Start describing notebooks for IPython architecture doc

docs/source/development/how_ipython_works.rst

@@ -1,56 +1,71 b''
 How IPython Works
 =================
 
 Terminal IPython
 ----------------
 
 When you type ``ipython``, you get the original IPython interface, running in
 the terminal. It does something like this::
 
     while True:
         code = input(">>> ")
         exec(code)
 
 Of course, it's much more complicated, because it has to deal with multi-line
 code, tab completion using :mod:`readline`, magic commands, and so on. But the
 model is like that: prompt the user for some code, and when they've entered it,
 exec it in the same process.
 
 The IPython Kernel
 ------------------
 
 All the other interfaces—the Notebook, the Qt console, ``ipython console`` in
 the terminal, and third party interfaces—use the IPython Kernel. This is a
 separate process which is responsible for running user code, and things like
 computing possible completions. Frontends communicate with it using JSON
 messages sent over ZeroMQ sockets; the protocol they use is described in
 :doc:`messaging`.
 
 The core execution machinery for the kernel is shared with terminal IPython:
 
 .. image:: figs/ipy_kernel_and_terminal.png
 
 A kernel process can be connected to more than one frontend simultaneously. In
 this case, the different frontends will have access to the same variables.
 
 .. TODO: Diagram illustrating this?
 
 This design was intended to allow easy development of different frontends based
 on the same kernel, but it also made it possible to support new languages in the
 same frontends, by developing kernels in those languages, and we are refining
 IPython to make that more practical.
 
 Today, there are two ways to develop a kernel for another language. Wrapper
 kernels reuse the communications machinery from IPython, and implement only the
 core execution part. Native kernels implement execution and communications in
 the target language:
 
 .. image:: figs/other_kernels.png
 
 .. seealso::
 
    :doc:`kernels`
    
    :doc:`wrapperkernels`
 
-
+Notebooks
+---------
+
+The Notebook frontend does something extra. In addition to running your code, it
+stores code and output, together with markdown notes, in an editable document
+called a notebook. When you save it, this is sent from your browser to the
+notebook server, which saves it on disk as a JSON file with a ``.ipynb``
+extension.
+
+.. TODO: Diagram of how these pieces fit together.
+
+The notebook server, not the kernel, is responsible for saving and loading
+notebooks, so you can edit notebooks even if you don't have the kernel for that
+language—you just won't be able to run code. The kernel doesn't know anything
+about the notebook document: it just gets sent cells of code to execute when the
+user runs them.