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Re: Queries about rep cache: get_shared_rep()

From: Julian Foad <julianfoad_at_gmail.com>
Date: Wed, 27 May 2015 17:35:22 +0100

Stefan Fuhrmann wrote:
> Alright. I gave it a bit more thought now.
> Whenever we encounter this mismatch, something pretty
> bad likely happened to the repo - such as a failed restore
> attempt. In turn, we can expect those situations to be
> very rare - which means we can afford some disruption
> for the user.
> I suggest that we do 3 things:
> * log the warning - for future reference, for being picked
> up by monitoring tools etc.

We already do that.

> * clear the rep-cache.db

Clearing the cache and continuing operation may make subsequent
commits much larger than they should be, and there is no easy way to
undo that if it happens.

Attempting to clear the rep cache might itself fail in some way,
depending on what kind of corruption has happened to it. It would also
destroy the evidence of what went wrong.

> * fail the current commit
> That way, we can be quite sure that only valid data gets
> committed.

Failing the current commit will ensure that no potentially bad (but
undiagnosed) response from the rep cache has already been used in an
earlier part of the transaction. I suppose that's what you're thinking
of. That makes sense to me.

> Alternatively, we could block any commit
> (inventing some new repo state) until the admin resolves
> the situation manually. Not sure which one I would prefer.

I suggest this is the best option, unless we specifically design and
the administrator specifically chooses an option to have higher
availability at the expense of disk space, fault diagnosis, and so on.

> On top of that, we should handle the other rep-cache.db
> consistency checks (e.g. head vs. rev of latest entry)
> the same way.

That makes sense.

I suggest all of this should be treated as a possible future
enhancement, not anything urgent.

- Julian
Received on 2015-05-27 18:35:47 CEST

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