Riak consistency state hook
Kresten Krab Thorup
krab at trifork.com
Tue Jul 9 09:48:22 EDT 2013
I've also been pondering these things.
The mere presence of out-of-sync handling begs the question of wether we should show some information about that to "end users", like much like some systems may attaching a "certainty" to a given data point.
It would indeed make sense for some front-end operations to only be enabled when the backing system has a high degree of in-sync, or perhaps the front-end system should display a warning of sorts.
It would be good to know how much we're currently "out of date". For a single key, you can force a check by doing a lookup with a high PR value (reads from primaries), and possibly you can even get that info by getting the details structure back from a read/write.
For the "system overall", it would be nice to have some kind of dashboard that monitors stats for handoffs, read repairs, entropy work, etc. From these stats you can probably compute or display something that says "how in sync" the riak system as a whole (or part) is right now. Without such a system it is hard to know "what is normal".
In general however, it is very difficult to guarantee anything about such an evaluation since it may change at any time, even before you get your response back.
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On Jul 9, 2013, at 3:09 PM, Evgeniy Shishkin <evgeniy.shishkin at gmail.com<mailto:evgeniy.shishkin at gmail.com>> wrote:
Lets say we have a cluster of 6 machines. Somewhere in time 3 machines (A part) got separated from 3 other machines (B part). After that I did some commits on B part (w = 3, handoff occured).
Later the network partitioning has gone. A and B parts are now connected again. One of consistency mechanisms will start to do its job to make DB consistent.
1. Do I have any chance to be acknowledged when this consistency thing will finish its job?
2. More generally, do you think that such mechanism might be useful for end users?
Thank you for your effort.
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