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AW: FSFS rep-cache validation

From: Markus Schaber <m.schaber_at_codesys.com>
Date: Mon, 27 Jan 2014 12:35:47 +0000


Von: Philip Martin [mailto:philip.martin_at_wandisco.com]
> Philip Martin <philip.martin_at_wandisco.com> writes:
> > Stefan Fuhrmann <stefan.fuhrmann_at_wandisco.com> writes:
> >
> >> But we do write the database strictly in revision order.
> >> So, if we could simply read the latest record once upon opening the
> >> DB. If that refers to a future revision, read "current" and compare.
> >> If the DB it still "in the future", abort txn, i.e. prevent any
> >> future commit until rep-cache.db gets deleted by the admin.
> >
> > That might work. The rep-cache for a new revision is not written
> > strictly in revision order but is written after updating HEAD. So
> > such a check would not be as strong as "highest revision" but would be
> > a useful extra check if we can implement it efficiently it without a
> > table scan. Is sqlite3_last_insert_rowid() the function we want?
> Bert pointed out that is the wrong function and there isn't really a suitable
> function. So to do this check we need something like
> Julian's suggestion: a one row table containing the most recent revision added
> to the rep-cache that gets updated with each write. It doesn't catch every
> possible failure but it does catch some.

To increase the backwards compatibility: Could this row be updated by a trigger?

Best regards

Markus Schaber

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Received on 2014-01-27 13:36:37 CET

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