Add docs on writing kernels

docs/source/development/kernels.rst

@@ -0,0 +1,145 b''
+==========================
+Making kernels for IPython
+==========================
+
+A 'kernel' is a program that runs and introspects the user's code. IPython
+includes a kernel for Python code, and people have written kernels for
+`several other languages <https://github.com/ipython/ipython/wiki/Projects-using-IPython#list-of-some-ipython-compatible-kernels>`_.
+
+When IPython starts a kernel, it passes it a connection file. This specifies
+how to set up communications with the frontend.
+
+There are two options for writing a kernel:
+
+1. You can reuse the IPython kernel machinery to handle the communications, and
+   just describe how to execute your code. This is much simpler if the target
+   language can be driven from Python. See :doc:`wrapperkernels` for details.
+2. You can implement the kernel machinery in your target language. This is more
+   work initially, but the people using your kernel might be more likely to
+   contribute to it if it's in the language they know.
+
+Connection files
+================
+
+Your kernel will be given the path to a connection file when it starts (see
+:ref:`kernelspecs` for how to specify the command line arguments for your kernel).
+This file, which is accessible only to the current user, will contain a JSON
+dictionary looking something like this::
+
+    {
+      "control_port": 50160,
+      "shell_port": 57503,
+      "transport": "tcp",
+      "signature_scheme": "hmac-sha256",
+      "stdin_port": 52597,
+      "hb_port": 42540,
+      "ip": "127.0.0.1",
+      "iopub_port": 40885,
+      "key": "a0436f6c-1916-498b-8eb9-e81ab9368e84"
+    }
+
+The ``transport``, ``ip`` and five ``_port`` fields specify five ports which the
+kernel should bind to using `ZeroMQ <http://zeromq.org/>`_. For instance, the
+address of the shell socket in the example above would be::
+
+    tcp://127.0.0.1:57503
+
+New ports are chosen at random for each kernel started.
+
+``signature_scheme`` and ``key`` are used to cryptographically sign messages, so
+that other users on the system can't send code to run in this kernel. See
+:ref:`wire_protocol` for the details of how this signature is calculated.
+
+Handling messages
+=================
+
+After reading the connection file and binding to the necessary sockets, the
+kernel should go into an event loop, listening on the hb (heartbeat), control
+and shell sockets.
+
+:ref:`Heartbeat <kernel_heartbeat>` messages should be echoed back immediately
+on the same socket - the frontend uses this to check that the kernel is still
+alive.
+
+Messages on the control and shell sockets should be parsed, and their signature
+validated. See :ref:`wire_protocol` for how to do this.
+
+The kernel will send messages on the iopub socket to display output, and on the
+stdin socket to prompt the user for textual input.
+
+.. seealso::
+
+   :doc:`messaging`
+     Details of the different sockets and the messages that come over them.
+
+
+.. _kernelspecs:
+
+Kernel specs
+============
+
+A kernel identifies itself to IPython by creating a directory, the name of which
+is used as an identifier for the kernel. These may be created in a number of
+locations:
+
++--------+--------------------------------------+-----------------------------------+
+|        | Unix                                 | Windows                           |
++========+======================================+===================================+
+| System | ``/usr/share/ipython/kernels``       | ``%PROGRAMDATA%\ipython\kernels`` |
+|        |                                      |                                   |
+|        | ``/usr/local/share/ipython/kernels`` |                                   |
++--------+--------------------------------------+-----------------------------------+
+| User   |                     ``~/.ipython/kernels``                               |
++--------+--------------------------------------+-----------------------------------+
+
+The user location takes priority over the system locations, and the case of the
+names is ignored, so selecting kernels works the same way whether or not the
+filesystem is case sensitive.
+
+Inside the directory, the most important file is *kernel.json*. This should be a
+JSON serialised dictionary containing the following keys and values:
+
+- **argv**: A list of command line arguments used to start the kernel. The text
+  ``{connection_file}`` in any argument will be replaced with the path to the
+  connection file.
+- **display_name**: The kernel's name as it should be displayed in the UI.
+  Unlike the kernel name used in the API, this can contain arbitrary unicode
+  characters.
+- **language**: The programming language which this kernel runs. This will be 
+  stored in notebook metadata. This may be used by syntax highlighters to guess
+  how to parse code in a notebook, and frontends may eventually use it to
+  identify alternative kernels that can run some code.
+- **codemirror_mode** (optional): The `codemirror mode <http://codemirror.net/mode/index.html>`_
+  to use for code in this language. This can be a string or a dictionary, as
+  passed to codemirror config. The string from *language* will be used if this is
+  not provided.
+- **env** (optional): A dictionary of environment variables to set for the kernel.
+  These will be added to the current environment variables before the kernel is
+  started.
+- **help_links** (optional): A list of dictionaries, each with keys 'text' and
+  'url'. These will be displayed in the help menu in the notebook UI.
+
+For example, the kernel.json file for IPython looks like this::
+
+    {
+     "argv": ["python3", "-c", "from IPython.kernel.zmq.kernelapp import main; main()", 
+              "-f", "{connection_file}"], 
+     "codemirror_mode": {
+      "version": 3, 
+      "name": "ipython"
+     }, 
+     "display_name": "IPython (Python 3)", 
+     "language": "python"
+    }
+
+To see the available kernel specs, run::
+
+    ipython kernelspec list
+
+To start the terminal console or the Qt console with a specific kernel::
+
+    ipython console --kernel bash
+    ipython qtconsole --kernel bash
+
+To use different kernels in the notebook, select a different kernel from the
+dropdown menu in the top-right of the UI.

docs/source/development/messaging.rst

@@ -126,6 +126,8 b' A message is defined by the following four-dictionary structure::'
 
     ``version`` key added to the header.
 
+.. _wire_protocol:
+
 The Wire Protocol
 =================
 
@@ -971,6 +973,7 b' When ``password`` is True, the frontend should not echo the input as it is enter'
    transported over the zmq connection), raw ``stdin`` isn't expected to be
    available.
 
+.. _kernel_heartbeat:
 
 Heartbeat for kernels
 =====================