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From Peter Schuller <>
Subject Re: Question about consitency level & data propagation & eventually consistent
Date Wed, 10 Nov 2010 15:27:54 GMT
> 1) If all nodes are up:
>  - Will all writes eventually reach all nodes (of the 3 nodes)?

I believe that if read repair is completely off, then for data that
was written that did *not* get saved by hinted hand-off, would not
propagate until anti-entropy as part of a 'nodetool repair' or perhaps
as part of node movement in the ring (as a side-effect).

Also see under "Consistency".

>  - What will be the maximal time until the last write reaches the last node
> (of the 3 nodes)? (e.g. Assume one of the node is doing compactation at that
> time)

There is no particular time guarantee, unless you yourself take steps
that would imply such a guarantee (such as by running repair with a
certain frequency).

> 2) If one or two nodes are down
> - As I understood it, one node will buffer the writes for the remaining
> nodes.

AFAIK not all. I.e., only when a node is marked as down will hinted
hand-off start eating writes for the node (right, anyone?).

Hinted hand-off is not supposed to be a guarantee that all data will
become up-to-date; it's rather a way to lessen the impact of nodes
going down by decreasing the amount of data that remains out of synch.

> - If the nodes go up again: When will these writes be propagated, at
> compactation?, what will be the maximal time until the writes reach the 2
> nodes? Will these writes be propagated at all?

Again there's no time guarantee as such. As for the writes, I believe
hinted hand-off sends those along independently of compaction (but I'm
not sure).

/ Peter Schuller

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