\chapter{Architecture and Design}

\epigraph{In general, when modeling phenomena in science and
  engineering, we begin with simplified, incomplete models. As we
  examine things in greater detail, these simple models become
  inadequate and must be replaced by more refined
  models.}{\textit{Structure and Interpretation of Computer Programs,
    Section 1.1.5}\cite{sicp}}

\section{Structure of combox}

combox consists of two main components -- the combox directory and the
node directories. The combox directory is the place where the user
stores all the files; the node directories are the directories under
which encrypted shards of the files (in the combox directory) are
scattered to. A node directory is the file storage provider's
directory, for instance, the Dropbox directory and the Google Drive
directory are node directories.

When a file \verb+humans.txt+ is created in the combox directory,
combox splits \verb+humans.txt+ into \verb+N+ shards, where \verb+N+
is the number of node directories; if there are two node directories
(Dropbox directory and Google Drive directory), then 2 shards are
created. Each shard of the file is then encrypted and the encrypted
shards are spread evenly across the node directories; if there are two
node directories -- Dropbox directory and Google Drive directory --
combox will create two encrypted shards of file \verb+humans.txt+ --
\verb+humans.txt.shard0+, \verb+humans.txt.shard1+ -- and place one
encrypted shard under the Dropbox directory and the other encrypted
shard under the Google Drive directory. Now, the Dropbox client and
the Google client will sync the respective shards that was place under
their directories to their respective servers.

\begin{figure}[h]
\includegraphics[scale=0.6]{3-combox-structure}
\caption{High level overview of how file creation works when combox is
  setup on two computers.}
\label{fig:3-combox-structure}
\end{figure}

Now, when the user moves to their second computer, the node clients
(Dropbox client and the Google Drive client) will sync the new
encrypted shards to their respective directories. Once the encrypted
shards are synced to the node directories, combox will pick the
encrypted shards -- \verb+humans.txt.shard0+, \verb+humans.txt.shard1+
-- decrypt them and reconstruct into \verb+humans.txt+ and place it in
the respective location under the combox directory; figure
\ref{fig:3-combox-structure} illustrates this. The process is similar
for file modification, deletion and rename/move.

\subsection{combox configuration}\label{sec:3-combox-config}

combox configuration wizard triggers automatically when combox finds
that it is not configured. The combox configuration setups up the
combox directory; asks the user to point to the location of the node
directories; reads the key (passphrase) to be used to encrypt file
shards that are spread across the node directories. The combox
configuration is written to
\verb+$HOME/.combox/config.yaml+; this YAML configuration file can be
manually edited by the user.

The
\verb+config_cb+\footnote{https://git.ricketyspace.net/combox/tree/combox/config.py?id=fb7fdd21\#n90}
function in the \verb+combox.config+ module is responsible for
carrying out the combox configuration. Prior to version \verb+0.2.0+,
the combox configuration was purely done through the CLI, from
\verb+0.2.0+ on wards, by default, the combox configuration done
through a graphical interface; it is still possible to configure
combox through the CLI with the \verb+--cli+ switch.

A demo of combox configuration using the graphical interface on
GNU/Linux can be viewed at
\url{https://ricketyspace.net/combox/combox-config-gui-glued-gnu.webm};
the same demo of combox configuration using the graphical interface on
OS X can be viewed at
\url{https://ricketyspace.net/combox/combox-config-gui-glued-osx.webm}.

\subsection{combox directory monitor}\label{sec:3-combox-cdirm}

combox directory monitor is an instance of
\verb+combox.events.ComboxDirMonitor+\footnote{https://git.ricketyspace.net/combox/tree/combox/events.py?id=fb7fdd21\#n42}
monitoring the combox directory for changes. When changes are made to
the combox directory, the combox directory monitor is responsible for
correctly detecting the type of change and doing the right thing at
that instance of time.

When a file is created in the combox directory, the combox directory
monitor will take that file, split it into \verb+N+ (equal to the
number of node directories) shards, encrypt the shards, spread the
encrypted shards to the node directories, and finally store the hash
of the file in the local combox data store.

When a file is modified in the combox directory, the combox directory
monitor will take that modified file, split it into \verb+N+ (equal to
the number of node directories) shards, encrypt the shards, spread the
encrypted shards to the node directories, and finally update the hash
of the file in the local combox data store.

When a file is deleted in the combox directory, the combox directory
monitor will remove the encrypted shards of the file in the node
directories and get rid of the file's hash from the local combox
data store.

When a file is moved/renamed in the combox directory, the combox
directory monitor will move/rename encrypted shards in all the node
directories, remove the file's hash from the local combox data store
and store the hash of file under its new name.

\subsection{Node directory monitor}\label{sec:3-combox-nodirm}

Node directory monitor is an instance of
\verb+combox.events.NodeDirMonitor+\footnote{https://git.ricketyspace.net/combox/tree/combox/events.py?id=fb7fdd21\#n352}
monitoring a node directory. When changes are made to the node
directory, the node directory monitor is responsible for correctly
detecting the type of change and doing the right thing at that
instance of time. Each node directory has a dedicated node directory
monitor; if there are 2 node directories, then combox will instantiate
2 node directory monitors.

When an encrypted shard is created in the node directory due to a file
created on another computer, the node directory first checks if the
respective file' encrypted shard(s) has/have arrived in other node
directory/directories. If all encrypted shards have arrived, then the
node directory takes all the encrypted shards, decrypts them,
reconstructs the file and puts the file in the combox directory on
this computer and stores the hash of the newly created file in the
local combox data store. If all the encrypted shards have not arrived,
then the node directory does not do anything. It must be observed here
that the node directory monitor of the last node directory which gets
the encrypted shard will be the one to perform the file reconstruction
and creation.

When an encrypted shard is modified in the node directory due to a
file modified on another computer, the node directory first checks if
the respective file' modified encrypted shard(s) has/have arrived in
other node directory/directories. If all modified encrypted shards
have arrived, then the node directory takes all the modified encrypted
shards, decrypts them, reconstructs the file and puts the modified
version of the file in the combox directory on this computer and
updates the file's hash in the local combox data store. If all the
modified encrypted shards have not arrived, then the node directory
does not do anything. It must be observed here that the node directory
monitor of the last node directory which gets the modified encrypted
shard will be the one to perform the file reconstruction and will
place the modified file in the combox directory.

When an encrypted shard is deleted in the node directory due to a file
deleted on another computer, the node directory first checks if the
respective file' encrypted shard(s) has/have been deleted in other
node directory/directories. If all encrypted shards have been deleted
from the node directories, then the node directory deletes the file in
the combox directory on this computer and removes its information from
the local combox data store. If all encrypted shards have not been
deleted, then the node directory does not do anything. It must be
observed here that the node directory monitor of the last node
directory in which the encrypted shard is deleted will be the one to
delete the file from the combox directory.

When an encrypted shard is moved/renamed in the node directory due to
a file moved/renamed on another computer, the node directory first
checks if the respective file' moved/renamed encrypted shard(s)
has/have arrived in other node directory/directories. If all
moved/renamed encrypted shards have arrived, then the node directory
takes all the moved/renamed encrypted shards, decrypts them,
reconstructs the moved/renamed file and puts the moved/renamed file in
the combox directory on this computer and stores the hash under the
file' new name in the local combox data store. If the all the
moved/renamed encrypted shards have not arrived, then the node
directory does not do anything. It must be observed here that the node
directory monitor of the last node directory which gets the
moved/renamed encrypted shard will be the one to perform the file
reconstruction and will place the moved/renamed file in the combox
directory.

\subsection{combox data store}\label{sec:3-combox-db}

To keep it simple, stupid, combox tracks bare minimum information
about the files, stored in the combox directory, and depends on file
system events to do the right thing when changes takes place in the
combox directory.

The only information that is stored in the combox data store, about a
file in the combox directory is its SHA-512 hash; The SHA-512 hash of
a file is enough information to detect changes in the file. In the
data store, there also four dictionaries -- \verb+file_moved+,
\verb+file_deleted+, \verb+file_created+, \verb+file_modified+ --
which tracks the number of shards of a file that was
moved/deleted/created/modified due the respective file being
moved/deleted/created/modified on another computer; these four
dictionaries are primarily used by the \verb+NodeDirMonitor+ to detect
remote file movement/deletion/creation/modification and triggering
file reconstruction from the encrypted shards at the right time.

The data store is a JSON file on the disk, stored by default at
\verb+$HOME/.combox/silo.db+. The
\verb+combox.silo.ComboxSilo+\footnote{https://git.ricketyspace.net/combox/tree/combox/silo.py?id=v0.2.2\#n29}
is the sole interface to read from and write to the data store. The
data store is primarily accessed and modified by the combox directory
monitor (\verb+ComboxDirMonitor+) and the node directory monitor
(\verb+NodeDirMonitor+) through a shared \verb+threading.Lock+ that ensures that only
one entity\footnote{An entity can be the combox directory monitor or
  one of the node directory monitors} can access/modify the database
at a time.

Below is an illustration of the structure of the combox data store:

\begin{verbatim}
{
  "/home/rsd/combox/ipsum.txt": "e3206df2bb2b3091103ab9d...",
  "/home/rsd/combox/tk-shot-osx.png": "7fcf1b44c15dd95e0...",
  "/home/rsd/combox/thgttg-21st.png": "0040eedfc3eeab546...",
  "/home/rsd/combox/lorem.txt": "5851dd7a4870ff165facb71...",
  "/home/rsd/combox/the-red-star.jpg": "4b818126d882e552...",
  "file_moved": {},
  "file_deleted": {},
  "file_created": {},
  "file_modified": {},
}
\end{verbatim}

The \verb+combox.silo.ComboxSilo+, which is the sole interface to read
from and write to the database, uses the pickleDB
library\cite{pylib:pickledb}. The pickleDB is a very basic key-value
store which allows one to store information in the JSON format.

It must be noted that the combox data store on each computer is
independent and does not communicate or make transactions with the
combox data store located in other computers.

\section{combox modules overview}

combox is spread into modules that have functions and/or classes. As
of \verb+2016-02-04+ combox is considerably a small program:

\begin{verbatim}
$ wc -l combox/*.py
  144 combox/cbox.py
  178 combox/config.py
  241 combox/crypto.py
  891 combox/events.py
  541 combox/file.py
  454 combox/gui.py
    0 combox/__init__.py
   71 combox/log.py
  278 combox/silo.py
   29 combox/_version.py
 2827 total
\end{verbatim}

This section gives an overview of each of the combox modules with
extreme brevity:

\begin{description}
\item[combox.cbox]\footnote{https://git.ricketyspace.net/combox/tree/combox/cbox.py?id=fb7fdd21}
  This module contains \verb+run_cb+ function that starts/initiates
  combox; this function creates an instance \verb+threading.Lock+ for
  combox data store access and another instance of
  \verb+threading.Lock+ which is shared by instances of
  \verb+combox.events.ComboxDirMonitor+ and
  \verb+combox.events.NodeDirMonitor+; it initializes an instance
  \verb+combox.events.ComboxDirMonitor+ that monitors the combox
  directory and an instance of \verb+combox.events.NodeDirMonitor+ for
  each node directory. This modules also houses the \verb+main+
  function that parses commandline arguments, starts combox
  configuration if needed or loads the combox configuration file to
  start running combox.
\item[combox.config]\footnote{https://git.ricketyspace.net/combox/tree/combox/config.py?id=fb7fdd21}
  Accommodates two import functions -- \verb+config_cb+ and
  \verb+get_nodedirs+. The \verb+config_cb+ is the combox
  configuration function that allows the user to configure combox;
  this function was designed in a such way that it could be used by
  both the commandline and graphical interfaces for configuring
  combox. The \verb+get_nodedirs+ function returns, as a list, the
  paths of the node directories; this function use used in numerous
  places in other combox modules.
\item[combox.crypto]\footnote{https://git.ricketyspace.net/combox/tree/combox/crypto.py?id=fb7fdd21}
  This has functions for encrypting and decrypting data; encrypting
  and decrypting shards (\verb+encrypt_shards+ and
  \verb+decrypt_shards+); a function for splitting a file into shards,
  encrypting those shards and spreading them across node directories
  (\verb+split_and_encrypt+); a function for decrypting the shards
  from the node directories, reconstructing the file from the
  decrypted shards and putting the file to the combox directory
  (\verb+decrypt_and_glue+). Functions \verb+split_and_encrypt+ and
  \verb+decrypt_and_glue+ are the two functions that that are
  extensively used by the \verb+combox.events+ module; all other
  functions in this module are pretty much helper functions for
  \verb+split_and_encrypt+ and \verb+decrypt_and_glue+ functions and
  are not used by other modules.
\item[combox.events]\footnote{https://git.ricketyspace.net/combox/tree/combox/events.py?id=fb7fdd21}
  This module took the most time to write and test and it is the most
  complex module in combox at the time of writing this report. It
  contains just two classes -- \verb+ComboxDirMonitor+ and
  \verb+NodeDirMonitor+. The \verb+ComboxDirMonitor+ inherits the
  \verb+watchdog.events.LoggingEventHandler+ and is responsible for
  monitoring for changes in the combox directory and doing the right
  thing when a change happens in the combox directory. The
  \verb+NodeDirMonitor+ also inherits
  \verb+watchdog.events.LoggingEventHandler+ and similarly responsible
  for monitoring a node directory and doing the right thing when a
  change happens in the node directory; subjectively,
  \verb+NodeDirMonitor+ is slightly more complex than the
  \verb+ComboxDirMonitor+.
\item[combox.file]\footnote{https://git.ricketyspace.net/combox/tree/combox/file.py?id=fb7fdd21} This is the second largest module in combox. It
  contains utility functions for reading, writing, moving
  files/directories, hashing files, splitting a file into shards, gluing
  shards into a file, manipulating directories inside combox and node
  directories.
\item[combox.gui]\footnote{https://git.ricketyspace.net/combox/tree/combox/gui.py?id=fb7fdd21}
  Contains the \verb+ComboxConfigDialog+ class; it is the graphical
  interface for configuring combox. The class uses the Tkinter
  library\cite{pylib:tkinter} for spawning graphical elements. Other
  graphical libraries including PyQt\cite{pylib:qt} were considered,
  Tkinter was chosen over others due to compatibility with all Unix,
  Unix-like systems and Microsoft Windows and it is part of the
  standard python library from python version 3 on wards.
\item[combox.log]\footnote{https://git.ricketyspace.net/combox/tree/combox/log.py?id=fb7fdd21}
  All the messages to \verb+stdout+ and \verb+stderr+ are sent through
  the \verb+log_i+ and \verb+log_e+ functions defined in this module.
\item[combox.silo]\footnote{https://git.ricketyspace.net/combox/tree/combox/silo.py?id=fb7fdd21} Contains the \verb+ComboxSilo+ class which is the
  canonical interface for combox for managing information about the
  files in the combox directory. Internally, the \verb+ComboxSilo+
  class uses the pickleDB library\cite{pylib:pickledb}.
\item[combox.\_version]\footnote{https://git.ricketyspace.net/combox/tree/combox/\_version.py?id=fb7fdd21} This is \emph{private} module that contains
  variables that contain the value of the present version and release
  of combox. The \verb+get_version+ function in this module returns
  the full version number; this function used by \verb+setup.py+.
\end{description}

\section{DRY}

The core functionality of combox is to split, encrypt file shards,
spread them across node directories (Google Drive and Dropbox) and
decrypt, glue shards and put them back to the combox directory when a
file is created/modified/deleted/moved in another computer. The plan
was to use external libraries to accomplish things that fell outside
the realm of the ``core functionality of combox''; the main reason
behind this decision was to not indulge in trying to solve problems
that others have already solved.

The \verb+watchdog+\cite{pylib:watchdog} library was chosen for file
monitoring; this library is compatible with Unix, Unix-like systems
and Microsoft Windows. The \verb+pycrypto+
library\cite{pylib:pycrypto} was used for encrypting data; combox uses
AES encryption scheme to encrypt file shards. The
\verb+pickleDB+\cite{pylib:pickledb} library was used to store
information about files in the combox directory.

Looking back, the decision to use external libraries reduced the
complexity of combox, reduced the time to complete the initial working
version of combox and made it possible to spend more than 3 months
just testing and fixing issues in combox.

\section{Operating system compatibility}\label{3-os-compat}

combox was developed on a GNU/Linux machine, a conscious effort was
made to write in an operating system independent way. The top criteria
for choosing a library to use in combox was that it had to be
compatible on \emph{all} of the three major computing
platforms\footnote{GNU/Linux, OS X and, Microsoft Windows}.

Prior to the \verb+0.1.0+ release, combox was tested on OS X (See
chapter \ref{ch:4}) and OS X specific issues that were found were
eventually fixed. The initial \verb+0.1.0+ release of combox was
compatible with GNU/Linux and OS X.

After the initial release of combox, it was seen if combox would be
compatible with Microsoft Windows out of the box. it was found that:

\begin{itemize}
\item Setting up the paraphernalia to run combox was
  non-trivial\cite{doc:combox-setup-windoze}.
\item The unit tests for the \verb+combox.file+ module royally failed.
\end{itemize}

At the time of writing the report, combox is at version \verb+0.2.3+
and it is not compatible with Microsoft Windows. Comprehensive
documentation for setting up the development environment for combox on
Microsoft Windows was written\cite{doc:combox-setup-windoze} to make
it less cumbersome for anyone who would want to work on making combox
compatible with Microsoft Windows.

\section{combox as a python package}\label{3-pypi}

Before version \verb+0.2.0+, the canonical way to install combox was
to pull the source from the \verb+git+ repository with:

\begin{verbatim}
  git clone git://ricketyspace.net/combox.git
\end{verbatim}

Then, do:

\begin{verbatim}
  cd combox
\end{verbatim}

Finally install combox with:

\begin{verbatim}
  python setup.py install
\end{verbatim}

Python has a package registry called CheeseShop\footnote{code name for
  Python Package Index, see https://wiki.python.org/moin/CheeseShop};
all packages registered at the CheeseShop can be installed using
\verb+pip+ -- Python's platform independent package management
system\cite{py:pip} -- with:

\begin{verbatim}
  pip install packagename
\end{verbatim}

To make it easier for (python) users to install combox on their
machine, an effort was made to make it a python
package\cite{py:package-guide}. From version \verb+0.2.0+, combox has
been registered as a python package at the CheeseShop. (Python) Users
can now easily get a copy of combox on their machine with:

\begin{verbatim}
  pip install combox
\end{verbatim}

All versions of combox that is available through the CheeseShop are
digitally signed using the following GPG key:

\begin{verbatim}
pub   4096R/00B252AF 2014-09-08 [expires: 2017-09-07]
      Key fingerprint = C174 1162 CEED 5FE8 9954  A4B9 9DF9 7838 00B2 52AF
uid                  Siddharth Ravikumar (sravik) <sravik@bgsu.edu>
sub   4096R/09CECEDB 2014-09-08 [expires: 2017-09-07]
\end{verbatim}

All versions of combox's source are also available as a compressed
\verb+TAR+ ball and as a \verb+ZIP+ archive; they can be downloaded
from \url{https://ricketyspace.net/combox/releases.html}.
