On 16/02/2009, at 2:55 PM, Chris Anderson wrote:
Maybe someone could tell the story of
what happens if you just use CouchDB, with it's weak consistency
'weak consistency' is a class of guarantees, not a guarantee per se,
so CouchDB doesn't 'have' weak consistency. In a replicated (as
opposed to local) context, I think it provides virtually no
consistency guarantees.
, and
don't bother even thinking about these issues. Is the naive approach
doomed? Does it suffice to tell the developers who have more relaxed
demands, "keep your documents independent - eg don't use multi-doc
transactions" or can consistency issues bite back in the single-doc
case as well?
It would suffice to say "don't presume that just because you update A
before you update B your code will see A if it can see B". This is the
essence of not having Monotonic Writes. It's not about transactions at
all. Isolated write groups are something that would a) optimise
replication; and b) be useful to application developers (especially
with a fail-on-commit option), but they aren't transactions in the
traditional sense.
The current replication system is fragile for single-doc cases,
because it depends on replication succeeding i.e. AFAICT there's no
guarantee that a given document will ever replicate, unless
replication completes. With Monotonic Writes, the anti-entropy nature
of replication will guarantees that document updates will be seen as
replication makes incremental progress. But I've still not analysed
the replication code, so I don't know whether document replication is
FIFO or not.
I think a Monotonic Writes guarantee makes applications easier to
write, and given that it can be provided, why not do it? By adopting a
formal model, it means we can draw on existing research, and by
adopting a formal conceptual framework, we can mnore fruitfully
discuss these issues.
Antony Blakey
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