This thread has gone rather long. So I might not be fully clear on everybody's positions.
To make sure I understand the possible proposals. Here is a concrete and 'must have' scenario for LinkedIn. We want to bootstrap from say a Database dump (espresso) into a kafka topic. (Databus bootstrap) What should be the creationTime that we stamp when we send into the Kafka topic ? Would we use the time at which the database record was touched (event time) ? or should it just be the local system time when we are publishing into the Kafka bootstrap topic. If it is the former (original database update time), then if I understand the proposal then we would set the max.append.delay to be large and allow the message to be produced into Kafka. However this would mean that older events could possibly (in the worst case) be cleaned up as soon as they are published ? So this option wouldn't work. If it is the later (when we publish into kafka.. we just stamp the current local time ), then that would work.. it is however as good as stamping it on the broker.. In this world the "event time" will be in the avro payload.. in case a downstream application wants to know when the event actually happened. On Thu, Oct 22, 2015 at 11:02 AM, Jay Kreps <j...@confluent.io> wrote: > Hey Kartik, > > Yes, I agree exactly with your characterization. > > The question is "what is the meaning of retention?" It could mean either: > 1. "Retain data that is no more than 7 days old" > 2. "Retain data for 7 days from when you get it" > > I don't know if either is actually a clear winner. > > Each is intuitive and easily expressible: > 1. "We have the last 7 days of data in Kafka" > 2. "You have 7 days to get your data from Kafka from whenever it arrives" > > Each has corner cases: > 1. May lead to retaining data for too little time in the bootstrap case > 2. May lead to over-retention in the bootstrap case > > Which failure is worse probably depends on who you ask: > 1. May lead to not retaining data long enough for a consumer to get it, > which the consumer would say is very bad > 2. May lead to running out of disk space, which ops would say is very bad > > -Jay > > > > > On Tue, Oct 20, 2015 at 12:02 AM, Kartik Paramasivam < > kparamasi...@linkedin.com.invalid> wrote: > > > Joel or Becket will probably respond back in more detail.. but here are > my > > 2c. > > > > From the standpoint of LinkedIN, the suggested proposal works.. in > essence > > max.appenddelay can be used to turn "creationTime" into "logAppendTime". > > this does mean that at LinkedIn we won't be able to use > "creationTime".. > > however that might also be fine because we anyways use the timeStamp that > > is set inside the avro payload. > > > > Keeping LI aside though, it looks like there are two distinct possible > > goals. > > 1. The broker will retain messages for x days after a message shows up at > > the broker. This behavior would super deterministic and would never > > change depending on the contents of the message or anything else. > > > > 2. The client is in "partial" control of how long a message stays in the > > broker based on the creationTime stamped by the client. > > > > Although (2) could be a feature in some scenarios..but in many scenarios > it > > can be pretty unintuitive and be perceived as an anti-feature. For e.g > say > > a mobile client buffered up some messages because the device was offline > > (maybe in a plane).. and then sent the message after say 23 hours on a > > plane. The message shows up in a Kafka topic with 24 hour retention.. > and > > now the message gets deleted in 1 hour. > > > > Kartik > > > > > > On Tue, Oct 13, 2015 at 12:32 AM, Jay Kreps <j...@confluent.io> wrote: > > > > > Here's my basic take: > > > - I agree it would be nice to have a notion of time baked in if it were > > > done right > > > - All the proposals so far seem pretty complex--I think they might make > > > things worse rather than better overall > > > - I think adding 2x8 byte timestamps to the message is probably a > > > non-starter from a size perspective > > > - Even if it isn't in the message, having two notions of time that > > control > > > different things is a bit confusing > > > - The mechanics of basing retention etc on log append time when that's > > not > > > in the log seem complicated > > > > > > To that end here is a possible 4th option. Let me know what you think. > > > > > > The basic idea is that the message creation time is closest to what the > > > user actually cares about but is dangerous if set wrong. So rather than > > > substitute another notion of time, let's try to ensure the correctness > of > > > message creation time by preventing arbitrarily bad message creation > > times. > > > > > > First, let's see if we can agree that log append time is not something > > > anyone really cares about but rather an implementation detail. The > > > timestamp that matters to the user is when the message occurred (the > > > creation time). The log append time is basically just an approximation > to > > > this on the assumption that the message creation and the message > receive > > on > > > the server occur pretty close together and the reason to prefer . > > > > > > But as these values diverge the issue starts to become apparent. Say > you > > > set the retention to one week and then mirror data from a topic > > containing > > > two years of retention. Your intention is clearly to keep the last > week, > > > but because the mirroring is appending right now you will keep two > years. > > > > > > The reason we are liking log append time is because we are > (justifiably) > > > concerned that in certain situations the creation time may not be > > > trustworthy. This same problem exists on the servers but there are > fewer > > > servers and they just run the kafka code so it is less of an issue. > > > > > > There are two possible ways to handle this: > > > > > > 1. Just tell people to add size based retention. I think this is not > > > entirely unreasonable, we're basically saying we retain data based > on > > > the > > > timestamp you give us in the data. If you give us bad data we will > > > retain > > > it for a bad amount of time. If you want to ensure we don't retain > > "too > > > much" data, define "too much" by setting a time-based retention > > setting. > > > This is not entirely unreasonable but kind of suffers from a "one > bad > > > apple" problem in a very large environment. > > > 2. Prevent bad timestamps. In general we can't say a timestamp is > bad. > > > However the definition we're implicitly using is that we think there > > > are a > > > set of topics/clusters where the creation timestamp should always be > > > "very > > > close" to the log append timestamp. This is true for data sources > that > > > have > > > no buffering capability (which at LinkedIn is very common, but is > more > > > rare > > > elsewhere). The solution in this case would be to allow a setting > > along > > > the > > > lines of max.append.delay which checks the creation timestamp > against > > > the > > > server time to look for too large a divergence. The solution would > > > either > > > be to reject the message or to override it with the server time. > > > > > > So in LI's environment you would configure the clusters used for > direct, > > > unbuffered, message production (e.g. tracking and metrics local) to > > enforce > > > a reasonably aggressive timestamp bound (say 10 mins), and all other > > > clusters would just inherent these. > > > > > > The downside of this approach is requiring the special configuration. > > > However I think in 99% of environments this could be skipped entirely, > > it's > > > only when the ratio of clients to servers gets so massive that you need > > to > > > do this. The primary upside is that you have a single authoritative > > notion > > > of time which is closest to what a user would want and is stored > directly > > > in the message. > > > > > > I'm also assuming there is a workable approach for indexing > non-monotonic > > > timestamps, though I haven't actually worked that out. > > > > > > -Jay > > > > > > On Mon, Oct 5, 2015 at 8:52 PM, Jiangjie Qin <j...@linkedin.com.invalid > > > > > wrote: > > > > > > > Bumping up this thread although most of the discussion were on the > > > > discussion thread of KIP-31 :) > > > > > > > > I just updated the KIP page to add detailed solution for the option > > > (Option > > > > 3) that does not expose the LogAppendTime to user. > > > > > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-32+-+Add+CreateTime+and+LogAppendTime+to+Kafka+message > > > > > > > > The option has a minor change to the fetch request to allow fetching > > time > > > > index entry as well. I kind of like this solution because its just > > doing > > > > what we need without introducing other things. > > > > > > > > It will be great to see what are the feedback. I can explain more > > during > > > > tomorrow's KIP hangout. > > > > > > > > Thanks, > > > > > > > > Jiangjie (Becket) Qin > > > > > > > > On Thu, Sep 10, 2015 at 2:47 PM, Jiangjie Qin <j...@linkedin.com> > > wrote: > > > > > > > > > Hi Jay, > > > > > > > > > > I just copy/pastes here your feedback on the timestamp proposal > that > > > was > > > > > in the discussion thread of KIP-31. Please see the replies inline. > > > > > The main change I made compared with previous proposal is to add > both > > > > > CreateTime and LogAppendTime to the message. > > > > > > > > > > On Tue, Sep 8, 2015 at 10:57 AM, Jay Kreps <j...@confluent.io> > wrote: > > > > > > > > > > > Hey Beckett, > > > > > > > > > > > > I was proposing splitting up the KIP just for simplicity of > > > discussion. > > > > > You > > > > > > can still implement them in one patch. I think otherwise it will > be > > > > hard > > > > > to > > > > > > discuss/vote on them since if you like the offset proposal but > not > > > the > > > > > time > > > > > > proposal what do you do? > > > > > > > > > > > > Introducing a second notion of time into Kafka is a pretty > massive > > > > > > philosophical change so it kind of warrants it's own KIP I think > it > > > > > isn't > > > > > > just "Change message format". > > > > > > > > > > > > WRT time I think one thing to clarify in the proposal is how MM > > will > > > > have > > > > > > access to set the timestamp? Presumably this will be a new field > in > > > > > > ProducerRecord, right? If so then any user can set the timestamp, > > > > right? > > > > > > I'm not sure you answered the questions around how this will work > > for > > > > MM > > > > > > since when MM retains timestamps from multiple partitions they > will > > > > then > > > > > be > > > > > > out of order and in the past (so the max(lastAppendedTimestamp, > > > > > > currentTimeMillis) override you proposed will not work, right?). > If > > > we > > > > > > don't do this then when you set up mirroring the data will all be > > new > > > > and > > > > > > you have the same retention problem you described. Maybe I missed > > > > > > something...? > > > > > lastAppendedTimestamp means the timestamp of the message that last > > > > > appended to the log. > > > > > If a broker is a leader, since it will assign the timestamp by > > itself, > > > > the > > > > > lastAppenedTimestamp will be its local clock when append the last > > > > message. > > > > > So if there is no leader migration, max(lastAppendedTimestamp, > > > > > currentTimeMillis) = currentTimeMillis. > > > > > If a broker is a follower, because it will keep the leader's > > timestamp > > > > > unchanged, the lastAppendedTime would be the leader's clock when it > > > > appends > > > > > that message message. It keeps track of the lastAppendedTime only > in > > > case > > > > > it becomes leader later on. At that point, it is possible that the > > > > > timestamp of the last appended message was stamped by old leader, > but > > > the > > > > > new leader's currentTimeMillis < lastAppendedTime. If a new message > > > > comes, > > > > > instead of stamp it with new leader's currentTimeMillis, we have to > > > stamp > > > > > it to lastAppendedTime to avoid the timestamp in the log going > > > backward. > > > > > The max(lastAppendedTimestamp, currentTimeMillis) is purely based > on > > > the > > > > > broker side clock. If MM produces message with different > > LogAppendTime > > > in > > > > > source clusters to the same target cluster, the LogAppendTime will > be > > > > > ignored re-stamped by target cluster. > > > > > I added a use case example for mirror maker in KIP-32. Also there > is > > a > > > > > corner case discussion about when we need the max(lastAppendedTime, > > > > > currentTimeMillis) trick. Could you take a look and see if that > > answers > > > > > your question? > > > > > > > > > > > > > > > > > My main motivation is that given that both Samza and Kafka > streams > > > are > > > > > > doing work that implies a mandatory client-defined notion of > time, > > I > > > > > really > > > > > > think introducing a different mandatory notion of time in Kafka > is > > > > going > > > > > to > > > > > > be quite odd. We should think hard about how client-defined time > > > could > > > > > > work. I'm not sure if it can, but I'm also not sure that it > can't. > > > > Having > > > > > > both will be odd. Did you chat about this with Yi/Kartik on the > > Samza > > > > > side? > > > > > I talked with Kartik and realized that it would be useful to have a > > > > client > > > > > timestamp to facilitate use cases like stream processing. > > > > > I was trying to figure out if we can simply use client timestamp > > > without > > > > > introducing the server time. There are some discussion in the KIP. > > > > > The key problem we want to solve here is > > > > > 1. We want log retention and rolling to depend on server clock. > > > > > 2. We want to make sure the log-assiciated timestamp to be retained > > > when > > > > > replicas moves. > > > > > 3. We want to use the timestamp in some way that can allow > searching > > by > > > > > timestamp. > > > > > For 1 and 2, an alternative is to pass the log-associated timestamp > > > > > through replication, that means we need to have a different > protocol > > > for > > > > > replica fetching to pass log-associated timestamp. It is actually > > > > > complicated and there could be a lot of corner cases to handle. > e.g. > > > what > > > > > if an old leader started to fetch from the new leader, should it > also > > > > > update all of its old log segment timestamp? > > > > > I think actually client side timestamp would be better for 3 if we > > can > > > > > find a way to make it work. > > > > > So far I am not able to convince myself that only having client > side > > > > > timestamp would work mainly because 1 and 2. There are a few > > > situations I > > > > > mentioned in the KIP. > > > > > > > > > > > > When you are saying it won't work you are assuming some > particular > > > > > > implementation? Maybe that the index is a monotonically > increasing > > > set > > > > of > > > > > > pointers to the least record with a timestamp larger than the > index > > > > time? > > > > > > In other words a search for time X gives the largest offset at > > which > > > > all > > > > > > records are <= X? > > > > > It is a promising idea. We probably can have an in-memory index > like > > > > that, > > > > > but might be complicated to have a file on disk like that. Imagine > > > there > > > > > are two timestamps T0 < T1. We see message Y created at T1 and > > created > > > > > index like [T1->Y], then we see message created at T1, supposedly > we > > > > should > > > > > have index look like [T0->X, T1->Y], it is easy to do in memory, > but > > we > > > > > might have to rewrite the index file completely. Maybe we can have > > the > > > > > first entry with timestamp to 0, and only update the first pointer > > for > > > > any > > > > > out of range timestamp, so the index will be [0->X, T1->Y]. Also, > the > > > > range > > > > > of timestamps in the log segments can overlap with each other. That > > > means > > > > > we either need to keep a cross segments index file or we need to > > check > > > > all > > > > > the index file for each log segment. > > > > > I separated out the time based log index to KIP-33 because it can > be > > an > > > > > independent follow up feature as Neha suggested. I will try to make > > the > > > > > time based index work with client side timestamp. > > > > > > > > > > > > For retention, I agree with the problem you point out, but I > think > > > what > > > > > you > > > > > > are saying in that case is that you want a size limit too. If you > > use > > > > > > system time you actually hit the same problem: say you do a full > > dump > > > > of > > > > > a > > > > > > DB table with a setting of 7 days retention, your retention will > > > > actually > > > > > > not get enforced for the first 7 days because the data is "new to > > > > Kafka". > > > > > I kind of think the size limit here is orthogonal. It is a valid > use > > > case > > > > > where people only want to use time based retention only. In your > > > example, > > > > > depending on client timestamp might break the functionality - say > it > > > is a > > > > > bootstrap case people actually need to read all the data. If we > > depend > > > on > > > > > the client timestamp, the data might be deleted instantly when they > > > come > > > > to > > > > > the broker. It might be too demanding to expect the broker to > > > understand > > > > > what people actually want to do with the data coming in. So the > > > guarantee > > > > > of using server side timestamp is that "after appended to the log, > > all > > > > > messages will be available on broker for retention time", which is > > not > > > > > changeable by clients. > > > > > > > > > > > > -Jay > > > > > > > > > > On Thu, Sep 10, 2015 at 12:55 PM, Jiangjie Qin <j...@linkedin.com> > > > > wrote: > > > > > > > > > >> Hi folks, > > > > >> > > > > >> This proposal was previously in KIP-31 and we separated it to > KIP-32 > > > per > > > > >> Neha and Jay's suggestion. > > > > >> > > > > >> The proposal is to add the following two timestamps to Kafka > > message. > > > > >> - CreateTime > > > > >> - LogAppendTime > > > > >> > > > > >> The CreateTime will be set by the producer and will change after > > that. > > > > >> The LogAppendTime will be set by broker for purpose such as > enforce > > > log > > > > >> retention and log rolling. > > > > >> > > > > >> Thanks, > > > > >> > > > > >> Jiangjie (Becket) Qin > > > > >> > > > > >> > > > > > > > > > > > > > > >
