Hi Guozhang, I left my comments inline.
On Thu, Jul 18, 2019 at 8:28 PM Guozhang Wang <wangg...@gmail.com> wrote: > > Hello Bruno, > > Thanks for the feedbacks, replied inline. > > On Mon, Jul 1, 2019 at 7:08 AM Bruno Cadonna <br...@confluent.io> wrote: > > > Hi Guozhang, > > > > Thank you for the KIP. > > > > 1) As far as I understand, the StreamsMetrics interface is there for > > user-defined processors. Would it make sense to also add a method to > > the interface to specify a sensor that records skipped records? > > > > Not sure I follow.. if users want to add a specific skipped records > sensor, she can still do that as a "throughput" sensor via " > addThroughputSensor" and then "record" right? > > As an after-thought, maybe it's better to rename `throughput` to `rate` in > the public APIs since it is really meant for the latter semantics. I did > not change it just to make less API changes / deprecate fewer functions. > But if we feel it is important we can change it as well. > I see now that a user can record the rate of skipped records. However, I was referring to the total number of skipped records. Maybe my question should be more general: should we allow the user to also specify sensors for totals or combinations of rate and totals? Regarding the naming, I like `rate` more than `throughput`, but I would not fight for it. > > > 2) What are the semantics of active-task-process and standby-task-process > > > > Ah good catch, I think I made it in the wrong column. Just some > explanations here: Within a thread's looped iterations, it will first try > to process some records from the active tasks, and then see if there are > any standby-tasks that can be processed as well (i.e. just reading from the > restore consumer and apply to the local stores). The ratio metrics are for > indicating 1) what tasks (active or standby) does this thread own so far, > and 2) how much time in percentage does it spend on each of them. > > But this metric should really be a task-level one that includes both the > thread-id and task-id, and upon task migrations they will be dynamically > deleted / (re)-created. For each task-id it may be owned by multiple > threads as one active and others standby, and hence the separation of > active / standby seems still necessary. > Makes sense. > > > > 3) How do dropped-late-records and expired-window-record-drop relate > > to each other? I guess the former is for records that fall outside the > > grace period and the latter is for records that are processed after > > the retention period of the window. Is this correct? > > > > Yes, that's correct. The names are indeed a bit confusing since they are > added at different releases historically.. > > More precisely, the `grace period` is a notion of the operator (hence the > metric is node-level, though it would only be used for DSL operators) while > the `retention` is a notion of the store (hence the metric is store-level). > Usually grace period will be smaller than store retention though. > > Processor node is aware of `grace period` and when received a record that > is older than grace deadline, it will be dropped immediately; otherwise it > will still be processed a maybe a new update is "put" into the store. The > store is aware of its `retention period` and then upon a "put" call if it > realized it is older than the retention deadline, that put call would be > ignored and metric is recorded. > > We have to separate them here since the window store can be used in both > DSL and PAPI, and for the former case it would likely to be already ignored > at the processor node level due to the grace period which is usually > smaller than retention; but for PAPI there's no grace period and hence the > processor would likely still process and call "put" on the store. > Alright! Got it! > > > 4) Is there an actual difference between skipped and dropped records? > > If not, shall we unify the terminology? > > > > > There is. Dropped records are only due to lateness; where as skipped > records can be due to serde errors (and user's error handling indicate > "skip and continue"), timestamp errors, etc. > > I've considered maybe a better (more extensible) way would be defining a > single metric name, say skipped-records, but use different tags to indicate > if its skipping reason (errors, windowing semantics, etc). But there's > still a tricky difference: for serde caused skipping for example, they will > be skipped at the very beginning and there's no effects taken at all. For > some others e.g. null-key / value at the reduce operator, it is only > skipped at the middle of the processing, i.e. some effects may have already > been taken in up-stream sub-topologies. And that's why for skipped-records > I've defined it on both task-level and node-level and the aggregate of the > latter may still be smaller than the former, whereas for dropped-records it > is only for node-level. > > So how about an even more significant change then: we enlarge the > `dropped-late-records` to `dropped-records` which is node-level only, but > includes reasons form lateness to semantics (like null-key) as well; and > then we have a task-level-only `skipped-records` which only record those > dropped at the very beginning and did not make it at all to the processing > topology. I feel this is a clearer distinguishment but also a bigger change > to users. > I like the way you dropped-records and skipped-records are now defined. My follow-up question is whether we should give names to those metrics that better describe their semantics, like: dropped-records-at-source and dropped-records-at-processor or records-dropped-at-source and records-dropped-at-processor or source-dropped-records and processor-dropped-records or alternatively with skipped. However, I would use the same term as in expired-window-record-drop Maybe, we should also consider to rename expired-window-record-drop to expired-window-record-dropped to be consistent. WDYT? > > > 5) What happens with removed metrics when the user sets the version of > > "built.in.metrics.version" to 2.2- > > > > I think for those redundant ones like ""forward-rate" and "destroy-rate" > we can still remove them with 2.2- as well; for other ones that are removed > / replaced like thread-level skipped-records we should still maintain them. > Could you add this comment about removal of redundant metrics to the KIP such that is documented somewhere? Best, Bruno
