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From Jan Filipiak <Jan.Filip...@trivago.com>
Subject Re: Reliably implementing global KeyValueStore#get
Date Wed, 07 Jun 2017 21:20:10 GMT

have you thought about using connect to put data into a store that is 
more reasonable for your kind of query requirements?

Best Jan

On 07.06.2017 00:29, Steven Schlansker wrote:
>> On Jun 6, 2017, at 2:52 PM, Damian Guy <damian.guy@gmail.com> wrote:
>> Steven,
>> In practice, data shouldn't be migrating that often. If it is then you
>> probably have bigger problems.
> Understood and agreed, but when designing distributed systems, it usually
> helps to model for the worst case rather than the "well that should never
> happen" case, lest you find yourself fixing those bugs at 3am instead :)
> I'd like to be able to induce extreme pain at the Kafka layer (change leader
> every 3 seconds and migrate all partitions around randomly) and still have
> my app behave correctly.
>> You should be able to use the metadata api
>> to find the instance the key should be on and then when you check that node
>> you can also check with the metadata api that the key should still be on
>> this host. If streams is rebalancing while you query an exception will be
>> raised and you'll need to retry the request once the rebalance has
>> completed.
> Agreed here as well.  But let's assume I have a very fast replication
> setup (assume it takes zero time, for the sake of argument) -- I'm fairly
> sure there's still a race here as this exception only fires *during a migration*
> not *after a migration that may have invalidated your metadata lookup completes*
>> HTH,
>> Damian
>> On Tue, 6 Jun 2017 at 18:11 Steven Schlansker <sschlansker@opentable.com>
>> wrote:
>>>> On Jun 6, 2017, at 6:16 AM, Eno Thereska <eno.thereska@gmail.com> wrote:
>>>> Hi Steven,
>>>> Do you know beforehand if a key exists? If you know that and are getting
>>> null() the code will have to retry by refreshing the metadata and going to
>>> the new instance. If you don’t know beforehand if a key exists or not you
>>> might have to check all instances of a store to make sure.
>>> No, I am not presupposing that the key can exist -- this is a user visible
>>> API and will
>>> be prone to "accidents" :)
>>> Thanks for the insight.  I worry that even checking all stores is not
>>> truly sufficient,
>>> as querying different all workers at different times in the presence of
>>> migrating data
>>> can still in theory miss it given pessimal execution.
>>> I'm sure I've long wandered off into the hypothetical, but I dream of some
>>> day being
>>> cool like Jepsen :)
>>>> Eno
>>>>> On Jun 5, 2017, at 10:12 PM, Steven Schlansker <
>>> sschlansker@opentable.com> wrote:
>>>>> Hi everyone, me again :)
>>>>> I'm still trying to implement my "remoting" layer that allows
>>>>> my clients to see the partitioned Kafka Streams state
>>>>> regardless of which instance they hit.  Roughly, my lookup is:
>>>>> Message get(Key key) {
>>>>>   RemoteInstance instance = selectPartition(key);
>>>>>   return instance.get(key); // http remoting
>>>>> }
>>>>> RemoteInstance.get(Key key) { // http endpoint
>>>>>   return readOnlyKeyValueStore.get(key);
>>>>> }
>>>>> However, the mapping of partitions to instances may change.
>>>>> If you call KeyValueStore.get(K) where K is on a partition you
>>>>> don't own, it returns null.  This is indistinguishable from a
>>>>> successful get on a key that doesn't exist.
>>>>> If one instance selects a sibling instance right as the partition is
>>> failing
>>>>> off of that instance, it may get routed there and by the time it gets
>>>>> the request no longer "owns" the partition -- returns a false 'null'.
>>>>> You can try re-checking after you get a null value, but that's
>>> susceptible
>>>>> to the same race -- it's unlikely but possible that the data migrates
>>> *back*
>>>>> before you do this re-check.
>>>>> Is there any way to correctly implement this without races?  I'd imagine
>>>>> you need a new primitive like KeyValueStore#get that atomically finds
>>>>> the key or throws an exception if it is not in an owned partition
>>>>> at the time of lookup so you know to recheck the partition and retry.
>>>>> Thoughts?
>>>>> Thanks again,
>>>>> Steven

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