Re: [infinispan-dev] The "Triangle" pattern for reducing Put latency

You're talking about the case where P applies the PUT, and sends an ACK back to O, but the async updates to the Bs are received by only a subset (or none) of the Bs, and then P crashes. As I was referring about the non-transactional case, wouldn't this be fine? Or do we want the *non-transactional* case to be an atomic update of P and all Bs? IMO, the latter should be done as part of a TX, not for the non-transactional case.

So I think we need to come up with a concise definition of what the transactional versus non-transaction semantics are. But even if we go with a design where O waits for ACKs from *all* Bs, we can still end up with inconsistencies; e.g. when not all Bs received the updates. O will fail the PUT, but the question is what do we do in such a case? Re-submit the PUT?

On 27/11/15 11:12, Radim Vansa wrote: > The update needs to be applied to *all* owners before the call returns > on O. With your strategy, P could apply update, send ACK but the async > backup updates would not be delivered on Bs; so an ACKed update would > get completely lost. > I don't say that these async Bs are not possible, but not in the basic > case - for default configuration, we need to keep the guarantees. > > Radim > > On 11/27/2015 10:34 AM, Bela Ban wrote: >> Adding to what Radim wrote (below), would the following make sense >> (conditions: non-TX, P != O && O != B)? >> >> The lock we acquire on P is actually used to establish an ordering for >> updates to the Bs. So this is very similar to SEQUENCER, expect that we >> have a sequencer (P) *per key*. >> >> Phase 1 >> ------- >> - O sends a PUT(x) message to P >> >> Phase 2 >> ------- >> - P adds PUT(x) to a queue and returns (freeing the up-thread) >> - A thread dequeues PUT(x) and sends an (async) UPDATE message to all Bs >> (possible optimization: send updates to the same key sets as batches) >> - PUT(x) is applied locally and an ACK is sent back to O >> >> O times out and throws an exception if it doesn't receive the ack from P. >> >> This would reduce the current 4 phases (for the above conditions) to 2, >> plus the added latency of processing PUT(x) in the queue. However, we'd >> get rid of the put-while-holding-the-lock issue. >> >> P's updates to the Bs are FIFO ordered, therefore all we need to do is >> send the update down into UNICAST3 (or NAKACK2, if we use multicasts) >> which guarantees ordering. Subsequent updates are ordered according to >> send order. The updates are guaranteed to be retransmitted as long as P >> is alive. >> >> If P crashes before returning the ack to O, or while updating the Bs, >> then O will time out and throw an exception. And, yes, there can be >> inconsistencies, but we're talking about the non-TX case. Perhaps O >> could resubmit PUT(x) to the new P. >> >> I don't know how this behaves wrt rebalancing: are we flushing pending >> updates before installing the new CH? >> >> Thoughts? >> >> >>> I think that the source of optimization is that once primary decides to >>> backup the operation, he can forget about it and unlock the entry. So, >>> we don't need any ACK from primary unless it's an exception/noop >>> notification (as with conditional ops). If primary waited for ACK from >>> backup, we wouldn't save anything. >