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Plan 9 from Bell Labs is a distributed operating system, primarily used as a research vehicle. It was developed as the research successor to Unix by the Computing Sciences Research Center at Bell Labs between the mid-1980s and 2002. Plan 9 is most notable for representing all system interfaces, including those required for networking and the user-interface, through the filesystem rather than specialized interfaces. Plan 9 aimed to provide users with a workstation-independent working environment through the use of the 9P protocols. Plan 9 continues to be used in some circles as a research operating system and by hobbyists.

The name "Plan 9 from Bell Labs" was coined from the science fiction movie Plan 9 from Outer Space.

History

Plan 9 replaced Unix at Bell Labs as the organization's primary platform for research and explores several changes to the original Unix model that improve the experience of using and programming the system, notably in distributed multi-user environments. Plan 9 was a Bell Labs internal project from its start during the mid 1980s. In 1992, the first public release was made available to universities. In 1995, a commercial second release version was made available to the general public. In the late 1990s, Lucent Technologies, who had inherited Bell Labs, dropped commercial interest for the project. In 2000, a non-commercial third release was made under an open source license. In 2002, a non-commercial fourth release was made under a new free software license.

A user and development community, including current and former Bell Labs and MIT members, continues to produce daily minor releases as ISO images. Bell Labs still hosts development. The development source tree is accessible over the 9P and HTTP protocols and are used to keep an installation up to date.{{cite web | year=2006 | url=http://cm.bell-labs.com/wiki/plan9/Staying_up_to_date/index.html | title=Staying up to date | publisher=Plan 9 community | accessdate=April 27 | accessyear=2006} -->

Overview

All resources as files

One of the key features adopted from Unix was the use of the file system to access resources. Before Unix, most operating systems had different mechanisms for accessing different types of devices. For example, the API to access a disk drive was vastly different from the API used to send and receive data from a serial port, which in turn was different from the API used to send data to a printer. Unix attempted to remove these distinctions. All device drivers were required to support meaningful read and write operations as a means of control. This lets programmers create utilities like mv and cp to send data from one device to another without being aware of the underlying implementation details.

However, at the time, many key concepts (such as the control of process state) did not seem to map neatly onto files. As new features like Berkeley sockets and the X Window System were added, they were incorporated to exist outside the file system. New hardware features (such as the ability to eject a CD in software) also encouraged the use of hardware-specific control mechanisms like the ioctl system call.

The Plan 9 research project rejected these different approaches. Each Plan 9 program views all available resources, including networking and the user-interface resources (like the window it is running in), as part of a hierarchical file system, rather than specialized interfaces.

Unicode support

Plan 9 uses Unicode throughout the system. UTF-8 was invented by Ken Thompson to be used as the native encoding in Plan 9 and the whole system was converted to use it everywhere in 1992.

Research team

Plan 9 from Bell Labs was developed by members of the Computing Science Research Center at Bell Labs, the same group that developed UNIX and C. The Plan 9 team was originally led by Rob Pike, Ken Thompson, Dave Presotto and Phil Winterbottom with support from Dennis Ritchie as head of the Computing Techniques Research Department. Over the years many notable developers have contributed to the project including Brian Kernighan, Tom Duff, Doug McIlroy, Bjarne Stroustrup and many others.{{cite web | last=McIlroy | first=Doug | year=1995 | url=http://cm.bell-labs.com/sys/man/preface.html | title=Preface to the Second (1995) Edition | publisher=Lucent Technologies | accessdate=April 2 | accessyear=2006} -->

Design concepts

Plan 9's designers were interested in goals similar to those of microkernels, but made different architecture and design choices to achieve them. Plan 9's design goals included:
* Resources as files: all resources are represented as files within a hierarchical file system
* Namespaces: the application view of the network is a single, coherent namespace that appears as a hierarchical file system but may represent physically separated (locally or remotely) resources
* Standard communication protocol: a standard protocol, called 9P, is used to access all resources, both local and remote

Filesystems, files, and names

Plan 9 extended the system beyond files to "names", that is, a unique path to any object whether it be a file, screen, user, or computer. All were handled using the existing Unix standards, but extended such that any object could be named and addressed (similar in concept to the more widely known URI system of the world wide web). In Unix, devices such as printers had been represented by names using software converters in /dev, but these addressed only devices attached by hardware, and did not address networked devices. Under Plan 9 printers were as virtualized as files, and both could be accessed over the network from any workstation.

Another Plan 9 innovation was the ability for users to have different names for the same "real world" objects. Each user could create a personalized environment by collecting various objects into their namespace. Unix has a similar concept in which users gain privileges by being copied from another user, but Plan 9 extends this to all objects. Users can easily spawn "clones" of themselves, modify them, and then remove them without affecting the resources from which they were created.

Union directories

Plan 9 also introduced the idea of union directories, directories that combine resources across different media or across a network, binding transparently to other directories. For example, another computer's /bin (applications) directory can be bound to one's own, and then this directory will hold both local and remote applications and the user can access both transparently. Unix links and filesystem mounts, made the original directory disappear. Using the same system, under Plan 9 external devices and resources can be bound to /dev, making all devices network devices without additional code.

/proc

To illustrate how these features work together to produce a greater whole, consider the rather interesting /proc directory, in which all running processes are listed. Processes are named objects (ie., files) under Plan 9, like most other system resources, and therefore it makes sense to list them in a directory, like other objects. This simple decision has useful side effects, allowing the user to use tools such as ls to search and sort the process list, which was previously available only through specialized tools. But of even greater interest, users can union remote processes into their namespace as well, interacting with them as if they are local, making network-wide processing almost trivial.

The result is a distributed computing environment assembled from separate machines — terminals that sit on users' desks, file servers that store permanent data, and other servers that provide faster CPUs, user authentication, and network gateways, all using the existing hierarchical directory/name system familiar to most computer users. A user can "build" a system by collecting up directories on fileservers, applications running on servers, printers on the network and then bind them all together into their personal namespace running on a terminal.

Networking and distributed computing

Plan 9 is based on UNIX but was developed to demonstrate the concept of making communication the central function of the computing system. All system resources are named and accessed as if they were files and multiple views of the distributed system can be defined dynamically for each program running on a particular machine. This approach improves generality and modularity of application design by encouraging servers that hold any information to appear to users and to applications just like collections of ordinary files.

Key to supporting the network transparency of Plan 9 was a new low-level networking protocol known as 9P. The 9P protocol and its implementation connected named network objects and presented a file-like system interface. 9P is a fast byte-oriented (rather than block-oriented) distributed file system that can virtualize any object, not only those presented by an NFS server on a remote machine. The protocol is used to refer to and communicate with processes, programs, and data, including both the user interface and the network. With the release of the 4th edition, it was modified and renamed 9P2000.

Implementations

An installable runtime environment exists for x86, and Plan 9 has been ported to MIPS, Alpha, SPARC, PowerPC, ARM and other architectures. The system is written in a dialect of ISO/ANSI C. Several applications were originally written in a language called Alef, but have since been rewritten in the same C dialect. Plan 9 can import POSIX applications and can emulate the Berkeley socket interface through APE. Recently, a new application called linuxemu was developed that can be used to run Linux binaries; it is, however, still a work in progress.

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