[infinispan-dev] TopologySafe Map / Reduce

[William Burns]

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 <emmanuel at hibernate.org> 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
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