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

[Pedro Ruivo]

On 11/25/2015 01:20 PM, Radim Vansa wrote:
> On 11/25/2015 12:07 PM, Sanne Grinovero wrote:
>> On 25 November 2015 at 10:48, Pedro Ruivo <pedro at infinispan.org> wrote:
>>>>
>>>> An alternative is to wait for all ACKs, but I think this could still
>>>> be optimised in "triangle shape" too by having the Originator only
>>>> wait for the ACKs from the non-primary replicas?
>>>> So backup owners have to send a confirmation message to the
>>>> Originator, while the Primary owner isn't expecting to do so.
>>> IMO, we should wait for all ACKs to keep our read design.
>
> What exactly is our 'read design'?

If we don't wait for all the ACKs, then we have to go to the primary 
owner for reads, even if the originator is a Backup owner.

>
> 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.

About the iteration between P -> B, you're right. We don't need to wait 
for the ACKs if the messages are sent in FIFO (and JGroups guarantee that)

About the O -> P, IMO, the Originator should wait for the reply from 
Backup. At least, the Primary would be the only one who needs to return 
the previous value (if needed) and it can return if the operation 
succeed or not. This way, it would avoid forking the code for each type 
of command without any benefit (I'm thinking sending the reply to 
originator in parallel with the update message to the backups).

>
> The gains are:
> * less hops (3 instead of 4 if O != P && O != B)
> * less messages (primary ACK is transitive based on ack from B)
> * shorter lock times (not locking during P -> B RPC)
>
>>> However, the
>>> Originator needs to wait for the ACK from Primary because of conditional
>>> operations and functional API.
>> If the operation is successful, Primary will have to let the
>> secondaries know so these can reply to the Originator directly: still
>> saves an hop.

As I said above: "I'm thinking sending the reply to originator in 
parallel with the update message to the backups"

>>
>>> In this first case, if the conditional operation fail, the Backups are
>>> not bothered. The latter case, we may need the return value from the
>>> function.
>> Right, for a failed or rejected operation the secondaries won't even
>> know about it,
>> so the Primary is in charge of letting the Originator know.
>> Essentially you're highlighting that the Originator needs to wait for
>> either the response from secondaries (all of them?)
>> or from the Primary.
>>
>>>> I suspect the tricky part is what happens when the Primary owner rules
>>>> +1 to apply the change, but then the backup owners (all or some of
>>>> them) somehow fail before letting the Originator know. The Originator
>>>> in this case should seek confirmation about its operation state
>>>> (success?) with the Primary owner; this implies that the Primary owner
>>>> needs to keep track of what it's applied and track failures too, and
>>>> this log needs to be pruned.
>
> Currently, in case of lost (timed out) ACK from B to P, we just report
> exception and don't care about synchronizing P and B - B can already
> store updated value.
> So we don't have to care about rollback on P if replication to B fails
> either - we just report that it's broken, sorry.
> Better consolidation API would be nice, though, something like
> cache.getAllVersions().
>
> Radim
>
>
>>>>
>>>> Sounds pretty nice, or am I missing other difficulties?
>>>>
>>>> Thanks,
>>>> Sanne
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