Pipeline (software)

In software engineering, a pipeline consists of a chain of processes or other data processing entities, arranged so that the output of each element of the chain is the input of the next. Usually some amount of buffer is provided between consecutive elements. The information that flow in these pipelines are often byte streams or bit streams.

The concept is also called the pipes and filters design pattern. It was named by analogy to a physical pipeline.

Multiprocessed pipelines

Pipelines are most efficiently implemented in a multitasking operating system, by launching all processes at the same time, and automatically servicing the data read requests by each process with the data written by the upstream process. In this way, the CPU will be naturally switched among the processes by the scheduler so as to minimize its idle time.

Usually, read and write requests are blocking operations, which means that the execution of the source process, upon writing, is suspended until all data could be written to the destination process, and, likewise, the execution of the destination process, upon reading, is suspended until at least some of the requested data could be obtained from the source process. Obviously, this cannot lead to a deadlock, where both processes would wait indefinitely for each other to respond, since at least one of the two processes will soon thereafter have its request serviced by the operating system, and continue to run.

For performance, most operating systems implementing pipes use pipe buffers, which allow the source process to provide more data than the destination process is currently able or willing to receive. Under most Unices and Unix-like operating systems, a special command is also available which implements a pipe buffer of potentially much larger and configurable size, typically called "buffer". This command can be useful if the destination process is significantly slower than the source process, but it is anyway desired that the source process can complete its task as soon as possible. E.g., if the source process consists of a command which reads an audio track from a CD and the destination process consists of a command which compresses the waveform audio data to a format like OGG Vorbis. In this case, buffering the entire track in a pipe buffer would allow the CD drive to spin down more quickly, and enable the user to remove the CD from the drive before the encoding process has finished.

Such a buffer command can be implemented using available operating system primitives for reading and writing data. Wasteful active waiting can be avoided by using facilities such as poll or select or multithreading.

VM/CMS and MVS

CMS Pipelines is a port of the pipeline idea to VM/CMS and MVS systems. It supports much more complex pipeline structures than Unix shells, with steps taking multiple input streams and producing multiple output streams. (Such functionality is supported by the Unix kernel, but few programs use it.) Due to the different nature of IBM mainframe operating systems, it implements many steps inside CMS Pipelines which in Unix are separate external programs, but can also call separate external programs for their functionality. Also, due to the record-oriented nature of files on IBM mainframes, pipelines operate in a record-oriented, rather than stream-oriented manner.

Pseudo-pipelines

On single-tasking operating systems, the processes of a pipeline have to be executed one by one in sequential order; thus the output of each process must be saved to a temporary file, which is then read by the next process. Since there is no parallelism or CPU switching, this version is called a "pseudo-pipeline".

For example, the command line interpreter of MS-DOS ('COMMAND.COM') provides pseudo-pipelines with a syntax superficially similar to that of Unix pipelines. The command "dir | sort | more" would have been executed like this (albeit with more complicated temporary file names):

Create temporary file 1.tmp
Run command "dir", redirecting its output to 1.tmp
Create temporary file 2.tmp
Run command "sort", redirecting its input to 1.tmp and its output to 2.tmp
Run command "more", redirecting its input to 2.tmp, and presenting its output to the user
Delete 1.tmp and 2.tmp, which are no longer needed
Return to the command prompt

Thus, pseudo-pipes acted like true pipes with a pipe buffer of unlimited size (disk space limitations notwithstanding), with the significant restriction that a receiving process could not read any data from the pipe buffer until the sending process finished completely. Besides causing disk traffic that would have been unnecessary under multi-tasking operating systems, this implementation also made pipes unsuitable for applications requiring real-time response, like, for example, interactive purposes (where the user enters commands that the first process in the pipeline receives via stdin, and the last process in the pipeline presents its output to the user via stdout).

Also, commands that produce a potentially infinite amount of output, such as the yes command, cannot be used in a pseudo-pipeline, since they would run until the temporary disk space is exhausted, so the following processes in the pipeline could not even start to run.

Pipelines in GUIs

Graphical environments such as RISC OS and ROX Desktop also make use of pipelines. Rather than providing a save dialog box containing a file manager to let the user specify where a program should write data, RISC OS and ROX provide a save dialog box containing an icon (and a field to specify the name). The destination is specified by dragging and dropping the icon. The user can drop the icon anywhere an already-saved file could be dropped, including onto icons of other programs. If the icon is dropped onto a program's icon, it's loaded and the contents that would otherwise have been saved are passed in on the new program's standard input stream.

For instance, a user browsing the world-wide web might come across a .gz compressed image which they want to edit and re-upload. Using GUI pipelines, they could drag the link to their de-archiving program, drag the icon representing the extracted contents to their image editor, edit it, open the save as dialog, and drag its icon to their uploading software.

Conceptually, this method could be used with a conventional save dialog box, but this would require the users programs to have an obvious and easily-accessible location in the filesystem that can be navigated to. In practice, this is often not the case, so GUI pipelines are rare.

Other considerations

A pipeline only allows information to flow in one direction, like water flows in a pipe.

Pipes and filters can be viewed as a form of functional programming, using byte streams as data objects.

The concept of pipeline is also central to the Cocoon web development framework where it allows a source stream to be modified before eventual display.

The pipeline concept is also used by data mining tools such as TextPipe.

This pattern encourages the use of text streams as the input and output of programs. This reliance on text has to be accounted when creating graphic shells to text programs. See XMLTerm for an approach to this problem.

History

Process pipelines were invented by Douglas McIlroy, one of the designers of the first Unix shells, and greatly contributed to the popularity of that operating system. It can be considered the first non-trivial instance of software componentry.

The idea was eventually ported to other operating systems, such as DOS, OS/2, Windows NT, BeOS, and Mac OS X.