javlyne Architectures.docx - Architectures These software architectures tell us how the various software components are to be organized and how they

javlyne Architectures.docx - Architectures These software...

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Architectures These software architectures tell us how the various software components are to be organized and how they should interact. In this chapter we will first pay attention to some commonly applied approaches toward organizing (distributed) computer systems. The actual realization of a distributed system requires that we instantiate and place software components on real machines. There are many different choices that can be made in doing so. The final instantiation of a software architecture is also referred to as a system architecture . Architectural styles The notion of an architectural style is important. Such a style is formulated in terms of components, the way that components are connected to each other, the data exchanged between components, and finally how these elements are jointly configured into a system. A component is a modular unit with well- defined required and provided interfaces that is replaceable within its environment. Several styles have by now been identified, of which the most important ones for distributed systems are: 1. Layered architectures 2. Object-based architectures 3. Data-centered architectures 4. Event-based architectures Layered architectures The basic idea for the layered style is simple: components are organized in a layered fashion where a component at layer Li is allowed to call components at the underlying layer Li 􀀀1, but not the other way around, as A key observation is that control generally flows from layer to layer: requests go down the hierarchy whereas the results flow upward. object-based architectures In essence, each object corresponds to what we have defined as a component, and these components are connected through a (remote) procedure call mechanism.
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Data-centered architectures Evolve around the idea that processes communicate through a common (passive or active) repository. It can be argued that for distributed systems these architectures are as important as the layered and object-based architectures. For example, a wealth of networked applications have been developed that rely on a shared distributed file system in which virtually all communication takes place through files. Likewise, Web-based distributed systems, are largely data-centric: processes communicate through the use of shared Web-based data services. In event-based architectures , processes essentially communicate through the propagation of events, which optionally also carry data, for distributed systems, event propagation has generally been associated with what are known as publish/subscribe systems. The basic idea is that processes publish events after which the middleware ensures that only those processes that subscribed to those events will receive them. The main advantage of event-based systems is that processes are loosely coupled. In principle, they need not explicitly refer to each other. This is also referred to as being decoupled in space, or referentially decoupled .
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Event-based architectures can be combined with data-centered architectures, yielding what is
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  • Spring '16
  • Magu
  • Client-server

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