Hi Anna and Jun, You are right. We should allocate up to the quota for each old sample.
I have revamped the Throttling Algorithm section to better explain our thought process and the token bucket inspiration. I have also added a chapter with few guidelines about how to define the quota. There is no magic formula for this but I give few insights. I don't have specific numbers that can be used out of the box so I think that it is better to not put any for the time being. We can always complement later on in the documentation. Please, take a look and let me know what you think. Cheers, David On Fri, Jun 5, 2020 at 8:37 AM Anna Povzner <a...@confluent.io> wrote: > Hi David and Jun, > > I dug a bit deeper into the Rate implementation, and wanted to confirm that > I do believe that the token bucket behavior is better for the reasons we > already discussed but wanted to summarize. The main difference between Rate > and token bucket is that the Rate implementation allows a burst by > borrowing from the future, whereas a token bucket allows a burst by using > accumulated tokens from the previous idle period. Using accumulated tokens > smoothes out the rate measurement in general. Configuring a large burst > requires configuring a large quota window, which causes long delays for > bursty workload, due to borrowing credits from the future. Perhaps it is > useful to add a summary in the beginning of the Throttling Algorithm > section? > > In my previous email, I mentioned the issue we observed with the bandwidth > quota, where a low quota (1MB/s per broker) was limiting bandwidth visibly > below the quota. I thought it was strictly the issue with the Rate > implementation as well, but I found a root cause to be different but > amplified by the Rate implementation (long throttle delays of requests in a > burst). I will describe it here for completeness using the following > example: > > - > > Quota = 1MB/s, default window size and number of samples > - > > Suppose there are 6 connections (maximum 6 outstanding requests), and > each produce request is 5MB. If all requests arrive in a burst, the > last 4 > requests (20MB over 10MB allowed in a window) may get the same throttle > time if they are processed concurrently. We record the rate under the > lock, > but then calculate throttle time separately after that. So, for each > request, the observed rate could be 3MB/s, and each request gets > throttle > delay = 20 seconds (instead of 5, 10, 15, 20 respectively). The delay is > longer than the total rate window, which results in lower bandwidth than > the quota. Since all requests got the same delay, they will also arrive > in > a burst, which may also result in longer delay than necessary. It looks > pretty easy to fix, so I will open a separate JIRA for it. This can be > additionally mitigated by token bucket behavior. > > > For the algorithm "So instead of having one sample equal to 560 in the last > window, we will have 100 samples equal to 5.6.", I agree with Jun. I would > allocate 5 per each old sample that is still in the overall window. It > would be a bit larger granularity than the pure token bucket (we lose 5 > units / mutation once we move past the sample window), but it is better > than the long delay. > > Thanks, > > Anna > > > On Thu, Jun 4, 2020 at 6:33 PM Jun Rao <j...@confluent.io> wrote: > > > Hi, David, Anna, > > > > Thanks for the discussion and the updated wiki. > > > > 11. If we believe the token bucket behavior is better in terms of > handling > > the burst behavior, we probably don't need a separate KIP since it's just > > an implementation detail. > > > > Regarding "So instead of having one sample equal to 560 in the last > window, > > we will have 100 samples equal to 5.6.", I was thinking that we will > > allocate 5 to each of the first 99 samples and 65 to the last sample. > Then, > > 6 new samples have to come before the balance becomes 0 again. > Intuitively, > > we are accumulating credits in each sample. If a usage comes in, we first > > use all existing credits to offset that. If we can't, the remaining usage > > will be recorded in the last sample, which will be offset by future > > credits. That seems to match the token bucket behavior the closest. > > > > 20. Could you provide some guidelines on the typical rate that an admin > > should set? > > > > Jun > > > > On Thu, Jun 4, 2020 at 8:22 AM David Jacot <dja...@confluent.io> wrote: > > > > > Hi all, > > > > > > I just published an updated version of the KIP which includes: > > > * Using a slightly modified version of our Rate. I have tried to > > formalize > > > it based on our discussion. As Anna suggested, we may find a better way > > to > > > implement it. > > > * Handling of ValidateOnly as pointed out by Tom. > > > > > > Please, check it out and let me know what you think. > > > > > > Best, > > > David > > > > > > On Thu, Jun 4, 2020 at 4:57 PM Tom Bentley <tbent...@redhat.com> > wrote: > > > > > > > Hi David, > > > > > > > > As a user I might expect the validateOnly option to do everything > > except > > > > actually make the changes. That interpretation would imply the quota > > > should > > > > be checked, but the check should obviously be side-effect free. I > think > > > > this interpretation could be useful because it gives the caller > either > > > some > > > > confidence that they're not going to hit the quota, or tell them, via > > the > > > > exception, when they can expect the call to work. But for this to be > > > useful > > > > it would require the retry logic to not retry the request when > > > validateOnly > > > > was set. > > > > > > > > On the other hand, if validateOnly is really about validating only > some > > > > aspects of the request (which maybe is what the name implies), then > we > > > > should clarify in the Javadoc that the quota is not included in the > > > > validation. > > > > > > > > On balance, I agree with what you're proposing, since the extra > utility > > > of > > > > including the quota in the validation seems to be not worth the extra > > > > complication for the retry. > > > > > > > > Thanks, > > > > > > > > Tom > > > > > > > > > > > > > > > > On Thu, Jun 4, 2020 at 3:32 PM David Jacot <dja...@confluent.io> > > wrote: > > > > > > > > > Hi Tom, > > > > > > > > > > That's a good question. As the validation does not create any load > on > > > the > > > > > controller, I was planning to do it without checking the quota at > > all. > > > > Does > > > > > that > > > > > sound reasonable? > > > > > > > > > > Best, > > > > > David > > > > > > > > > > On Thu, Jun 4, 2020 at 4:23 PM David Jacot <dja...@confluent.io> > > > wrote: > > > > > > > > > > > Hi Jun and Anna, > > > > > > > > > > > > Thank you both for your replies. > > > > > > > > > > > > Based on our recent discussion, I agree that using a rate is > better > > > to > > > > > > remain > > > > > > consistent with other quotas. As you both suggested, it seems > that > > > > > changing > > > > > > the way we compute the rate to better handle spiky workloads and > > > > behave a > > > > > > bit more similarly to the token bucket algorithm makes sense for > > all > > > > > > quotas as > > > > > > well. > > > > > > > > > > > > I will update the KIP to reflect this. > > > > > > > > > > > > Anna, I think that we can explain this in this KIP. We can't just > > say > > > > > that > > > > > > the Rate > > > > > > will be updated in this KIP. I think that we need to give a bit > > more > > > > > info. > > > > > > > > > > > > Best, > > > > > > David > > > > > > > > > > > > On Thu, Jun 4, 2020 at 6:31 AM Anna Povzner <a...@confluent.io> > > > wrote: > > > > > > > > > > > >> Hi Jun and David, > > > > > >> > > > > > >> Regarding token bucket vs, Rate behavior. We recently observed a > > > > couple > > > > > of > > > > > >> cases where a bursty workload behavior would result in long-ish > > > pauses > > > > > in > > > > > >> between, resulting in lower overall bandwidth than the quota. I > > will > > > > > need > > > > > >> to debug this a bit more to be 100% sure, but it does look like > > the > > > > case > > > > > >> described by David earlier in this thread. So, I agree with Jun > > -- I > > > > > think > > > > > >> we should make all quota rate behavior consistent, and probably > > > > similar > > > > > to > > > > > >> the token bucket one. > > > > > >> > > > > > >> Looking at KIP-13, it doesn't describe rate calculation in > enough > > > > > detail, > > > > > >> but does mention window size. So, we could keep "window size" > and > > > > > "number > > > > > >> of samples" configs and change Rate implementation to be more > > > similar > > > > to > > > > > >> token bucket: > > > > > >> * number of samples define our burst size > > > > > >> * Change the behavior, which could be described as: If a burst > > > happens > > > > > >> after an idle period, the burst would effectively spread evenly > > over > > > > the > > > > > >> (now - window) time period, where window is (<number of > samples> - > > > 1)* > > > > > >> <window size>. Which basically describes a token bucket, while > > > keeping > > > > > the > > > > > >> current quota configs. I think we can even implement this by > > > changing > > > > > the > > > > > >> way we record the last sample or lastWindowMs. > > > > > >> > > > > > >> Jun, if we would be changing Rate calculation behavior in > > bandwidth > > > > and > > > > > >> request quotas, would we need a separate KIP? Shouldn't need to > if > > > we > > > > > >> keep window size and number of samples configs, right? > > > > > >> > > > > > >> Thanks, > > > > > >> Anna > > > > > >> > > > > > >> On Wed, Jun 3, 2020 at 3:24 PM Jun Rao <j...@confluent.io> > wrote: > > > > > >> > > > > > >> > Hi, David, > > > > > >> > > > > > > >> > Thanks for the reply. > > > > > >> > > > > > > >> > 11. To match the behavior in the Token bucket approach, I was > > > > thinking > > > > > >> that > > > > > >> > requests that don't fit in the previous time windows will be > > > > > >> accumulated in > > > > > >> > the current time window. So, the 60 extra requests will be > > > > accumulated > > > > > >> in > > > > > >> > the latest window. Then, the client also has to wait for 12 > more > > > > secs > > > > > >> > before throttling is removed. I agree that this is probably a > > > better > > > > > >> > behavior and it's reasonable to change the existing behavior > to > > > this > > > > > >> one. > > > > > >> > > > > > > >> > To me, it seems that sample_size * num_windows is the same as > > max > > > > > burst > > > > > >> > balance. The latter seems a bit better to configure. The thing > > is > > > > that > > > > > >> the > > > > > >> > existing quota system has already been used in quite a few > > places > > > > and > > > > > >> if we > > > > > >> > want to change the configuration in the future, there is the > > > > migration > > > > > >> > cost. Given that, do you feel it's better to adopt the new > > token > > > > > bucket > > > > > >> > terminology or just adopt the behavior somehow into our > existing > > > > > >> system? If > > > > > >> > it's the former, it would be useful to document this in the > > > rejected > > > > > >> > section and add a future plan on migrating existing quota > > > > > >> configurations. > > > > > >> > > > > > > >> > Jun > > > > > >> > > > > > > >> > > > > > > >> > On Tue, Jun 2, 2020 at 6:55 AM David Jacot < > dja...@confluent.io > > > > > > > > wrote: > > > > > >> > > > > > > >> > > Hi Jun, > > > > > >> > > > > > > > >> > > Thanks for your reply. > > > > > >> > > > > > > > >> > > 10. I think that both options are likely equivalent from an > > > > accuracy > > > > > >> > point > > > > > >> > > of > > > > > >> > > view. If we put the implementation aside, conceptually, I am > > not > > > > > >> > convinced > > > > > >> > > by the used based approach because resources don't have a > > clear > > > > > owner > > > > > >> > > in AK at the moment. A topic can be created by (Principal A, > > no > > > > > client > > > > > >> > id), > > > > > >> > > then can be extended by (no principal, Client B), and > finally > > > > > deleted > > > > > >> by > > > > > >> > > (Principal C, Client C). This does not sound right to me > and I > > > > fear > > > > > >> that > > > > > >> > it > > > > > >> > > is not going to be easy to grasp for our users. > > > > > >> > > > > > > > >> > > Regarding the naming, I do agree that we can make it more > > future > > > > > >> proof. > > > > > >> > > I propose `controller_mutations_rate`. I think that > > > > differentiating > > > > > >> the > > > > > >> > > mutations > > > > > >> > > from the reads is still a good thing for the future. > > > > > >> > > > > > > > >> > > 11. I am not convinced by your proposal for the following > > > reasons: > > > > > >> > > > > > > > >> > > First, in my toy example, I used 101 windows and 7 * 80 > > > requests. > > > > We > > > > > >> > could > > > > > >> > > effectively allocate 5 * 100 requests to the previous > windows > > > > > assuming > > > > > >> > that > > > > > >> > > they are empty. What shall we do with the remaining 60 > > requests? > > > > > >> Shall we > > > > > >> > > allocate them to the current window or shall we divide it > > among > > > > all > > > > > >> the > > > > > >> > > windows? > > > > > >> > > > > > > > >> > > Second, I don't think that we can safely change the behavior > > of > > > > all > > > > > >> the > > > > > >> > > existing > > > > > >> > > rates used because it actually changes the computation of > the > > > rate > > > > > as > > > > > >> > > values > > > > > >> > > allocated to past windows would expire before they would > > today. > > > > > >> > > > > > > > >> > > Overall, while trying to fit in the current rate, we are > going > > > to > > > > > >> build a > > > > > >> > > slightly > > > > > >> > > different version of the rate which will be even more > > confusing > > > > for > > > > > >> > users. > > > > > >> > > > > > > > >> > > Instead, I think that we should embrace the notion of burst > as > > > it > > > > > >> could > > > > > >> > > also > > > > > >> > > be applied to other quotas in the future. Users don't have > to > > > know > > > > > >> that > > > > > >> > we > > > > > >> > > use the Token Bucket or a special rate inside at the end of > > the > > > > day. > > > > > >> It > > > > > >> > is > > > > > >> > > an > > > > > >> > > implementation detail. > > > > > >> > > > > > > > >> > > Users would be able to define: > > > > > >> > > - a rate R; and > > > > > >> > > - a maximum burst B. > > > > > >> > > > > > > > >> > > If we change the metrics to be as follow: > > > > > >> > > - the actual rate; > > > > > >> > > - the burst balance in %, 0 meaning that the user is > > throttled; > > > > > >> > > It remains disattach from the algorithm. > > > > > >> > > > > > > > >> > > I personally prefer this over having to define a rate and a > > > number > > > > > of > > > > > >> > > windows > > > > > >> > > while having to understand that the number of windows > > implicitly > > > > > >> defines > > > > > >> > > the > > > > > >> > > allowed burst. I think that it is clearer and easier to > grasp. > > > > Don't > > > > > >> you? > > > > > >> > > > > > > > >> > > Best, > > > > > >> > > David > > > > > >> > > > > > > > >> > > On Fri, May 29, 2020 at 6:38 PM Jun Rao <j...@confluent.io> > > > wrote: > > > > > >> > > > > > > > >> > > > Hi, David, Anna, > > > > > >> > > > > > > > > >> > > > Thanks for the response. Sorry for the late reply. > > > > > >> > > > > > > > > >> > > > 10. Regarding exposing rate or usage as quota. Your > argument > > > is > > > > > that > > > > > >> > > usage > > > > > >> > > > is not very accurate anyway and is harder to implement. > So, > > > > let's > > > > > >> just > > > > > >> > > be a > > > > > >> > > > bit loose and expose rate. I am sort of neutral on that. > (1) > > > It > > > > > >> seems > > > > > >> > to > > > > > >> > > me > > > > > >> > > > that overall usage will be relatively more accurate than > > rate. > > > > All > > > > > >> the > > > > > >> > > > issues that Anna brought up seem to exist for rate too. > Rate > > > has > > > > > the > > > > > >> > > > additional problem that the cost of each request may not > be > > > > > uniform. > > > > > >> > (2) > > > > > >> > > In > > > > > >> > > > terms of implementation, a usage based approach requires > > > > tracking > > > > > >> the > > > > > >> > > user > > > > > >> > > > info through the life cycle of an operation. However, as > you > > > > > >> mentioned, > > > > > >> > > > things like topic creation can generate additional > > > > > >> > > > LeaderAndIsr/UpdateMetadata requests. Longer term, we > > probably > > > > > want > > > > > >> to > > > > > >> > > > associate those cost to the user who initiated the > > operation. > > > If > > > > > we > > > > > >> do > > > > > >> > > > that, we sort of need to track the user for the full life > > > cycle > > > > of > > > > > >> the > > > > > >> > > > processing of an operation anyway. (3) If you prefer rate > > > > > strongly, > > > > > >> I > > > > > >> > > don't > > > > > >> > > > have strong objections. However, I do feel that the new > > quota > > > > name > > > > > >> > should > > > > > >> > > > be able to cover all controller related cost longer term. > > This > > > > KIP > > > > > >> > > > currently only covers topic creation/deletion. It would > not > > be > > > > > >> ideal if > > > > > >> > > in > > > > > >> > > > the future, we have to add yet another type of quota for > > some > > > > > other > > > > > >> > > > controller related operations. The quota name in the KIP > has > > > > > >> partition > > > > > >> > > > mutation. In the future, if we allow things like topic > > > renaming, > > > > > it > > > > > >> may > > > > > >> > > not > > > > > >> > > > be related to partition mutation directly and it would be > > > > trickier > > > > > >> to > > > > > >> > fit > > > > > >> > > > those operations in the current quota. So, maybe sth more > > > > general > > > > > >> like > > > > > >> > > > controller_operations_quota will be more future proof. > > > > > >> > > > > > > > > >> > > > 11. Regarding the difference between the token bucket > > > algorithm > > > > > and > > > > > >> our > > > > > >> > > > current quota mechanism. That's a good observation. It > seems > > > > that > > > > > we > > > > > >> > can > > > > > >> > > > make a slight change to our current quota mechanism to > > > achieve a > > > > > >> > similar > > > > > >> > > > thing. As you said, the issue was that we allocate all 7 * > > 80 > > > > > >> requests > > > > > >> > in > > > > > >> > > > the last 1 sec window in our current mechanism. This is a > > bit > > > > > >> > unintuitive > > > > > >> > > > since each sec only has a quota capacity of 5. An > > alternative > > > > way > > > > > >> is to > > > > > >> > > > allocate the 7 * 80 requests to all previous windows, each > > up > > > to > > > > > the > > > > > >> > > > remaining quota capacity left. So, you will fill the > > current 1 > > > > sec > > > > > >> > window > > > > > >> > > > and all previous ones, each with 5. Then, it seems this > will > > > > give > > > > > >> the > > > > > >> > > same > > > > > >> > > > behavior as token bucket? The reason that I keep asking > this > > > is > > > > > that > > > > > >> > from > > > > > >> > > > an operational perspective, it's simpler if all types of > > > quotas > > > > > >> work in > > > > > >> > > the > > > > > >> > > > same way. > > > > > >> > > > > > > > > >> > > > Jun > > > > > >> > > > > > > > > >> > > > On Tue, May 26, 2020 at 9:52 AM David Jacot < > > > > dja...@confluent.io> > > > > > >> > wrote: > > > > > >> > > > > > > > > >> > > > > Hi folks, > > > > > >> > > > > > > > > > >> > > > > I have updated the KIP. As mentioned by Jun, I have made > > the > > > > > >> > > > > quota per principal/clientid similarly to the other > > quotas. > > > I > > > > > have > > > > > >> > > > > also explained how this will work in conjunction with > the > > > auto > > > > > >> > > > > topics creation. > > > > > >> > > > > > > > > > >> > > > > Please, take a look at it. I plan to call a vote for it > in > > > the > > > > > >> next > > > > > >> > few > > > > > >> > > > > days if there are no comments in this thread. > > > > > >> > > > > > > > > > >> > > > > Best, > > > > > >> > > > > David > > > > > >> > > > > > > > > > >> > > > > On Wed, May 13, 2020 at 10:57 AM Tom Bentley < > > > > > tbent...@redhat.com > > > > > >> > > > > > > >> > > > wrote: > > > > > >> > > > > > > > > > >> > > > > > Hi David, > > > > > >> > > > > > > > > > > >> > > > > > Thanks for the explanation and confirmation that > > evolving > > > > the > > > > > >> APIs > > > > > >> > is > > > > > >> > > > not > > > > > >> > > > > > off the table in the longer term. > > > > > >> > > > > > > > > > > >> > > > > > Kind regards, > > > > > >> > > > > > > > > > > >> > > > > > Tom > > > > > >> > > > > > > > > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > > > > >> > > > > > > >> > > > > > > > > > > > > > > > > > > > > >
