an automobile or airplane. Real-time systems and TP systems both have predictable loads with periodic peaks. So not surprisingly, there are many similarities between the two kinds of systems. It responds to a real-world process consisting of end-users interacting with display devices, which communicate with application programs accessing a shared database. TP essentially is a kind of real-time system, with a real-time response time demand of 1 to 2 seconds. TP systems are similar to real-time systems, such as a system collecting input from a satellite or controlling a factory’s shop floor equipment. Bernstein, Eric Newcomer, in Principles of Transaction Processing (Second Edition), 2009 Real-Time Systems
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