nextcloud-desktop/doc/csync.txt

CSYNC User Guide
================
Andreas Schneider <mail@cynapses.org>
:Author Initials: ADS

csync is a bidirectional file synchronizer for Linux and allows to keep two
copies of files and directories in sync.  It uses uses widly adopted protocols
like smb or sftp so that there is no need for a server component of csync. It
is a user-level program which means you don't need to be a superuser.

Introduction
------------

It is often the case that we have multiple copies (called replicas) of a
filesystem or part of a filesystem (for example on a notebook and on a desktop
    computer). Changes to each replica are often made independently and as a
result they do not contain the same information. In that case a file
synchronizer is used to make them consistent again, without loosing any
information.

The goal is to detect conflicting <<X13, updates>> (files which has been
    modified) and propagate non-conflicting updates to each replica. If there
are no conflicts left we are done and the replicas are identical.

Basics
------

This section describes some basics you might need to understand how file
synchronization works.

Paths
~~~~~
A path normally refers to a point with a set of files which should be
synchronized. It is specified relative to the root of the replica. The path is
just a sequence of names separated by '/'.

NOTE: The path separator is always a forward slash '/', even for Windows.

csync is always using the absolute path. This could be '/home/gladiac' or
for sftp 'sftp://gladiac:secret@myserver/home/gladiac'.


[[X13]]
What is an update?
~~~~~~~~~~~~~~~~~~
The contents of a path could be a file, a directory or a symbolic link
(symbolic links are not supported yet). To be more precise, if the path refers
to:

- a regular file, the the contents of the file are the byte stream and the
  metatdata of the file.
- a directory, then the content is the metadata of the directory.
- a symbolic link, then the content is the string where the link points to.

csync keeps a record of each path which has been successfully synchronized. The
path gets compared with the record and if it has changed since the last
synchronization, we have an update. This is done by comparing the modification
or change (modification time of the metadata) time.

What is a conflict?
~~~~~~~~~~~~~~~~~~~
A path is conflicting if it fulfills the following conditions:

1. it has been updated in one replica,
2. it or any of its descendants has been updated on the other replica too, and
3. its contents in are not identical.

File Synchronization
--------------------

The main goal of a file synchronizer is correctness. It changes whole or
separated pieces of a users file system. So a user is not able to monitor the
complete file synchronization process. So the synchronizer is in a position
where it can damage the file system. It is important that the implementation
behaves correctly under all conditions, even if there is an unexpected error
(for example disk full).

On problem concerning correctness is the handling of conflicts. Each file
synchronizer tries to propagate conflicting changes to the other replica. At
the end both replicas should be identical. There are different strategies to
fulfill these goals.

csync is a 3-phase file synchronizer. The desicion for this design was that
user interaction should be possible and it should be easy to understand the
process. The 3 phases are update detection, reconciliation and propagation.
These will be described in the following sections.

Update detection
~~~~~~~~~~~~~~~~
There are differnt strategies to do update detection. csync uses a state-based
modtime-inode update detector. This means it uses a the modification time to
detect updates. It doesn't require much resources. A record of each file is
stored in a database (called statedb) and compared with the current
modification time during update detection. If the file has changed since the
last synchronization a instruction is set to evaluate it during the
reconcilation phase. If we don't have a record for a file we invastigate, it is
marked as new.

There is a problem to detect renaming of files. This is sovled by the record we
store in the statedb too. If we don't find the file by the name in the database
we search for the inode number. If the inode number is found then the file has
been renamed.

Reconciliation
~~~~~~~~~~~~~~
The most improtant component is the update detector cause the reconciler depends
on it. The correctness of reconciler is mandatory cause it can damage a
filesystem. It decides which file:

* keeps untouched
* has a conflict
* gets synchronized
* or gets *deleted*

A wrong decision of the reconciler leads in most cases to a loss of data. So there
are several conditions a the file synchronizer has to follow.

Specification
^^^^^^^^^^^^^

TODO

Propagation
~~~~~~~~~~~

The next instance of the file synchronizer the propagator. It uses the
calculated records to apply them on the current replica.

* 2-phase-copy
* merge trees and write journal

Robustness
~~~~~~~~~~

TODO

Crash resistance
^^^^^^^^^^^^^^^^

TODO

Transfer errors
^^^^^^^^^^^^^^^

TODO

Database loss
^^^^^^^^^^^^^

TODO

Getting started
---------------

Installing csync
~~~~~~~~~~~~~~~~

See the `README` and `INSTALL` files for install prerequisites and
procedures. Packagers take a look at <<X90, Appendix B: Packager Notes>>.

Using the commandline client
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
TODO
csync /home/csync sftp://TODO:secret@server:port/profile/TODO

The PAM module
~~~~~~~~~~~~~~
TODO


[[X90]]
Appendix A: Packager Notes
--------------------------

Read the `README` and `INSTALL` files (in the distribution root
directory).