My sync setup looks roughly like this: An application server cyclically reads values from a database, generates HTML files and some images, and dumps everything in the /source directory. I use NFS 4 to mount the /dest1 and /dest2 directories. When any new data arrives in /source, I want it to reach the two target directories /dest1 and /dest2.

Simple and Been Here for Years

Because complex solutions are out of the question, actually the only option left is to dump the task on inotify subsystem's plate. Inotify has been part of the kernel since 2.6.13 and provides an interface to filesystem events for userspace programs.

Apart from incron [1], I've hardly found a use for it so far, but it should be ideal for quick syncing of directories – or at least, these were my ideas on the matter. What was missing was the right tool for the task. Some quick research brought to light two candidates: lsyncd [2] and iWatch [3], but I'll focus on iWatch here.

The monitor is suitable for operation in the foreground or as a daemon. In the simplest case, I just monitor a single directory without triggering an action:

iwatch -r /var

iWatch then reports:

[ 6/May/2014 20:49:30] IN_CREATE/var/tmp/etilqs_SqorfaOvdiBaBI7
[ 6/May/2014 20:49:30] IN_DELETE/var/tmp/etilqs_SqorfaOvdiBaBI7

The -c <action> parameter tells iWatch to respond to events. To avoid all these parameters from hell, a configuration file, as shown in Figure 1, seemed to be a better option. The exciting part of the configuration is in the <path> tag. Here type = recursive tells iWatch to include directories below /source as well.

Figure 1: iWatch can be controlled better by a configuration file than at the command line. The directories to be monitored must be added to the <path> tags.

In case of a filesystem event, the mechanism starts the /home/charly/bash/sync.sh shell script. iWatch passes in the %f variable to the script. The variable resolves to the full path of the file that has changed. The script sync.sh, in turn, is a lean two-liner:

#!/bin/bash
rsync -a --delete $1 /dest1/$1 &
rsync -a --delete $1 /dest2/$1 &

This method works quite well as long as the number of events to be processed does not increase exorbitantly. If it does  – such as when syncing a six-digit number of very small files that the server writes to disk at maximum speed on the plate – you could experience a considerable number of queued rsync processes. Not that this would ever happen to me <cough>!