add label to the two-process section

there used to be a whole file which talked about the work-in-progress on
moving toward the two-process ZeroMQ model, and the qtconsole docs
reference it, but after that work was finished, the work-in-progress
docs were removed.

docs/source/overview.txt

@@ -1,269 +1,271 b''
 .. _overview:
 
 ============
 Introduction
 ============
 
 Overview
 ========
 
 One of Python's most useful features is its interactive interpreter.
 This system allows very fast testing of ideas without the overhead of
 creating test files as is typical in most programming languages.
 However, the interpreter supplied with the standard Python distribution
 is somewhat limited for extended interactive use.
 
 The goal of IPython is to create a comprehensive environment for
 interactive and exploratory computing.  To support this goal, IPython
 has three main components:
 
 * An enhanced interactive Python shell.
 * A decoupled two-process communication model, which allows for multiple
   clients to connect to a computation kernel, most notably the web-based
   :ref:`notebook <htmlnotebook>`
 * An architecture for interactive parallel computing.
 
 All of IPython is open source (released under the revised BSD license).
 
 Enhanced interactive Python shell
 =================================
 
 IPython's interactive shell (:command:`ipython`), has the following goals,
 amongst others:
 
 1. Provide an interactive shell superior to Python's default. IPython
    has many features for object introspection, system shell access,
    and its own special command system for adding functionality when
    working interactively. It tries to be a very efficient environment
    both for Python code development and for exploration of problems
    using Python objects (in situations like data analysis).
   
 2. Serve as an embeddable, ready to use interpreter for your own
    programs. IPython can be started with a single call from inside
    another program, providing access to the current namespace. This
    can be very useful both for debugging purposes and for situations
    where a blend of batch-processing and interactive exploration are
    needed.  New in the 0.9 version of IPython is a reusable wxPython
    based IPython widget.
   
 3. Offer a flexible framework which can be used as the base
    environment for other systems with Python as the underlying
    language. Specifically scientific environments like Mathematica,
    IDL and Matlab inspired its design, but similar ideas can be
    useful in many fields.
   
 4. Allow interactive testing of threaded graphical toolkits. IPython
    has support for interactive, non-blocking control of GTK, Qt and
    WX applications via special threading flags. The normal Python
    shell can only do this for Tkinter applications.
 
 Main features of the interactive shell
 --------------------------------------
 
 * Dynamic object introspection. One can access docstrings, function
   definition prototypes, source code, source files and other details
   of any object accessible to the interpreter with a single
   keystroke (:samp:`?`, and using :samp:`??` provides additional detail).
   
 * Searching through modules and namespaces with :samp:`*` wildcards, both
   when using the :samp:`?` system and via the :samp:`%psearch` command.
 
 * Completion in the local namespace, by typing :kbd:`TAB` at the prompt.
   This works for keywords, modules, methods, variables and files in the
   current directory. This is supported via the readline library, and
   full access to configuring readline's behavior is provided. 
   Custom completers can be implemented easily for different purposes
   (system commands, magic arguments etc.)
 
 * Numbered input/output prompts with command history (persistent
   across sessions and tied to each profile), full searching in this
   history and caching of all input and output.
 
 * User-extensible 'magic' commands. A set of commands prefixed with
   :samp:`%` is available for controlling IPython itself and provides
   directory control, namespace information and many aliases to
   common system shell commands.
 
 * Alias facility for defining your own system aliases.
 
 * Complete system shell access. Lines starting with :samp:`!` are passed
   directly to the system shell, and using :samp:`!!` or :samp:`var = !cmd` 
   captures shell output into python variables for further use.
 
 * Background execution of Python commands in a separate thread.
   IPython has an internal job manager called jobs, and a
   convenience backgrounding magic function called :samp:`%bg`.
 
 * The ability to expand python variables when calling the system shell. In a
   shell command, any python variable prefixed with :samp:`$` is expanded. A
   double :samp:`$$` allows passing a literal :samp:`$` to the shell (for access
   to shell and environment variables like :envvar:`PATH`).
 
 * Filesystem navigation, via a magic :samp:`%cd` command, along with a
   persistent bookmark system (using :samp:`%bookmark`) for fast access to
   frequently visited directories.
 
 * A lightweight persistence framework via the :samp:`%store` command, which
   allows you to save arbitrary Python variables. These get restored
   automatically when your session restarts.
 
 * Automatic indentation (optional) of code as you type (through the
   readline library).
 
 * Macro system for quickly re-executing multiple lines of previous
   input with a single name. Macros can be stored persistently via
   :samp:`%store` and edited via :samp:`%edit`.
 
 * Session logging (you can then later use these logs as code in your
   programs). Logs can optionally timestamp all input, and also store
   session output (marked as comments, so the log remains valid
   Python source code).
 
 * Session restoring: logs can be replayed to restore a previous
   session to the state where you left it.
 
 * Verbose and colored exception traceback printouts. Easier to parse
   visually, and in verbose mode they produce a lot of useful
   debugging information (basically a terminal version of the cgitb
   module).
 
 * Auto-parentheses: callable objects can be executed without
   parentheses: :samp:`sin 3` is automatically converted to :samp:`sin(3)`.
 
 * Auto-quoting: using :samp:`,`, or :samp:`;` as the first character forces
   auto-quoting of the rest of the line: :samp:`,my_function a b` becomes
   automatically :samp:`my_function("a","b")`, while :samp:`;my_function a b`
   becomes :samp:`my_function("a b")`.
 
 * Extensible input syntax. You can define filters that pre-process
   user input to simplify input in special situations. This allows
   for example pasting multi-line code fragments which start with
   :samp:`>>>` or :samp:`...` such as those from other python sessions or the
   standard Python documentation.
 
 * Flexible configuration system. It uses a configuration file which
   allows permanent setting of all command-line options, module
   loading, code and file execution. The system allows recursive file
   inclusion, so you can have a base file with defaults and layers
   which load other customizations for particular projects.
 
 * Embeddable. You can call IPython as a python shell inside your own
   python programs. This can be used both for debugging code or for
   providing interactive abilities to your programs with knowledge
   about the local namespaces (very useful in debugging and data
   analysis situations).
 
 * Easy debugger access. You can set IPython to call up an enhanced version of
   the Python debugger (pdb) every time there is an uncaught exception. This
   drops you inside the code which triggered the exception with all the data
   live and it is possible to navigate the stack to rapidly isolate the source
   of a bug. The :samp:`%run` magic command (with the :samp:`-d` option) can run
   any script under pdb's control, automatically setting initial breakpoints for
   you.  This version of pdb has IPython-specific improvements, including
   tab-completion and traceback coloring support. For even easier debugger
   access, try :samp:`%debug` after seeing an exception. winpdb is also
   supported, see ipy_winpdb extension.
 
 * Profiler support. You can run single statements (similar to
   :samp:`profile.run()`) or complete programs under the profiler's control.
   While this is possible with standard cProfile or profile modules,
   IPython wraps this functionality with magic commands (see :samp:`%prun`
   and :samp:`%run -p`) convenient for rapid interactive work.
 
 * Doctest support. The special :samp:`%doctest_mode` command toggles a mode
   that allows you to paste existing doctests (with leading :samp:`>>>`
   prompts and whitespace) and uses doctest-compatible prompts and
   output, so you can use IPython sessions as doctest code.
 
+.. _ipythonzmq:
+
 Decoupled two-process model
 ==============================
 
 IPython has abstracted and extended the notion of a traditional
 *Read-Evaluate-Print Loop* (REPL) environment by decoupling the *evaluation*
 into its own process. We call this process a kernel: it receives execution
 instructions from clients and communicates the results back to them.
 
 This decoupling allows us to have several clients connected to the same
 kernel, and even allows clients and kernels to live on different machines.
 With the exclusion of the traditional single process terminal-based IPython
 (what you start if you run ``ipython`` without any subcommands), all
 other IPython machinery uses this two-process model. This includes ``ipython
 console``,  ``ipython qtconsole``, and ``ipython notebook``.
 
 As an example, this means that when you start ``ipython qtconsole``, you're
 really starting two processes, a kernel and a Qt-based client can send
 commands to and receive results from that kernel. If there is already a kernel
 running that you want to connect to, you can pass the  ``--existing`` flag
 which will skip initiating a new kernel and connect to the most recent kernel,
 instead. To connect to a specific kernel once you have several kernels
 running, use the ``%connect_info`` magic to get the unique connection file,
 which will be something like ``--existing kernel-19732.json`` but with
 different numbers which correspond to the Process ID of the kernel.
 
 You can read more about using :ref:`ipython qtconsole <qtconsole>`, and
 :ref:`ipython notebook <htmlnotebook>`. There is also a :ref:`message spec
 <messaging>` which documents the protocol for communication between kernels
 and clients.
 
 
 Interactive parallel computing
 ==============================
 
 Increasingly, parallel computer hardware, such as multicore CPUs, clusters and
 supercomputers, is becoming ubiquitous. Over the last 8 years, we have
 developed an architecture within IPython that allows such hardware to be used
 quickly and easily from Python. Moreover, this architecture is designed to
 support interactive and collaborative parallel computing.
 
 The main features of this system are:
 
 * Quickly parallelize Python code from an interactive Python/IPython session.
 
 * A flexible and dynamic process model that be deployed on anything from 
   multicore workstations to supercomputers.
 
 * An architecture that supports many different styles of parallelism, from
   message passing to task farming.  And all of these styles can be handled
   interactively.
 
 * Both blocking and fully asynchronous interfaces.
 
 * High level APIs that enable many things to be parallelized in a few lines
   of code.
 
 * Write parallel code that will run unchanged on everything from multicore
   workstations to supercomputers.
 
 * Full integration with Message Passing libraries (MPI).
 
 * Capabilities based security model with full encryption of network connections.
 
 * Share live parallel jobs with other users securely.  We call this
   collaborative parallel computing.
 
 * Dynamically load balanced task farming system.
 
 * Robust error handling.  Python exceptions raised in parallel execution are
   gathered and presented to the top-level code.
 
 For more information, see our :ref:`overview <parallel_index>` of using IPython
 for parallel computing.
 
 Portability and Python requirements
 -----------------------------------
 
 As of the 0.11 release, IPython works with Python 2.6 and 2.7. Versions 0.9 and
 0.10 worked with Python 2.4 and above. IPython now also supports Python 3,
 although for now the code for this is separate, and kept up to date with the
 main IPython repository. In the future, these will converge to a single codebase
 which can be automatically translated using 2to3.
 
 IPython is known to work on the following operating systems:
 
 	* Linux
 	* Most other Unix-like OSs (AIX, Solaris, BSD, etc.)
 	* Mac OS X
 	* Windows (CygWin, XP, Vista, etc.)
 
 See :ref:`here <install_index>` for instructions on how to install IPython.