Summary (Top)
See text Table 4.1 on page 126

Example (Top)

The architecture for one of the event processors that was shown in the architecture on the viewtype page is shown in Figure 5:

Figure 5: Architecture for Event Processor 2

Component Types (Top)

There are three basic types of components that participate in a pipe-and-filter processing chain:

• Pump (Producer)
  Produces data and puts it to an output port that is connected to the input end of a pipe.

• Filter
  Gets data from an inport port connected to the output end of a pipe, transforms the data, and puts it to an output port that is connected to the input end of a different pipe. A filter may have multiple input and/or output ports.

• Sink (Consumer)
  Gets data from the input port that is connected to the output end of a pipe and consumes the data.

In the abstract, a filter is a combination of a sink and a pump.

Connector Middleware (Top)

Some possible ways to implement pipes are:

• Procedure Invokation
  Filters that are running synchronously on a single processor may be connected by procedure invokations. The output port of a filter is a call to a putData method in the next filter. The input port of a filter is parameter set that is passed to its putData method. A consequence of this type of pipe is that one unit of data from the producer passes through the entire filter chain to the consumer before control returns to the produce to put the next data item.

  If threading is available, a buffered invokation pipe can be created with a filter running in one thread and the following filter running in another thread.

• Character and Byte Streams
  Many operating systems and some object oriented languages provide pipes that are streams of bytes. The software sockets that are used to implement connections between two (possibly remote) applications are basically two way streams of bytes.

Advantages (Top)

Some advantages of the pipe-and-connector style are [Bass98, p97] :

• Filters stand alone and can be treated as black boxes. This isolation of functionality helps to ensure quality attributes such as information hiding, high cohesion, modifiability, and reuse.

• Filters interact with other components in limited ways. This connection simplicity helps to ensure low coupling.

• Pipes and filters can be hierarchically composed. Higher order filters can be created from any combination of lower order pipes and filters.

• The construction of the pipe and filter sequence can often be delayed until runtime (late binding). This permits a controller component to tailor a process based on the current state of the applcation.

• Because the process performed by the filter is isolated from the other components in the system, it is relatively easy to run a pipe-and-filter system on parallel processors or in multiple threads on a single processor.

Disadvantages (Top)

Some disadvantages of the pipe-and-connector style are [Bass98, p97] :

• Because the problem is decomposed into sequential steps, a batch mentality is encouraged. It is difficult to create entire interactive applications using this style.

• Filters often force the data to be represented in the lowest common denominator, typically byte or character streams. This means that if processing must be based on information tokens (words, lines, records), every filter may introduce overhead for parsing and unparsing the data stream.

• If a filter can not produce any output until it has received all of its input, the filter will require a buffer of unlimited size. If fixed size buffers are used, the system could deadlock. A sort filter has this problem.