34 lines
1.5 KiB
ReStructuredText
34 lines
1.5 KiB
ReStructuredText
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==========
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Watchers
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==========
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A datasync "watcher" is responsible for checking a given system to see
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if any records are in need of sync (e.g. have changed recently).
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If the given system has a SQL DB, and applicable tables include a
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"last modified" column, then the most common/simple way is for the
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watcher to simply query the table, looking for records which were
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modified since the last check. There are a couple of downsides to
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this approach:
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* must query each table separately, to find all "changed" records
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* not possible to detect "deleted" records in this way
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Rattail itself uses a different approach though. The Rattail DB
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includes a ``change`` table, which in normal circumstances is empty.
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If so configured, Rattail can insert records into this ``change``
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table whenever (almost) *any* data record is changed or deleted etc.
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Then a datasync watcher will look for records in the ``change`` table
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instead of having to query each applicable table separately. The
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watcher also removes ``change`` records as they are processed.
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A similar effect can be achieved if the watched system has a SQL DB to
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which you can install (tables and) triggers. In this case you can
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create a new table (e.g. ``datasync``) and then add triggers for the
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various tables you need to watch. When any record is changed or
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deleted the trigger should add a record to your new ``datasync``
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table. Then the actual watcher can check your ``datasync`` table, and
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remove any found after they are processed.
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Those are just the patterns used thus far; more are likely possible.
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