==========
 Watchers
==========

A datasync "watcher" is responsible for checking a given system to see
if any records are in need of sync (e.g. have changed recently).

If the given system has a SQL DB, and applicable tables include a
"last modified" column, then the most common/simple way is for the
watcher to simply query the table, looking for records which were
modified since the last check.  There are a couple of downsides to
this approach:

* must query each table separately, to find all "changed" records
* not possible to detect "deleted" records in this way

Rattail itself uses a different approach though.  The Rattail DB
includes a ``change`` table, which in normal circumstances is empty.
If so configured, Rattail can insert records into this ``change``
table whenever (almost) *any* data record is changed or deleted etc.
Then a datasync watcher will look for records in the ``change`` table
instead of having to query each applicable table separately.  The
watcher also removes ``change`` records as they are processed.

A similar effect can be achieved if the watched system has a SQL DB to
which you can install (tables and) triggers.  In this case you can
create a new table (e.g. ``datasync``) and then add triggers for the
various tables you need to watch.  When any record is changed or
deleted the trigger should add a record to your new ``datasync``
table.  Then the actual watcher can check your ``datasync`` table, and
remove any found after they are processed.

Those are just the patterns used thus far; more are likely possible.