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From "Matthias J. Sax" <matth...@confluent.io>
Subject Re: [DISCUSS] KIP-280: Enhanced log compaction
Date Sun, 06 May 2018 16:37:21 GMT
Hi,

I just updated myself on this KIP. One question (maybe it was discussed
and I missed it). What is the motivation to not use the offset as tie
breaker for the "timestamp" case? Isn't this inconsistent behavior?


-Matthias


On 5/2/18 2:07 PM, Guozhang Wang wrote:
> Hello Luís,
> 
> Sorry for the late reply.
> 
> My understanding is that such duplicates will only happen if the non-offset
> version value, either the timestamp or some long-typed header key, are the
> same (i.e. we cannot break ties).
> 
> 1. For timestamp, which is in milli-seconds, I think in practice the
> likelihood of records with the same key and the same milli-sec timestamp
> are very small. And hence the duplicate amount should be very small.
> 
> 2. For long-typed header key, it is arguably out of Kafka's control, and
> indeed users may (mistakenly) generate many records with the same key and
> the same header value.
> 
> 
> So I'd like to propose a counter-offer: for 1), we still use only 8 bytes
> and allows for potential duplicates due to ties; for 2) we use 16 bytes to
> always break ties. The motivation for distinguishing 1) and 2), is that my
> expectation for 1) would be much common, and hence worth special handling
> it to be more effective in cleaning. WDYT?
> 
> 
> Guozhang
> 
> 
> 
> On Wed, May 2, 2018 at 2:36 AM, Luís Cabral <luis_cabral@yahoo.com.invalid>
> wrote:
> 
>>  Hi Guozhang,
>>
>> Have you managed to have a look at my reply?
>> How do you feel about this?
>>
>> Kind Regards,
>> Luís Cabral
>>     On Monday, April 30, 2018, 9:27:15 AM GMT+2, Luís Cabral <
>> luis_cabral@yahoo.com> wrote:
>>
>>   Hi Guozhang,
>>
>> I understand the argument, but this is a hazardous compromise for using
>> Kafka as an event store (as is my original intention).
>>
>> I expect to have many duplicated messages in Kafka as the overall
>> architecture being used allows for the producer to re-send a fresh state of
>> the backed data into Kafka.Though this scenario is not common, as the
>> intention is for Kafka to bear the weight of replaying all the records for
>> new consumers, but it will occasionally happen.
>>
>> As there are plenty of records which are not updated frequently, this
>> would leave the topic with a surplus of quite a few million duplicate
>> records (and increasing every time the above mentioned function is applied).
>>
>> I would prefer to have the temporary memory footprint of 8 bytes per
>> record whenever the compaction is run (only when not in 'offset' mode),
>> than allowing for the topic to run into this state.
>>
>> What do you think? Is this scenario too specific for me, or do you believe
>> that it could happen to other clients as well?
>>
>> Thanks again for the continued discussion!
>> Cheers,
>> Luis    On Friday, April 27, 2018, 8:21:13 PM GMT+2, Guozhang Wang <
>> wangguoz@gmail.com> wrote:
>>
>>  Hello Luis,
>>
>> When the comparing the version returns `equal`, the original proposal is to
>> use the offset as the tie breaker. My previous comment is that
>>
>> 1) when we build the map calling `put`, if there is already an entry for
>> the key, compare its stored version, and replace if the put record's
>> version is "no smaller than" the stored record: this is because when
>> building the map we are always going from smaller offsets to larger ones.
>>
>> 2) when making a second pass to determine if each record should be retained
>> based on the map, we do not try to break the tie if the map's returned
>> version is the same but always treat it as "keep". In this case when we are
>> comparing a record with itself stored in the offset map, version comparison
>> would return `equals`. As I mentioned in the PR, one caveat is that we may
>> indeed have multiple records with the same key and the same version, but
>> once a new versioned record is appended it will be deleted.
>>
>>
>> Does that make sense?
>>
>> Guozhang
>>
>>
>> On Fri, Apr 27, 2018 at 4:20 AM, Luís Cabral <luis_cabral@yahoo.com.invalid
>>>
>> wrote:
>>
>>>  Hi,
>>>
>>> I was updating the PR to match the latest decisions and noticed (or
>>> rather, the integration tests noticed) that without storing the offset,
>>> then the cache doesn't know when to keep the record itself.
>>>
>>> This is because, after the cache is populated, all the records are
>>> compared against the stored ones, so "Record{key:A,offset:1,version:1}"
>>> will compare against itself and be flagged as "don't keep", since we only
>>> compared based on the version and didn't check to see if the offset was
>> the
>>> same or not.
>>>
>>> This sort of invalidates not storing the offset in the cache,
>>> unfortunately, and the binary footprint increases two-fold when "offset"
>> is
>>> not used as a compaction strategy.
>>>
>>> Guozhang: Is it ok with you if we go back on this decision and leave the
>>> offset as a tie-breaker?
>>>
>>>
>>> Kind Regards,Luis
>>>
>>>    On Friday, April 27, 2018, 11:11:55 AM GMT+2, Luís Cabral
>>> <luis_cabral@yahoo.com.INVALID> wrote:
>>>
>>>  Hi,
>>>
>>> The KIP is now updated with the results of the byte array discussion.
>>>
>>> This is my first contribution to Kafka, so I'm not sure on what the
>>> processes are. Is it now acceptable to take this into a vote, or should I
>>> ask for more contributors to join the discussion first?
>>>
>>> Kind Regards,Luis    On Friday, April 27, 2018, 6:12:03 AM GMT+2,
>> Guozhang
>>> Wang <wangguoz@gmail.com> wrote:
>>>
>>>  Hello Luís,
>>>
>>>> Offset is an integer? I've only noticed it being resolved as a long so
>>> far.
>>>
>>> You are right, offset is a long.
>>>
>>> As for timestamp / other types, I left a comment in your PR about
>> handling
>>> tie breakers.
>>>
>>>> Given these arguments, is this point something that you absolutely must
>>> have?
>>>
>>> No I do not have a strong use case in mind to go with arbitrary byte
>>> arrays, was just thinking that if we are going to enhance log compaction
>>> why not generalize it more :)
>>>
>>> Your concern about the memory usage makes sense. I'm happy to take my
>>> suggestion back and enforce only long typed fields.
>>>
>>>
>>> Guozhang
>>>
>>>
>>>
>>>
>>>
>>> On Thu, Apr 26, 2018 at 1:44 AM, Luís Cabral
>> <luis_cabral@yahoo.com.invalid
>>>>
>>> wrote:
>>>
>>>>  Hi,
>>>>
>>>> bq. have a integer typed OffsetMap (for offset)
>>>>
>>>> Offset is an integer? I've only noticed it being resolved as a long so
>>> far.
>>>>
>>>>
>>>> bq. long typed OffsetMap (for timestamp)
>>>>
>>>> We would still need to store the offset, as it is functioning as a
>>>> tie-breaker. Not that this is a big deal, we can be easily have both
>> (as
>>>> currently done on the PR).
>>>>
>>>>
>>>> bq. For the byte array typed offset map, we can use a general hashmap,
>>>> where the hashmap's CAPACITY will be reasoned from the given "val
>> memory:
>>>> Int" parameter
>>>>
>>>> If you have a map with 128 byte capacity, then store a value with 16
>>> bytes
>>>> and another with 32 bytes, how many free slots do you have left in this
>>> map?
>>>>
>>>> You can make this work, but I think you would need to re-design the
>> whole
>>>> log cleaner approach, which implies changing some of the already
>> existing
>>>> configurations (like "log.cleaner.io.buffer.load.factor"). I would
>>> rather
>>>> maintain backwards compatibility as much as possible in this KIP, and
>> if
>>>> this means that using "foo" / "bar" or "2.1-a" / "3.20-b" as record
>>>> versions is not viable, then so be it.
>>>>
>>>> Given these arguments, is this point something that you absolutely must
>>>> have? I'm still sort of hoping that you are just entertaining the idea
>>> and
>>>> are ok with having a long (now conceded to be unsigned, so the byte
>>> arrays
>>>> can be compared directly).
>>>>
>>>>
>>>> Kind Regards,Luis
>>>>
>>>
>>>
>>>
>>> --
>>> -- Guozhang
>>>
>>
>>
>>
>> --
>> -- Guozhang
>>
> 
> 
> 


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