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clsync
======
Contents
--------

1.  Name
2.  Motivation
3.  inotify vs fanotify
4.  Installing
5.  How to use
6.  Example of usage
7.  Other uses
8.  Clustering
9.  Known building issues
10. FreeBSD support
11. Support
12. Developing
13. Articles


1. Name
-------

Why "clsync"? The first name of the utility was "insync" (due to inotify), but
then I suggested to use "fanotify" instead of "inotify" and utility was been
renamed to "fasync". After that I started to intensively write the program.
However I faced with some problems in "fanotify", so I was have to temporary
fallback to "inotify", then I decided that the best name is "Runtime Sync" or
"Live Sync", but "rtsync" is a name of some corporation and "lsync" is busy
by "[lsyncd](https://github.com/axkibe/lsyncd)". So I called it
"clsync", that should be interpreted as "lsync, but on c" due to "lsyncd" that
written on "LUA" and may be used for the same purposes.

UPD: Also I was have to add somekind of clustering support. It's multicast
notifing subsystem to prevent loops on bidirection syncing. So "clsync" also
can be interpreted as "cluster live sync". ;)

2. Motivation
-------------

This utility was been writted for two purposes:
- for making high availability clusters
- for making backups of them

To do HA cluster I've tried a lot of different solutions, like "simple
rsync by cron", "glusterfs", "ocfs2 over drbd", "common mirrorable external
storage", "incron + perl + rsync", "inosync", "lsyncd" and so on. When I
started to write the utility we was using "lsyncd", "ceph" and
"ocfs2 over drbd". However all of this solutions doesn't arrange me, so I
was have to write own utility for this purpose.

To do backups we also tried a lot of different solution, and again I was have
to write own utility for this purpose.

The best known (for me) replacement for this utility is "lsyncd", however:
- It's code is `>½` on LUA. There a lot of problems connected with it,
for example:
    - It's more difficult to maintain the code with ordinary sysadmin.
    - It really eats 100% CPU sometimes.
    - It requires LUA libs, that cannot be easily installed to few
of our systems.
- It's a little buggy. That may be easily fixed for our cases,
but LUA. :(
- It doesn't support pthread or something like that. It's necessary
to serve huge directories with a lot of containers right.
- It cannot run rsync for a pack of files. It runs rsync for every
event. :(
- Sometimes, it's too complex in configuration for our situation.
- It can't set another event-collecting delay for big files. We don't
want to sync big files (`>1GiB`) so often as ordinary files.
- Shared object (.so file) cannot be used as rsync-wrapper.
- It doesn't support kqueue/bsm

Sorry, if I'm wrong. Let me know if it is, please :). "lsyncd" - is really
interesting and useful utility, just it's not appropriate for us.

UPD.: Also clsync was used to replace incron/csync2/etc in HPC-clusters for
syncing /etc/{passwd,shadow,group,shells} files.

3. inotify vs fanotify:
-----------------------

It's said, that fanotify is much better, than inotify. So I started to write
this program with using of fanotify. However I encountered the problem, that
fanotify was unable to catch some important events at the moment of writing
the program, like "directory creation" or "file deletion". So I switched to
"inotify", leaving the code for "fanotify" in the safety... So, don't use
"fanotify" in this utility ;).


4. Installing
-------------

Debian/ubuntu-users can try to install it directly with apt-get:

    apt-get install clsync

If it's required to install clsync from the source, first of all, you should
install dependencies to compile it. On debian-like systems you should
execute something like:

    apt-get install libglib2.0-dev autoreconf gcc

Next step is generating Makefile. To do that usually it's enought to execute:

    autoreconf -i && ./configure

Next step is compiling. To compile usually it's enough to execute:

    make

Next step is installing. To install usually it's enough to execute:

    su -c 'make install'


5. How to use
-------------

How to use is described in "man" ;). What is not described, you can ask me
personally (see "Support").


6. Example of usage
-------------------

Example of usage, that works on my PC is in directory "examples". Just run
"clsync-start-rsyncdirect.sh" and try to create/modify/delete files/dirs in
"example/testdir/from". All modifications should appear (with some delay) in
directory "example/testdir/to" ;)

For dummies:

    pushd /tmp
    git clone https://github.com/xaionaro/clsync
    cd clsync
    autoreconf -fi
    ./configure
    make
    export PATH_OLD="$PATH"
    export PATH="$(pwd):$PATH"
    cd examples
    ./clsync-start-rsyncdirect.sh
    export PATH="$PATH_OLD"

Now you can try to make changes in directory
"/tmp/clsync/examples/testdir/from" (in another terminal).
Wait about 7 seconds after the changes and check directory
"/tmp/clsync/examples/testdir/to". To finish the experiment press ^C
(control+c) in clsync's terminal.

    cd ../..
    rm -rf clsync
    popd

Note: There's no need to change PATH's value if clsync is installed
system-wide, e.g. with

    make install

For dummies, again (with "make install"):

    pushd /tmp
    git clone https://github.com/xaionaro/clsync
    cd clsync
    autoreconf -fi
    ./configure
    make
    sudo make install
    cd examples
    ./clsync-start-rsyncdirect.sh

Directory "/tmp/clsync/examples/testdir/from" is now synced to
"/tmp/clsync/examples/testdir/to" with 7 seconds delay. To terminate
the clsync press ^C (control+c) in clsync's terminal.

    cd ..
    sudo make uninstall
    cd ..
    rm -rf clsync
    popd

For really dummies or/and lazy users, there's a video demonstration:
[http://ut.mephi.ru/oss/clsync](http://ut.mephi.ru/oss/clsync)


7. Other uses
-------------

Also, clsync may be used to do nearly atomic directory recursive copy.

For example, command

    ionice -c 3 clsync -L /dev/shm/clsync --exit-on-no-events -x 23 -x 24 -M rsyncdirect -S $(which rsync) -W /path/from -D /path/to -d1

may be used to copy "/path/from" into "/path/to" with sync up of changes made (in "/path/from") while the copying. It will copy new changes over and over until there will be no changes, and then clsync will exit.


8. Clustering
-------------

I've started to implement support of bi-directional syncing with using
multicast notifing of other nodes. However it became a long task, so it was
suspended for next releases.

However let's solve next hypothetical problem. For example, you're using
LXC and trying to replicate containers between two servers (to make failover
and load balancing).

In this case you have to sync containers in both directions. However, if you
just run clsync to sync containers to neighboring node on both of them, you'll
get sync-loop [file-update on A causes file-update on B causes file-update
on A causes ...].

Well, in this case I with my colleagues were using separate directories for
every node of cluster (e.g. "`/srv/nodes/<NODE NAME>/containers/<CONTAINERS>`")
and syncing every directory only in one direction. That was failover with
load-balancing, but very unconvenient. So I've started to write code for
bi-directional syncing, however it's no time to complete it :(. So
Andrew Savchenko proposed to run one clsync-instance per container. And this's
really good solution. It's just need to start clsync-process when container
starts and stop the process when containers stops. The only problem is
split-brain, that can be solved two ways:
- by human every time;
- by scripts that chooses which variant of container to save.

Example of the script is just a script that calls "find" on both sides to
determine which side has the latest changes :)

9. Known building issues
------------------------

May be problems with "configuring" or compilation. In this case just try
next command:
    echo '#define REVISION "-custom"' > revision.h; gcc -std=gnu99 -D\_FORTIFY\_SOURCE=2 -DPARANOID -pipe -Wall -ggdb3 --param ssp-buffer-size=4 -fstack-check -fstack-protector-all -Xlinker -zrelro -pthread $(pkg-config --cflags glib-2.0) $(pkg-config --libs glib-2.0) -ldl \*.c -o /tmp/clsync


10. FreeBSD support
-------------------

clsync was been ported to FreeBSD.

FreeBSD doesn't support inotify, so there're 3 ways to use clsync on it:
* using [libinotify](https://github.com/dmatveev/libinotify-kqueue);
* using BSM API;
* using kqueue/kevent directly.

However:
* kqueue/kevent doesn't allow to catch file creation events. However it allows to catch an event of directory content change (without details). So clsync waits for such events and rescans (non-recursively) the whole dir on each such event. This algorithm is not tested and may be buggy. Moreover kqueue/kevent requires to open a file descriptor for every watched file. So this way may eat a lot of CPU and file descriptors.
* libinotify is not production ready. There may be problems with it. Moreover libinotify backends to kqueue API anyway. On the other hand inotify support is well tested in clsync, so this way should be stable (if libinotify is stable) in contrast to kqueue direct use.
* Using of BSM API requires auditd reconfiguration. It may hopple to real audit. Moreover this's a global OS setting. And using of this way forces clsync to catch all FS events of the whole system.

I recommend to use the BSM API at the moment. However when the libinotify will be production ready you should try that way.

I hope you will send me bugreports to make me able to improve the FreeBSD support :)


11. Support
-----------

To get support, you can contact with me this ways:
- Official IRC channel of "clsync": irc.freenode.net#clsync
- Where else can you find me: IRC:SSL+UTF-8 irc.campus.mephi.ru:6695#mephi,xaionaro,xai
- And e-mail: <dyokunev@ut.mephi.ru>, <xaionaro@gmail.com>; PGP pubkey: 0x8E30679C

12. Developing
--------------

I started to write "DEVELOPING" and "PROTOCOL" files.
You can look there if you wish. ;)

I'll be glad to receive code contribution :)

13. Articles
------------

Russian:
- [HA clustering](https://gitlab.ut.mephi.ru/ut/articles/blob/master/clsync/ha)
- [syncing to many nodes](https://gitlab.ut.mephi.ru/ut/articles/blob/master/clsync/inotify-to-many-nodes)
- [atomic sync](https://gitlab.ut.mephi.ru/ut/articles/blob/master/clsync/atomicsync)



                                               -- Dmitry Yu Okunev <dyokunev@ut.mephi.ru> 0x8E30679C