Actually this was something I was hoping to get to possibly in the near future.
I already have to do https://issues.jboss.org/browse/ISPN-4358 which will require rewriting parts of the distributed entry iterator. In doing so I was planning on breaking this out to a more generic framework where you could run a given operation by segment guaranteeing it was only ran once per entry. In doing so I was thinking I could try to move M/R on top of this to allow it to also be resilient to rehash events. Additional comments inline. On Thu, Oct 9, 2014 at 8:18 AM, Emmanuel Bernard <[email protected]> wrote: > Pedro and I have been having discussions with the LEADS guys on their > experience of Map / Reduce especially around stability during topology > changes. > > This ties to the .size() thread you guys have been exchanging on (I only > could read it partially). > > On the requirements, theirs is pretty straightforward and expected I think > from most users. > They are fine with inconsistencies with entries create/updated/deleted > between the M/R start and the end. There is no way we can fix this without adding a very strict isolation level like SERIALIZABLE. > They are *not* fine with seeing the same key/value several time for the > duration of the M/R execution. This AFAIK can happen when a topology change > occurs. This can happen if it was processed on one node and then rehash migrates the entry to another and runs it there. > > Here is a proposal. > Why not run the M/R job not per node but rather per segment? > The point is that segments are stable across topology changes. The M/R tasks > would then be about iterating over the keys in a given segment. > > The M/R request would send the task per segments on each node where the > segment is primary. This is exactly what the iterator does today but also watches for rehashes to send the request to a new owner when the segment moves between nodes. > (We can imagine interesting things like sending it to one of the backups for > workload optimization purposes or sending it to both primary and backups and > to comparisons). > The M/R requester would be in an interesting situation. It could detect that > a segment M/R never returns and trigger a new computation on another node > than the one initially sent. > > One tricky question around that is when the M/R job store data in an > intermediary state. We need some sort of way to expose the user indirectly to > segments so that we can evict per segment intermediary caches in case of > failure or retry. This was one place I was thinking I would need to take special care to look into when doing a conversion like this. > > But before getting ahead of ourselves, what do you thing of the general idea? > Even without retry framework, this approach would be more stable than our > current per node approach during topology changes and improve dependability. Doing it solely based on segment would remove the possibility of having duplicates. However without a mechanism to send a new request on rehash it would be possible to only find a subset of values (if a segment is removed while iterating on it). > > Emmanuel > _______________________________________________ > infinispan-dev mailing list > [email protected] > https://lists.jboss.org/mailman/listinfo/infinispan-dev _______________________________________________ infinispan-dev mailing list [email protected] https://lists.jboss.org/mailman/listinfo/infinispan-dev
