[infinispan-issues] [JBoss JIRA] (ISPN-9690) EXCEPTION eviction strategy should not require transactions

[ https://issues.jboss.org/browse/ISPN-9690?page=com.atlassian.jira.plugin.... ] Galder Zamarreño updated ISPN-9690: ----------------------------------- Comment: was deleted (was: A chat discussion: {code} William Burns: @Galder responded to your email, however we can talk here maybe William Burns: actually @Galder I can see not requiring transactions when the cache mode is SIMPLE, LOCAL or INVALIDATION William Burns: but that is not an option currently, would have to be added Galder: Hmmm, we want this for DIST William Burns: yeah then we would need two phase commit like I mentioned in my email William Burns: since back up owner may reject a write, but primary may be fine William Burns: unless we want to relax that requirement Galder: Hmmmm Galder: Assuming this is DIST, can't we start rejecting before the limit is reached? William Burns: the way it currently works is each owner checks if their current size + new entry size > max size and if any of the owners reject, we throw the ContainerFullException Galder: I mean: could an owner guess that adding something might end up topping non-owners and reject it? Dan: @Galder @William Burns we could split the limit per segment and throw an exception once a segment is full Galder: @William Burns Do you understand what I mean? Galder: @Dan Don't think that'd solve the problem Galder: You could still have issues where the primary owner is OK but the backup owner ends up failing due to segment full? William Burns: @Galder do you mean that each node has an idea of how full another owner is, updated every so often in the background? Galder: Or is the segment size fixed? Galder: @William Burns Maybe... Or some other way that does not required communication Galder: Segment based CH means we have more or less the same amount of data everywhere... Galder: Or at least relatively similar number of entries in each node Galder: In essence, could each primary owner probabilistically decide that doing a put would topple others? Dan: @Galder yeah, the number of entries is typically within 10% of the average Galder: There you go, 10% Galder: I mean with EXCEPTION strategy only Dan: But that's for all the entries on a node, a segment always has the same entries on all the owners that have it Galder: Yeah but each node has different segments Galder: That's where the variance comes in, right? Dan: yes, the segments are different Dan: sometimes by more than 10% :) Galder: @William Burns I'm not necessarily suggesting each node keeps track of others, i'm more suggesting that given how full a node is, it could decide that others are likely full... Dan: I was thinking of computing a fixed limit per segment, but you're right, that depends on how many nodes there are in the cluster William Burns: So we are thinking of some fixed percentage, that if a node is actually current > (max *(1-%)) it would throw the exception and not consult other nodes. In that case I would say why even have this %, just let the user configure the max directly knowing this is the behavior William Burns: and that some nodes can technically go over William Burns: this is also very problematic if you have a lot of writes to same nodes William Burns: ie. 3 nodes A, B, C if majority of writes are always to A and B and not C, C could get OOM Galder: CH should take care of too many writes in some nodes Galder: But true, some keys could be written more Dan: @William Burns do you mean only throw the exception on the primary owner, and accept any write on the backup? William Burns: well that is what Galder is after, so we don't have to use transactions William Burns: is my assumption Galder: So we are thinking of some fixed percentage, that if a node is actually current > (max *(1-%)) it would throw the exception and not consult other nodes. In that case I would say why even have this %, just let the user configure the max directly knowing this is the behavior But the user already configures the max, doesn't it? William Burns: yes and we use transactions to ensure max is never reach on any node Galder: Right, but using transactions for this seems too heavyweight Dan: @Galder transactions are the only way to make it precise, so we only insert an entry if all the owners have room for it William Burns: unfortunately without some sort of two phase operation we can some nodes updated and some others - and I figured don't reinvent the wheel Dan: Any non-2PC solution will have to either allow some inserts to fail on some nodes and succeed on others, or have some nodes go over the configured max William Burns: now I am open to possibly allowing others to go over, but we have to make sure the user knows this to possibly lower the max Galder: Any non-2PC solution will have to either allow some inserts to fail on some nodes and succeed on others, or have some nodes go over the configured max Would that be fine if partition handling was enabled? Dan: @William Burns I've been dreaming about lite transactions that don't need a xid and all that baggage ever since Mircea removed support for mixing tx and non-tx operations in the same cache :) Dan: @Galder Would that be fine if partition handling was enabled? Depends on what you mean by "fine" :) William Burns: @Dan yeah tbh I was thinking that would be nice for this use case Galder: So when a node fails due to memory issues, we switch the readiness probe to say node not healthy... then the user gives it more memory to the pods... and one at the time they get restarted... partition handling would kick in and address partial replications Galder: ^ I might be day dreaming :see_no_evil: William Burns: hrmm, I think just regular rebalance should repair it William Burns: don't need partition handling William Burns: assuming we haven't lost >= numOwners William Burns: and then nothing can save it :D Galder: Exactly Galder: TBH, the above should be done by the autoscaler Dan: assuming we haven't lost >= numOwners Yeah, that's the big assumption -- if one node is shut down because it wanted to insert too many entries, those entries will be redirected somewhere else Galder: Rather than manually by someone Dan: So an exception in one node may trigger the restart of the entire cluster William Burns: yeah actually the more I think about it losing that node could lose data then William Burns: since state transfer writes may be blocked William Burns: if this fills up another node Dan: @Galder does OS/K8S keep pods around if they're not healthy? Galder: Hmmmm, actually, I think it tries to restart them until they're healthy Dan: Yeah, that's what I thought William Burns: this is the age old issue of eviction without a store can lose data on a rehash, irrespective of numOwners William Burns: @Galder will you have a store? Galder: Actually Galder: @Dan Depends on the config... Galder: @Dan Depends how many times the readiness probe fails, at which point it'd restart it Galder: https://kubernetes.io/docs/tasks/configure-pod-container/configure-livene... Galder: @Galder will you have a store? Not by default William Burns: so in that case I would say we should err on the side of making sure no node has too much as much as possible Galder: Ok Galder: What if you had a store? William Burns: if we had a store then partial updates should be fine William Burns: ie. primary writes the value and backup ignores it Galder: But the stores would also have partial udpates? William Burns: but we would have to implement writing to a store but not memory Galder: exactly Galder: Ok Galder: I'll create a JIRA and summarise the discussion for now Galder: I have to head out in a few William Burns: alright William Burns: we could even optimize it for shared stores William Burns: :D William Burns: actually that is interesting... William Burns: we could have a store mode with shared where we never write the entry to memory on backups Galder: Local stores are preferable Galder: One less thing to worry about William Burns: yeah agreed for this use case, I was just thinking more in general Galder: @Dan @William Burns We want the default cache in data grid service to use EXCEPTION strategy, so that data is preserved (as opposed to the cache service where it evicts data, it's a cache), so would you make the default cache transactional? Or have a local file based store instead? Galder: I'd go for the latter, just wondering if you see any case where the former would be better Galder: Actually, with the current state you still need transactions because we don't write to store if memory full William Burns: yeah latter is only way to not lose data William Burns: former may be faster? would have to confirm Galder: Yeah, but local file based persistence alone is not enough today to make EXCEPTION strategy work? William Burns: no, tbh this seems like a different eviction mode at this point with a store William Burns: cause you would never get an EXCEPTION William Burns: tbh, I don't know if this is worth it Galder: true William Burns: this is essentially regular eviction with a store William Burns: just changes what elements are in memory Galder: True... William Burns: yeah now I remember before that I thought EXCEPTION based eviction and stores don't make that much sense together {code}) -- This message was sent by Atlassian Jira (v7.12.1#712002)