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From Ewen Cheslack-Postava <e...@confluent.io>
Subject Re: [DISCUSS] KIP-228 Negative record timestamp support
Date Wed, 03 Jan 2018 03:48:21 GMT
For `allow.negative.timestamps`, do you mean this as a broker config? I'm
not entirely clear on what the proposal would entail.

I think taking into account whether we're talking about compatibility with
existing data in Kafka vs enabling use of negative timestamps is important
here. If they're effectively not supported today (though admittedly this is
really client-specific), then we need only concern ourselves with data that
hasn't been produced into Kafka yet. In that case, we can always handle
sentinel values in special ways if we really want to. For example, the Java
producer does not accept any values < 0 and the API supports passing null
rather than the sentinels. The implementation could easily be made to map
those values into a range that is less likely to be utilized (e.g. use the
values near Long.MIN_VALUE and have the consumer convert back as needed).
The sentinel for NO_TIMESTAMP could be changed between versions as long as
it is handled consistently between client versions.

IMO we already have way too many configs, so we should think about where
the impact is and if a not ideal, but also not significant compromise can
be made and avoid most of the additional complexity. Introducing the new
config seems like it has significant compatibility concerns that need to be
sorted out. In contrast, I suspect the use cases we need to support that
have come up so far can handle 1 or 2 special cases and the necessary
munging could be handled safely by interceptors such that it is trivial to
make sure all your apps do the right thing. I appreciate the pain of a ton
of mailing list questions about an issue like this, but given the
likelihood of encountering that particular value, I just find it unlikely
it would be that common and I think it's a reasonable tradeoff to tell a
user they might need to handle that one special case.

-Ewen

On Thu, Dec 28, 2017 at 12:58 PM, Matthias J. Sax <matthias@confluent.io>
wrote:

> I agree that changing message format or using a flag bit might not be
> worth it.
>
> However, just keeping -1 as "unknown" leaving a time gap give me a lot
> of headache, too. Your arguments about "not an issue in practice" kinda
> make sense to me, but I see the number of question on the mailing list
> already if we really follow this path... It will confuse users that
> don't pay attention and "loose" data if Kafka Streams drops records with
> timestamp -1 but processes other records with negative timestamps.
>
> Thus, I was wondering if a new topic config (maybe
> `allow.negative.timestamps` with default `false`) that allows for enable
> negative timestamps would be the better solution? With this new config,
> we would not have any sentinel value for "unknown" and all timestamps
> would be valid. Old producers, can't write to those topics if they are
> configured with CREATE_TIME though; APPEND_TIME would still work for
> older producers but with APPEND_TIME no negative timestamps are possible
> in the first place, so this config would not have any impact anyway.
>
> Kafka Streams could check the topic config and only drop negative
> timestamps is they are not enabled. Or course, existing topic should not
> enable negative timestamps if there are records with -1 in them already
> -- otherwise, semantics break down -- but this would be a config error
> we cannot prevent. However, I would expect that mostly newly created
> topics would enable this config anyway.
>
>
> -Matthias
>
> On 12/18/17 10:47 PM, Ewen Cheslack-Postava wrote:
> > I think the trivial change of just recognizing using -1 was a mistake
> for a
> > sentinel value and special casing it while allowing other negative values
> > through is the most practical, reasonable change.
> >
> > Realistically, the scope of impact for that -1 is pretty tiny, as has
> been
> > pointed out. A single millisecond gap in available timestamps in 1969.
> For
> > producers that really want to be careful (as the NYT data might want to
> > be), having the producer layer adjust accordingly is unlikely to be an
> > issue (you can't assume these timestamps are unique anyway, so they
> cannot
> > reasonably used for ordering; adjusting by 1ms is a practical tradeoff).
> >
> > Other approaches where we modify the semantics of the timestamp from the
> > two existing modes require eating up valuable flags in the message
> format,
> > or ramping the message format version, all of which make things
> > significantly messier. Hell, timezones, leap seconds, and ms granularity
> > probably make that 1ms window pretty much moot for any practical
> > applications, and for the extremely rare case that an application might
> > care, they are probably willing to pay the cost of a secondary index if
> > they needed to store timestamp values in the payload rather than in the
> > metadata.
> >
> > Given that we have the current system in place, I suspect that any
> > translation to using Long.MIN_VALUE as the sentinel is probably just more
> > confusing to users, adds more implementation overhead to client
> libraries,
> > and is more likely to introduce bugs.
> >
> > Warts like these always feel wrong when approached from pure design
> > principles, but the fact is that the constraints are already there. To
> me,
> > none of the proposals to move to an encoding we'd prefer seem to add
> enough
> > value to outweigh the migration, compatibility, and implementation costs.
> >
> > @Dong -- your point about special timestamp values is a very good one.
> The
> > issue may extend to other cases in the protocol where we use timestamps.
> Is
> > this the scope we need to worry about (2 values instead of just 1) or are
> > there others? This also might be something we want to look out for in the
> > future -- using special values relative to <SignedIntType>.MIN_VALUE
> > instead of relative to 0.
> >
> > -Ewen
> >
> > On Tue, Dec 12, 2017 at 11:12 AM, Dong Lin <lindong28@gmail.com> wrote:
> >
> >> Hey Konstantin,
> >>
> >> Thanks for updating the KIP.
> >>
> >> If we were to support negative timestamp in the message, we probably
> also
> >> want to support negative timestamp in ListOffsetRequest. Currently in
> >> ListOffsetRequest, timestamp value -2 is used to indicate earliest
> >> timestamp and timestamp value -1 is used to indicate latest timestamp.
> It
> >> seems that we should make changes accordingly so that -1 and -2 can be
> >> supported as valid timestamp in ListOffsetRequest. What do you think?
> >>
> >> Thanks,
> >> Dong
> >>
> >>
> >>
> >> On Mon, Dec 11, 2017 at 12:55 PM, Konstantin Chukhlomin <
> >> chuhlomin@gmail.com
> >>> wrote:
> >>
> >>> Hi all,
> >>>
> >>> I've updated KIP with few more details:
> >>> Added (proposed) Changes in binary message format <
> >>> https://cwiki.apache.org/confluence/display/KAFKA/KIP-228+
> >>> Negative+record+timestamp+support#KIP-228Negativerecordtimes
> >>> tampsupport-Changesinbinarymessageformat>
> >>> Added Changes from producer perspective <https://cwiki.apache.org/conf
> >>> luence/display/KAFKA/KIP-228+Negative+record+timestamp+supp
> >>> ort#KIP-228Negativerecordtimestampsupport-
> Changesfromproducerperspective
> >>>
> >>> Added Changes from consumer perspective <https://cwiki.apache.org/conf
> >>> luence/display/KAFKA/KIP-228+Negative+record+timestamp+supp
> >>> ort#KIP-228Negativerecordtimestampsupport-
> Changesfromconsumerperspective
> >>>
> >>>
> >>> Let me know if it makes sense to you.
> >>>
> >>> -Konstantin
> >>>
> >>>> On Dec 7, 2017, at 2:46 PM, Konstantin Chukhlomin <
> chuhlomin@gmail.com
> >>>
> >>> wrote:
> >>>>
> >>>> Hi Matthias,
> >>>>
> >>>> Indeed for consumers it will be not obvious what −1 means: actual
> >>> timestamp
> >>>> or no timestamp. Nevertheless, it's just −1 millisecond, so I thought
> >> it
> >>> will be
> >>>> not a big deal to leave it (not clean, but acceptable).
> >>>>
> >>>> I agree that it will much cleaner to have a different type of topics
> >>> that support
> >>>> negative timestamp and/or threat Long.MIN_VALUE as a no-timestamp.
> >>>> I'll update KIP to make it a proposed solution.
> >>>>
> >>>> Thanks,
> >>>> Konstantin
> >>>>
> >>>>> On Dec 5, 2017, at 7:06 PM, Matthias J. Sax <matthias@confluent.io>
> >>> wrote:
> >>>>>
> >>>>> Thanks for the KIP Konstantin.
> >>>>>
> >>>>> From my understanding, you propose to just remove the negative
> >> timestamp
> >>>>> check in KafkaProducer and KafkaStreams. If topics are configured
> with
> >>>>> `CreateTime` brokers also write negative timestamps if they are
> >> embedded
> >>>>> in the message.
> >>>>>
> >>>>> However, I am not sure about the overlapping semantics for -1
> >> timestamp.
> >>>>> My concerns is, that this ambiguity might result in issues. Assume
> >> that
> >>>>> there is a topic (configured with `CreateTime`) for which an old
and
> a
> >>>>> new producer are writing. The old producer uses old message format
> and
> >>>>> does not include any timestamp in the message. The broker will
> >> "upgrade"
> >>>>> this message to the new format and set -1. At the same time, the
new
> >>>>> producer could write a message with valid timestamp -1. A consumer
> >> could
> >>>>> not distinguish between both cases...
> >>>>>
> >>>>> Also, there might be other Producer implementations that write
> >> negative
> >>>>> timestamps. Thus, those might already exist. For Streams, we don't
> >>>>> process those and we should make sure to keep it this way (to avoid
> >>>>> ambiguity).
> >>>>>
> >>>>> Thus, it might actually make sense to introduce a new timestamp
type
> >> to
> >>>>> express those new semantics. The question is still, how to deal
with
> >>>>> older producer clients that want to write to those topics.
> >>>>>
> >>>>> - We could either use `Long.MIN_VALUE` as "unknown" (this would
be
> way
> >>>>> better than -1 as it's not in the middle of the range but at the
very
> >>>>> end and it will also have well-defined semantics).
> >>>>> - Or we use a "mixed-mode" where we use broker wall-clock time for
> >>>>> older message formats (ie, append time semantics for older producers)
> >>>>> - Third, we would even give an error message back to older producers;
> >>>>> this might change the backward compatibility guarantees Kafka
> provides
> >>>>> so far when upgrading brokers. However, this would not affect exiting
> >>>>> topics, but only newly created ones (and we could disallow changing
> >> the
> >>>>> semantics to the new timestamp type to guard against miss
> >>>>> configuration). Thus, it might be ok.
> >>>>>
> >>>>> For Streams, we could check the topic config and process negative
> >>>>> timestamps only if the topic is configures with the new timestamp
> >> type.
> >>>>>
> >>>>>
> >>>>> Maybe I am a little bit to paranoid about overloading -1 semantics.
> >>>>> Curious to get feedback from others.
> >>>>>
> >>>>>
> >>>>>
> >>>>> -Matthias
> >>>>>
> >>>>>
> >>>>> On 12/5/17 1:24 PM, Konstantin Chukhlomin wrote:
> >>>>>> Hi Dong,
> >>>>>>
> >>>>>> Currently we are storing historical timestamp in the message.
> >>>>>>
> >>>>>> What we are trying to achieve is to make it possible to do Kafka
> >> lookup
> >>>>>> by timestamp. Ideally I would do `offsetsForTimes` to find articles
> >>> published
> >>>>>> in 1910s (if we are storing articles on the log).
> >>>>>>
> >>>>>> So first two suggestions aren't really covering our use-case.
> >>>>>>
> >>>>>> We could create a new timestamp type like "HistoricalTimestamp"
or
> >>> "MaybeNegativeTimestamp".
> >>>>>> And the only difference between this one and CreateTime is that
it
> >>> could be negative.
> >>>>>> I tend to use CreateTime for this purpose because it's easier
to
> >>> understand from
> >>>>>> user perspective as a timestamp which publisher can set.
> >>>>>>
> >>>>>> Thanks,
> >>>>>> Konstantin
> >>>>>>
> >>>>>>> On Dec 5, 2017, at 3:47 PM, Dong Lin <lindong28@gmail.com>
wrote:
> >>>>>>>
> >>>>>>> Hey Konstantin,
> >>>>>>>
> >>>>>>> Thanks for the KIP. I have a few questions below.
> >>>>>>>
> >>>>>>> Strictly speaking Kafka actually allows you to store historical
> >> data.
> >>> And
> >>>>>>> user are free to encode arbitrary timestamp field in their
Kafka
> >>> message.
> >>>>>>> For example, your Kafka message can currently have Json
or Avro
> >>> format and
> >>>>>>> you can put a timestamp field there. Do you think that could
> address
> >>> your
> >>>>>>> use-case?
> >>>>>>>
> >>>>>>> Alternatively, KIP-82 introduced Record Header in Kafka
and you can
> >>> also
> >>>>>>> define your customized key/value pair in the header. Do
you think
> >>> this can
> >>>>>>> address your use-case?
> >>>>>>>
> >>>>>>> Also, currently there are two types of timestamp according
to
> >> KIP-32.
> >>> If
> >>>>>>> the type is LogAppendTime then the timestamp value is the
time when
> >>> broker
> >>>>>>> receives the message. If the type is CreateTime then the
timestamp
> >>> value is
> >>>>>>> determined when producer produces message. With these two
> >>> definitions, the
> >>>>>>> timestamp should always be positive. We probably need a
new type
> >> here
> >>> if we
> >>>>>>> can not put timestamp in the Record Header or the message
payload.
> >>> Does
> >>>>>>> this sound reasonable?
> >>>>>>>
> >>>>>>> Thanks,
> >>>>>>> Dong
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>> On Tue, Dec 5, 2017 at 8:40 AM, Konstantin Chukhlomin <
> >>> chuhlomin@gmail.com>
> >>>>>>> wrote:
> >>>>>>>
> >>>>>>>> Hi all,
> >>>>>>>>
> >>>>>>>> I have created a KIP to support negative timestamp:
> >>>>>>>> https://cwiki.apache.org/confluence/display/KAFKA/KIP-
> >>>>>>>> 228+Negative+record+timestamp+support <https://cwiki.apache.org/
> >>>>>>>> confluence/display/KAFKA/KIP-228+Negative+record+timestamp+
> >> support>
> >>>>>>>>
> >>>>>>>> Here are proposed changes: https://github.com/apache/
> >>>>>>>> kafka/compare/trunk...chuhlomin:trunk <https://github.com/apache/
> >>>>>>>> kafka/compare/trunk...chuhlomin:trunk>
> >>>>>>>>
> >>>>>>>> I'm pretty sure that not cases are covered, so comments
and
> >>> suggestions
> >>>>>>>> are welcome.
> >>>>>>>>
> >>>>>>>> Thank you,
> >>>>>>>> Konstantin
> >>>>>>
> >>>>>
> >>>>
> >>>
> >>>
> >>
> >
>
>

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