Hi Cham, thanks for the feedback > Beam has a policy of no knobs (or keeping knobs minimum) to allow runners to optimize better. I think one concern might be that the addition of this option might be going against this.
I agree that less knobs is more. But if assignment of a key is specific per user request via API (optional), than runner should not optimize that but need to respect how data is requested to be laid down. It could however optimize number of shards as it is doing right now if numShards is not set explicitly. Anyway FileIO feels fluid here. You can leave shards empty and let runner decide or optimize, but only in batch and not in streaming. Then it allows you to set number of shards but never let you change the logic how it is assigned and defaults to round robin with random seed. It begs question for me why I can manipulate number of shards but not the logic of assignment. Are there plans to (or already case implemented) optimize around and have runner changing that under different circumstances? > Also looking at your original reasoning for adding this. > > I need to generate shards which are compatible with Hive bucketing and therefore need to decide shard assignment based on data fields of input element > This sounds very specific to Hive. Again I think the downside here is that we will be giving up the flexibility for the runner to optimize the pipeline and decide sharding. Do you think it's possible to somehow relax this requirement for > > > Hive or use a secondary process to format the output to a form that is suitable for Hive after being written to an intermediate storage. It is possible. But it is quite suboptimal because of extra IO penalty and I would have to use completely custom file writing as FileIO would not support this. Then naturally the question would be, should I use FileIO at all? I am quite not sure if I am doing something so rare that it is not and can not be supported. Maybe I am. I remember an old discussion from Scio guys when they wanted to introduce Sorted Merge Buckets to FileIO where sharding was also needed to be manipulated (among other things) > > When running e.g. on Spark and job encounters kind of failure which cause a loss of some data from previous stages, Spark does issue recompute of necessary task in necessary stages to recover data. Because the shard assignment function is random as default, some data will end up in different shards and cause duplicates in the final output. > I think this was already addressed. The correct solution here is to implement RequiresStableInput for runners that do not already support that and update FileIO to use that. How will @RequiresStableInput prevent this situation when running batch use case? On Mon, Jun 28, 2021 at 10:29 AM Chamikara Jayalath <chamik...@google.com> wrote: > > > On Sun, Jun 27, 2021 at 10:48 PM Jozef Vilcek <jozo.vil...@gmail.com> > wrote: > >> Hi, >> >> how do we proceed with reviewing MR proposed for this change? >> >> I sense there is a concern exposing existing sharding function to the >> API. But from the discussion here I do not have a clear picture >> about arguments not doing so. >> Only one argument was that dynamic destinations should be able to do the >> same. While this is true, as it is illustrated in previous commnet, it is >> not simple nor convenient to use and requires more customization than >> exposing sharding which is already there. >> Are there more negatives to exposing sharding function? >> > > Seems like currently FlinkStreamingPipelineTranslator is the only real > usage of WriteFiles.withShardingFunction() : > https://github.com/apache/beam/blob/90c854e97787c19cd5b94034d37c5319317567a8/runners/flink/src/main/java/org/apache/beam/runners/flink/FlinkStreamingPipelineTranslator.java#L281 > WriteFiles is not expected to be used by pipeline authors but exposing > this through the FileIO will make this available to pipeline authors (and > some may choose to do so for various reasons). > This will result in sharding being strict and runners being inflexible > when it comes to optimizing execution of pipelines that use FileIO. > > Beam has a policy of no knobs (or keeping knobs minimum) to allow runners > to optimize better. I think one concern might be that the addition of this > option might be going against this. > > Also looking at your original reasoning for adding this. > > > I need to generate shards which are compatible with Hive bucketing and > therefore need to decide shard assignment based on data fields of input > element > > This sounds very specific to Hive. Again I think the downside here is that > we will be giving up the flexibility for the runner to optimize the > pipeline and decide sharding. Do you think it's possible to somehow relax > this requirement for Hive or use a secondary process to format the output > to a form that is suitable for Hive after being written to an intermediate > storage. > > > When running e.g. on Spark and job encounters kind of failure which > cause a loss of some data from previous stages, Spark does issue recompute > of necessary task in necessary stages to recover data. Because the shard > assignment function is random as default, some data will end up in > different shards and cause duplicates in the final output. > > I think this was already addressed. The correct solution here is to > implement RequiresStableInput for runners that do not already support that > and update FileIO to use that. > > Thanks, > Cham > > >> >> On Wed, Jun 23, 2021 at 9:36 AM Jozef Vilcek <jozo.vil...@gmail.com> >> wrote: >> >>> The difference in my opinion is in distinguishing between - as written >>> in this thread - physical vs logical properties of the pipeline. I proposed >>> to keep dynamic destination (logical) and sharding (physical) separate on >>> API level as it is at implementation level. >>> >>> When I reason about using `by()` for my case ... I am using dynamic >>> destination to partition data into hourly folders. So my destination is >>> e.g. `2021-06-23-07`. To add a shard there I assume I will need >>> * encode shard to the destination .. e.g. in form of file prefix >>> `2021-06-23-07/shard-1` >>> * dynamic destination now does not span a group of files but must be >>> exactly one file >>> * to respect above, I have to. "disable" sharding in WriteFiles and make >>> sure to use `.withNumShards(1)` ... I am not sure what `runnner determined` >>> sharding would do, if it would not split destination further into more >>> files >>> * make sure that FileNameing will respect my intent and name files as I >>> expect based on that destination >>> * post process `WriteFilesResult` to turn my destination which targets >>> physical single file (date-with-hour + shard-num) back into the destination >>> with targets logical group of files (date-with-hour) so I hook it up do >>> downstream post-process as usual >>> >>> Am I roughly correct? Or do I miss something more straight forward? >>> If the above is correct then it feels more fragile and less intuitive to >>> me than the option in my MR. >>> >>> >>> On Tue, Jun 22, 2021 at 4:28 PM Reuven Lax <re...@google.com> wrote: >>> >>>> I'm not sure I understand your PR. How is this PR different than the >>>> by() method in FileIO? >>>> >>>> On Tue, Jun 22, 2021 at 1:22 AM Jozef Vilcek <jozo.vil...@gmail.com> >>>> wrote: >>>> >>>>> MR for review for this change is here: >>>>> https://github.com/apache/beam/pull/15051 >>>>> >>>>> On Fri, Jun 18, 2021 at 8:47 AM Jozef Vilcek <jozo.vil...@gmail.com> >>>>> wrote: >>>>> >>>>>> I would like this thread to stay focused on sharding FileIO only. >>>>>> Possible change to the model is an interesting topic but of a much >>>>>> different scope. >>>>>> >>>>>> Yes, I agree that sharding is mostly a physical rather than logical >>>>>> property of the pipeline. That is why it feels more natural to >>>>>> distinguish >>>>>> between those two on the API level. >>>>>> As for handling sharding requirements by adding more sugar to dynamic >>>>>> destinations + file naming one has to keep in mind that results of >>>>>> dynamic >>>>>> writes can be observed in the form of KV<DestinationT, String>, so >>>>>> written >>>>>> files per dynamic destination. Often we do GBP to post-process files per >>>>>> destination / logical group. If sharding would be encoded there, then it >>>>>> might complicate things either downstream or inside the sugar part to put >>>>>> shard in and then take it out later. >>>>>> From the user perspective I do not see much difference. We would >>>>>> still need to allow API to define both behaviors and it would only be >>>>>> executed differently by implementation. >>>>>> I do not see a value in changing FileIO (WriteFiles) logic to stop >>>>>> using sharding and use dynamic destination for both given that sharding >>>>>> function is already there and in use. >>>>>> >>>>>> To the point of external shuffle and non-deterministic user input. >>>>>> Yes users can create non-deterministic behaviors but they are in >>>>>> control. Here, Beam internally adds non-deterministic behavior and users >>>>>> can not opt-out. >>>>>> All works fine as long as external shuffle service (depends on >>>>>> Runner) holds to the data and hands it out on retries. However if data in >>>>>> shuffle service is lost for some reason - e.g. disk failure, node breaks >>>>>> down - then pipeline have to recover the data by recomputing necessary >>>>>> paths. >>>>>> >>>>>> On Thu, Jun 17, 2021 at 7:36 PM Robert Bradshaw <rober...@google.com> >>>>>> wrote: >>>>>> >>>>>>> Sharding is typically a physical rather than logical property of the >>>>>>> pipeline, and I'm not convinced it makes sense to add it to Beam in >>>>>>> general. One can already use keys and GBK/Stateful DoFns if some kind >>>>>>> of logical grouping is needed, and adding constraints like this can >>>>>>> prevent opportunities for optimizations (like dynamic sharding and >>>>>>> fusion). >>>>>>> >>>>>>> That being said, file output are one area where it could make sense. >>>>>>> I >>>>>>> would expect that dynamic destinations could cover this usecase, and >>>>>>> a >>>>>>> general FileNaming subclass could be provided to make this pattern >>>>>>> easier (and possibly some syntactic sugar for auto-setting num shards >>>>>>> to 0). (One downside of this approach is that one couldn't do dynamic >>>>>>> destinations, and have each sharded with a distinct sharing function >>>>>>> as well.) >>>>>>> >>>>>>> If this doesn't work, we could look into adding ShardingFunction as a >>>>>>> publicly exposed parameter to FileIO. (I'm actually surprised it >>>>>>> already exists.) >>>>>>> >>>>>>> On Thu, Jun 17, 2021 at 9:39 AM <je...@seznam.cz> wrote: >>>>>>> > >>>>>>> > Alright, but what is worth emphasizing is that we talk about batch >>>>>>> workloads. The typical scenario is that the output is committed once the >>>>>>> job finishes (e.g., by atomic rename of directory). >>>>>>> > Jan >>>>>>> > >>>>>>> > Dne 17. 6. 2021 17:59 napsal uživatel Reuven Lax <re...@google.com >>>>>>> >: >>>>>>> > >>>>>>> > Yes - the problem is that Beam makes no guarantees of determinism >>>>>>> anywhere in the system. User DoFns might be non deterministic, and have >>>>>>> no >>>>>>> way to know (we've discussed proving users with an @IsDeterministic >>>>>>> annotation, however empirically users often think their functions are >>>>>>> deterministic when they are in fact not). _Any_ sort of triggered >>>>>>> aggregation (including watermark based) can always be non deterministic. >>>>>>> > >>>>>>> > Even if everything was deterministic, replaying everything is >>>>>>> tricky. The output files already exist - should the system delete them >>>>>>> and >>>>>>> recreate them, or leave them as is and delete the temp files? Either >>>>>>> decision could be problematic. >>>>>>> > >>>>>>> > On Wed, Jun 16, 2021 at 11:40 PM Jan Lukavský <je...@seznam.cz> >>>>>>> wrote: >>>>>>> > >>>>>>> > Correct, by the external shuffle service I pretty much meant >>>>>>> "offloading the contents of a shuffle phase out of the system". Looks >>>>>>> like >>>>>>> that is what the Spark's checkpoint does as well. On the other hand (if >>>>>>> I >>>>>>> understand the concept correctly), that implies some performance >>>>>>> penalty - >>>>>>> the data has to be moved to external distributed filesystem. Which then >>>>>>> feels weird if we optimize code to avoid computing random numbers, but >>>>>>> are >>>>>>> okay with moving complete datasets back and forth. I think in this >>>>>>> particular case making the Pipeline deterministic - idempotent to be >>>>>>> precise - (within the limits, yes, hashCode of enum is not stable >>>>>>> between >>>>>>> JVMs) would seem more practical to me. >>>>>>> > >>>>>>> > Jan >>>>>>> > >>>>>>> > On 6/17/21 7:09 AM, Reuven Lax wrote: >>>>>>> > >>>>>>> > I have some thoughts here, as Eugene Kirpichov and I spent a lot >>>>>>> of time working through these semantics in the past. >>>>>>> > >>>>>>> > First - about the problem of duplicates: >>>>>>> > >>>>>>> > A "deterministic" sharding - e.g. hashCode based (though Java >>>>>>> makes no guarantee that hashCode is stable across JVM instances, so this >>>>>>> technique ends up not being stable) doesn't really help matters in Beam. >>>>>>> Unlike other systems, Beam makes no assumptions that transforms are >>>>>>> idempotent or deterministic. What's more, _any_ transform that has any >>>>>>> sort >>>>>>> of triggered grouping (whether the trigger used is watermark based or >>>>>>> otherwise) is non deterministic. >>>>>>> > >>>>>>> > Forcing a hash of every element imposed quite a CPU cost; even >>>>>>> generating a random number per-element slowed things down too much, >>>>>>> which >>>>>>> is why the current code generates a random number only in startBundle. >>>>>>> > >>>>>>> > Any runner that does not implement RequiresStableInput will not >>>>>>> properly execute FileIO. Dataflow and Flink both support this. I believe >>>>>>> that the Spark runner implicitly supports it by manually calling >>>>>>> checkpoint >>>>>>> as Ken mentioned (unless someone removed that from the Spark runner, >>>>>>> but if >>>>>>> so that would be a correctness regression). Implementing this has >>>>>>> nothing >>>>>>> to do with external shuffle services - neither Flink, Spark, nor >>>>>>> Dataflow >>>>>>> appliance (classic shuffle) have any problem correctly implementing >>>>>>> RequiresStableInput. >>>>>>> > >>>>>>> > On Wed, Jun 16, 2021 at 11:18 AM Jan Lukavský <je...@seznam.cz> >>>>>>> wrote: >>>>>>> > >>>>>>> > I think that the support for @RequiresStableInput is rather >>>>>>> limited. AFAIK it is supported by streaming Flink (where it is not >>>>>>> needed >>>>>>> in this situation) and by Dataflow. Batch runners without external >>>>>>> shuffle >>>>>>> service that works as in the case of Dataflow have IMO no way to >>>>>>> implement >>>>>>> it correctly. In the case of FileIO (which do not use >>>>>>> @RequiresStableInput >>>>>>> as it would not be supported on Spark) the randomness is easily >>>>>>> avoidable >>>>>>> (hashCode of key?) and I it seems to me it should be preferred. >>>>>>> > >>>>>>> > Jan >>>>>>> > >>>>>>> > On 6/16/21 6:23 PM, Kenneth Knowles wrote: >>>>>>> > >>>>>>> > >>>>>>> > On Wed, Jun 16, 2021 at 5:18 AM Jan Lukavský <je...@seznam.cz> >>>>>>> wrote: >>>>>>> > >>>>>>> > Hi, >>>>>>> > >>>>>>> > maybe a little unrelated, but I think we definitely should not use >>>>>>> random assignment of shard keys (RandomShardingFunction), at least for >>>>>>> bounded workloads (seems to be fine for streaming workloads). Many batch >>>>>>> runners simply recompute path in the computation DAG from the failed >>>>>>> node >>>>>>> (transform) to the root (source). In the case there is any >>>>>>> non-determinism >>>>>>> involved in the logic, then it can result in duplicates (as the >>>>>>> 'previous' >>>>>>> attempt might have ended in DAG path that was not affected by the fail). >>>>>>> That addresses the option 2) of what Jozef have mentioned. >>>>>>> > >>>>>>> > This is the reason we introduced "@RequiresStableInput". >>>>>>> > >>>>>>> > When things were only Dataflow, we knew that each shuffle was a >>>>>>> checkpoint, so we inserted a Reshuffle after the random numbers were >>>>>>> generated, freezing them so it was safe for replay. Since other engines >>>>>>> do >>>>>>> not checkpoint at every shuffle, we needed a way to communicate that >>>>>>> this >>>>>>> checkpointing was required for correctness. I think we still have many >>>>>>> IOs >>>>>>> that are written using Reshuffle instead of @RequiresStableInput, and I >>>>>>> don't know which runners process @RequiresStableInput properly. >>>>>>> > >>>>>>> > By the way, I believe the SparkRunner explicitly calls >>>>>>> materialize() after a GBK specifically so that it gets correct results >>>>>>> for >>>>>>> IOs that rely on Reshuffle. Has that changed? >>>>>>> > >>>>>>> > I agree that we should minimize use of RequiresStableInput. It has >>>>>>> a significant cost, and the cost varies across runners. If we can use a >>>>>>> deterministic function, we should. >>>>>>> > >>>>>>> > Kenn >>>>>>> > >>>>>>> > >>>>>>> > Jan >>>>>>> > >>>>>>> > On 6/16/21 1:43 PM, Jozef Vilcek wrote: >>>>>>> > >>>>>>> > >>>>>>> > >>>>>>> > On Wed, Jun 16, 2021 at 1:38 AM Kenneth Knowles <k...@apache.org> >>>>>>> wrote: >>>>>>> > >>>>>>> > In general, Beam only deals with keys and grouping by key. I think >>>>>>> expanding this idea to some more abstract notion of a sharding function >>>>>>> could make sense. >>>>>>> > >>>>>>> > For FileIO specifically, I wonder if you can use writeDynamic() to >>>>>>> get the behavior you are seeking. >>>>>>> > >>>>>>> > >>>>>>> > The change in mind looks like this: >>>>>>> > >>>>>>> https://github.com/JozoVilcek/beam/commit/9c5a7fe35388f06f72972ec4c1846f1dbe85eb18 >>>>>>> > >>>>>>> > Dynamic Destinations in my mind is more towards the need for >>>>>>> "partitioning" data (destination as directory level) or if one needs to >>>>>>> handle groups of events differently, e.g. write some events in FormatA >>>>>>> and >>>>>>> others in FormatB. >>>>>>> > Shards are now used for distributing writes or bucketing of events >>>>>>> within a particular destination group. More specifically, currently, >>>>>>> each >>>>>>> element is assigned `ShardedKey<Integer>` [1] before GBK operation. >>>>>>> Sharded >>>>>>> key is a compound of destination and assigned shard. >>>>>>> > >>>>>>> > Having said that, I might be able to use dynamic destination for >>>>>>> this, possibly with the need of custom FileNaming, and set shards to be >>>>>>> always 1. But it feels less natural than allowing the user to swap >>>>>>> already >>>>>>> present `RandomShardingFunction` [2] with something of his own choosing. >>>>>>> > >>>>>>> > >>>>>>> > [1] >>>>>>> https://github.com/apache/beam/blob/release-2.29.0/sdks/java/core/src/main/java/org/apache/beam/sdk/values/ShardedKey.java >>>>>>> > [2] >>>>>>> https://github.com/apache/beam/blob/release-2.29.0/sdks/java/core/src/main/java/org/apache/beam/sdk/io/WriteFiles.java#L856 >>>>>>> > >>>>>>> > Kenn >>>>>>> > >>>>>>> > On Tue, Jun 15, 2021 at 3:49 PM Tyson Hamilton <tyso...@google.com> >>>>>>> wrote: >>>>>>> > >>>>>>> > Adding sharding to the model may require a wider discussion than >>>>>>> FileIO alone. I'm not entirely sure how wide, or if this has been >>>>>>> proposed >>>>>>> before, but IMO it warrants a design doc or proposal. >>>>>>> > >>>>>>> > A couple high level questions I can think of are, >>>>>>> > - What runners support sharding? >>>>>>> > * There will be some work in Dataflow required to support >>>>>>> this but I'm not sure how much. >>>>>>> > - What does sharding mean for streaming pipelines? >>>>>>> > >>>>>>> > A more nitty-detail question: >>>>>>> > - How can this be achieved performantly? For example, if a >>>>>>> shuffle is required to achieve a particular sharding constraint, should >>>>>>> we >>>>>>> allow transforms to declare they don't modify the sharding property >>>>>>> (e.g. >>>>>>> key preserving) which may allow a runner to avoid an additional shuffle >>>>>>> if >>>>>>> a preceding shuffle can guarantee the sharding requirements? >>>>>>> > >>>>>>> > Where X is the shuffle that could be avoided: input -> shuffle >>>>>>> (key sharding fn A) -> transform1 (key preserving) -> transform 2 (key >>>>>>> preserving) -> X -> fileio (key sharding fn A) >>>>>>> > >>>>>>> > On Tue, Jun 15, 2021 at 1:02 AM Jozef Vilcek < >>>>>>> jozo.vil...@gmail.com> wrote: >>>>>>> > >>>>>>> > I would like to extend FileIO with possibility to specify a custom >>>>>>> sharding function: >>>>>>> > https://issues.apache.org/jira/browse/BEAM-12493 >>>>>>> > >>>>>>> > I have 2 use-cases for this: >>>>>>> > >>>>>>> > I need to generate shards which are compatible with Hive bucketing >>>>>>> and therefore need to decide shard assignment based on data fields of >>>>>>> input >>>>>>> element >>>>>>> > When running e.g. on Spark and job encounters kind of failure >>>>>>> which cause a loss of some data from previous stages, Spark does issue >>>>>>> recompute of necessary task in necessary stages to recover data. Because >>>>>>> the shard assignment function is random as default, some data will end >>>>>>> up >>>>>>> in different shards and cause duplicates in the final output. >>>>>>> > >>>>>>> > Please let me know your thoughts in case you see a reason to not >>>>>>> to add such improvement. >>>>>>> > >>>>>>> > Thanks, >>>>>>> > Jozef >>>>>>> > >>>>>>> > >>>>>>> >>>>>>
