Software Architecture
3.1. Pipes and Filters
In a pipe and filter style each component has a set of inputs and a set of outputs. A component reads streams of data on its inputs and produces streams of data on its outputs, delivering a complete instance of the result in a standard order. This is usually accomplished by applying a local transformation to the input streams and computing incrementally so output begins before input is consumed. Hence components are termed “filters”. The connectors of this style serve as conduits for the streams, transmitting outputs of one filter to inputs of another. Hence the connectors are termed “pipes”.
Among the important invariants of the style, filters must be independent entities: in particular, they should not share state with other filters. Another important invariant is that filters do not know the identity of their upstream and downstream filters. Their specifications might restrict what appears on the input pipes or make guarantees about what appears on the output pipes, but they may not identify the components at the ends of those pipes. Furthermore, the correctness of the output of a pipe and filter network should not depend on the order in which the filters perform their incremental processing—although fair scheduling can be assumed. (See [5] for an in-depth discussion of this style and its formal properties.) Figure 1 illustrates this style.
Common specializations of this style include pipelines, which restrict the topologies to linear sequences of filters; bounded pipes, which restrict the amount of data that can reside on a pipe; and typed pipes, which require that the data passed between two filters have a well-defined type.
[pic]
Figure 1: Pipes and Filters
A degenerate case of a pipeline architecture occurs when each filter processes all of its input data as a single entity.1 In this case the architecture becomes a “batch sequential” system. In these systems pipes no longer serve the function
In a pipe and filter style each component has a set of inputs and a set of outputs. A component reads streams of data on its inputs and produces streams of data on its outputs, delivering a complete instance of the result in a standard order. This is usually accomplished by applying a local transformation to the input streams and computing incrementally so output begins before input is consumed. Hence components are termed “filters”. The connectors of this style serve as conduits for the streams, transmitting outputs of one filter to inputs of another. Hence the connectors are termed “pipes”.
Among the important invariants of the style, filters must be independent entities: in particular, they should not share state with other filters. Another important invariant is that filters do not know the identity of their upstream and downstream filters. Their specifications might restrict what appears on the input pipes or make guarantees about what appears on the output pipes, but they may not identify the components at the ends of those pipes. Furthermore, the correctness of the output of a pipe and filter network should not depend on the order in which the filters perform their incremental processing—although fair scheduling can be assumed. (See [5] for an in-depth discussion of this style and its formal properties.) Figure 1 illustrates this style.
Common specializations of this style include pipelines, which restrict the topologies to linear sequences of filters; bounded pipes, which restrict the amount of data that can reside on a pipe; and typed pipes, which require that the data passed between two filters have a well-defined type.
[pic]
Figure 1: Pipes and Filters
A degenerate case of a pipeline architecture occurs when each filter processes all of its input data as a single entity.1 In this case the architecture becomes a “batch sequential” system. In these systems pipes no longer serve the function