nextcloud-desktop/doc/csync.txt

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

csync is a lightweight utility to synchronize files between two directories
on a system or between multiple systems.

It synchronizes bidirectionally and allows the user to keep two copies of files
and directories in sync.  csync uses widely adopted protocols, such as smb or
sftp, so that there is no need for a server component. It is a user-level
program which means you don't need to be a superuser or administrator.

Together with a Pluggable Authentication Module (PAM), the intent is to provide
Roaming Home Directories for Linux (see <<X80, The PAM Module>>).

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 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 losing any
information.

The goal is to detect conflicting updates (files which have been modified) and
propagate non-conflicting updates to each replica. If there are no conflicts
left, we are done, and the replicas are identical. To resolve or handle
conflicts there are several algorithms available. They will be discussed
one of the following sections.

Basics
------

This section describes some basics of file synchronization.

Paths
~~~~~
A path normally refers to a point which contains a set of files which should be
synchronized. It is specified relative to the root of the replica locally, but
has to be absolute if you use a protocol. The path is just a sequence of names
separated by '/'.

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

csync always uses the absolute path on remote replicas. This could
'sftp://gladiac:secret@myserver/home/gladiac' for sftp.

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 contents of the file are the byte stream and the
  metadata of the file.
- a directory: then the content is the metadata of the directory.
- a symbolic link: the content is the named file 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. This is the way how updates
are detected.

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 primary goal of the file synchronizer is correctness.  It may change
scattered or large parts of the filesystem.  Since this in mostly not monitored
by the user, and the file synchronizer is in a position to harm the system,
csync must be safe, even in the case of unexpected errors (e.g. disk full).
What was done to make csync safe is described in the following sections.


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

Update detection
~~~~~~~~~~~~~~~~
There are different strategies for update detection.  csync uses a state-based
modtime-inode update detector.  This means it uses the modification time to
detect updates.  It doesn't require many 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 an instruction is set to evaluate it during the
reconciliation phase. If we don't have a record for a file we investigate, it
is marked as new.

It can be difficult to detect renaming of files.  This problem is also solved
by the record we store in the statedb.  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 important component is the update detector, because the reconciler
depends on it. The correctness of reconciler is mandatory because it can damage
a filesystem. It decides which file:

* Stays untouched
* Has a conflict
* Gets synchronized
* or is *deleted*

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

Algorithms
^^^^^^^^^^

For conflict resolution several different algorithms could be implemented. The
most common algorithms are the merge and the conflict algorithm. The first
is a batch algorithm and the second is one which needs user interaction.

Merge algorithm
+++++++++++++++

The merge algorithm is an algorithm which doesn't need any user interaction. It
is simple and used for example by Microsoft for Roaming Profiles. If it detects
a conflict (the same file changed on both replicas) then it will use the most
recent file and overwrite the other. This means you can loose some data, but
normally you want the latest file.

Conflict algorithm
++++++++++++++++++

This is not implemented yet.

If a file has a conflict the user has to decide which file should be used.

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

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


The propagator uses a two-phase-commit mechanism to simulate an atomic
filesystem operation.

In the first phase we copy the file to a temporary file on the opposite
replica. This has the advantage that we can check if the file which has been
copied to the opposite replica has been transfered successfully. If the
connection gets interrupted during the transfer we still have the original
states of the file. This means no data will be lost.

In the second phase the file on the opposite replica will be overwritten by
the temporary file.

After a successful propagation we have to merge the trees to reflect the
current state of the filesystem tree.  This updated tree will be written as a
journal into the state database. It will be used during the update detection of
the next synchronization. See above for a description of the state database
during synchronization.

Robustness
~~~~~~~~~~

This is a very important topic. The file synchronizer should not crash, and if
it has crashed, there should be no loss of data. To achieve this goal there are
several mechanisms which will be discussed in the following sections.

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

The synchronization process can be interrupted by different events, this can
be:

* the system could be halted due to errors.
* the disk could be full or the quota exceeded.
* the network or power cable could be pulled out.
* the user could force a stop of the synchronization process.
* various communication errors could occur.

That no data will be lost due to an event we enforce the following invariant:

IMPORTANT: At every moment of the synchronization each file, has either its
original content or its correct final content.

This means that the original content can not be incorrect, no data can be lost
until we overwrite it after a successful synchronization.  Therefore, each
interrupted synchronization process is a partial sync and can be continued and
completed by simply running csync again. The only problem could be an error of
the filesystem, so we reach this invariant only approximately.

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

With the Two-Phase-Commit we check the file size after the file has transferred
and we are able to detect transfer errors. A more robust approach would be a
transfer protocol with checksums, but this is not doable at the moment. We may
add this in the future.

Future filesystems, like btrfs, will help to compare checksums instead of the
filesize. This will make the synchronization safer. This does not imply that it
is unsafe now, but checksums are safer than simple filesize checks.

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

It is possible that the state database could get corrupted. If this happens,
all files get evaluated. In this case the file synchronizer wont delete any
file, but it could occur that deleted files will be restored from the other
replica.

To prevent a corruption or loss of the database if an error occurs or the user
forces an abort, the synchronizer is working on a copy of the database and will
use a Two-Phase-Commit to save it at the end.

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

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

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

Using the commandline client
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The synopsis of the commandline client is

  csync [OPTION...] SOURCE DESTINATION

It synchronizes the content of SOURCE with DESTINATION and vice versa. The
DESTINATION can be a local directory or a remote file server.

  csync /home/csync scheme://user:password@server:port/full/path

Examples
^^^^^^^^

To synchronize two local directories:

  csync /home/csync/replica1 /home/csync/relplica2

Two synchronizer a local directory with an smb server, use

  csync /home/csync smb://rupert.galaxy.site/Users/csync

If you use kerberos, you don't have to specify a username or a password. If you
don't use kerberos, the commandline client will ask about the user and the
password.  If you don't want to be prompted, you can specify it on the
commandline:

  csync /home/csync smb://csync:secret@rupert.galaxy.site/Users/csync

If you use the sftp protocol and want to specify a port, you do it the
following way:

  csync /home/csync sftp://csync@krikkit.galaxy.site:2222/home/csync

The remote destination is supported by plugins. By default csync ships with smb
and sftp support. For more information, see the manpage of csync(1).

Exclude lists
~~~~~~~~~~~~~

csync provides exclude lists with simple shell wildcard patterns. There is a
global exclude list, which is normally located in
'/etc/csync/csync_exclude.conf' and it has already some sane defaults. If you
run csync the first time, it will create an empty exclude list for the user.
This file will be '~/.csync/csync_exclude.conf'. csync considers both
configuration files and an additional one if you specify it.

The entries in the file are newline separated. Use
'/etc/csync/csync_exclude.conf' as an example.

Debug messages and dry run
~~~~~~~~~~~~~~~~~~~~~~~~~~

By default the csync client logs to stderr and you can increase the debug
level with a commandline options.

To simulate a run of the file synchronizer, you should set the priority to
'debug' for the categories 'csync.updater' and 'csync.reconciler' in the config
file '~/.csync/csync_log.conf'. Then run csync with the '--dry-run' option.
This will only run update detection and reconciliation.

[[X80]]
The PAM module
~~~~~~~~~~~~~~

pam_csync is a PAM module to provide roaming home directories for a user
session. This module is aimed at environments with central file servers where a
user wishes to store his home directory. The Authentication Module verifies the
identity of a user and triggers a synchronization with the server on the first
login and the last logout. More information can be found in the manpage of the
module pam_csync(8) or pam itself pam(8).


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

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