I agree with this, but I think it's a significant rethinking of Beam that I didn't want to couple to schemas. In addition to rethinking the API, it might also require rethinking all of our runners.
Also while columnar can be a large perf win, I suspect that we currently have lower-hanging fruit to optimize when it comes to performance. Reuven On Tue, Jan 8, 2019 at 5:25 AM Robert Bradshaw <rober...@google.com> wrote: > On Fri, Jan 4, 2019 at 12:54 AM Reuven Lax <re...@google.com> wrote: > > > > I looked at Apache Arrow as a potential serialization format for Row > coders. At the time it didn't seem a perfect fit - Beam's programming model > is record-at-a-time, and Arrow is optimized for large batches of records > (while Beam has a concept of "bundles" they are completely non > deterministic, and records might bundle different on retry). You could use > Arrow with single-record batches, but I suspect that would end up adding a > lot of extra overhead. That being said, I think it's still something worth > investigating further. > > Though Beam's model is row-oriented, I think it'd make a lot of sense > to support column-oriented transfer of data across the data plane > (we're already concatenating serialized records lengthwise), with > Arrow as a first candidate, and (either as part of the public API or > as an implementation detail) columnar processing as well (e.g. > projections, maps, filters, and aggregations can often be done more > efficiently in a columnar fashion). While this is often a significant > win in C++ (and presumably Java), it's essential for doing > high-performance computing in Python (e.g. Numpy, SciPy, Pandas, > Tensorflow, ... all have batch-oriented APIs and avoid representing > records as individual objects, something we'll need to tackle for > BeamPython at least). > > > > > Reuven > > > > > > > > On Fri, Jan 4, 2019 at 12:34 AM Gleb Kanterov <g...@spotify.com> wrote: > >> > >> Reuven, it sounds great. I see there is a similar thing to Row coders > happening in Apache Arrow, and there is a similarity between Apache Arrow > Flight and data exchange service in portability. How do you see these two > things relate to each other in the long term? > >> > >> On Fri, Jan 4, 2019 at 12:13 AM Reuven Lax <re...@google.com> wrote: > >>> > >>> The biggest advantage is actually readability and usability. A > secondary advantage is that it means that Go will be able to interact > seamlessly with BeamSQL, which would be a big win for Go. > >>> > >>> A schema is basically a way of saying that a record has a specific set > of (possibly nested, possibly repeated) fields. So for instance let's say > that the user's type is a struct with fields named user, country, > purchaseCost. This allows us to provide transforms that operate on field > names. Some example (using the Java API): > >>> > >>> PCollection users = events.apply(Select.fields("user")); // Select > out only the user field. > >>> > >>> PCollection joinedEvents = > queries.apply(Join.innerJoin(clicks).byFields("user")); // Join two > PCollections by user. > >>> > >>> // For each country, calculate the total purchase cost as well as the > top 10 purchases. > >>> // A new schema is created containing fields total_cost and > top_purchases, and rows are created with the aggregation results. > >>> PCollection purchaseStatistics = events.apply( > >>> Group.byFieldNames("country") > >>> .aggregateField("purchaseCost", Sum.ofLongs(), > "total_cost")) > >>> .aggregateField("purchaseCost", Top.largestLongs(10), > "top_purchases")) > >>> > >>> > >>> This is far more readable than what we have today, and what unlocks > this is that Beam actually knows the structure of the record instead of > assuming records are uncrackable blobs. > >>> > >>> Note that a coder is basically a special case of a schema that has a > single field. > >>> > >>> In BeamJava we have a SchemaRegistry which knows how to turn user > types into schemas. We use reflection to analyze many user types (e.g. > simple POJO structs, JavaBean classes, Avro records, protocol buffers, > etc.) to determine the schema, however this is done only when the graph is > initially generated. We do use code generation (in Java we do bytecode > generation) to make this somewhat more efficient. I'm willing to bet that > the code generator you've written for structs could be very easily modified > for schemas instead, so it would not be wasted work if we went with schemas. > >>> > >>> One of the things I'm working on now is documenting Beam schemas. They > are already very powerful and useful, but since there is still nothing in > our documentation about them, they are not yet widely used. I expect to > finish draft documentation by the end of January. > >>> > >>> Reuven > >>> > >>> On Thu, Jan 3, 2019 at 11:32 PM Robert Burke <r...@google.com> wrote: > >>>> > >>>> That's an interesting idea. I must confess I don't rightly know the > difference between a schema and coder, but here's what I've got with a bit > of searching through memory and the mailing list. Please let me know if I'm > off track. > >>>> > >>>> As near as I can tell, a schema, as far as Beam takes it is a > mechanism to define what data is extracted from a given row of data. So in > principle, there's an opportunity to be more efficient with data with many > columns that aren't being used, and only extract the data that's meaningful > to the pipeline. > >>>> The trick then is how to apply the schema to a given serialization > format, which is something I'm missing in my mental model (and then how to > do it efficiently in Go). > >>>> > >>>> I do know that the Go client package for BigQuery does something like > that, using field tags. Similarly, the "encoding/json" package in the Go > Standard Library permits annotating fields and it will read out and > deserialize the JSON fields and that's it. > >>>> > >>>> A concern I have is that Go (at present) would require pre-compile > time code generation for schemas to be efficient, and they would still > mostly boil down to turning []bytes into real structs. Go reflection > doesn't keep up. > >>>> Go has no mechanism I'm aware of to Just In Time compile more > efficient processing of values. > >>>> It's also not 100% clear how Schema's would play with protocol > buffers or similar. > >>>> BigQuery has a mechanism of generating a JSON schema from a proto > file, but that's only the specification half, not the using half. > >>>> > >>>> As it stands, the code generator I've been building these last months > could (in principle) statically analyze a user's struct, and then generate > an efficient dedicated coder for it. It just has no where to put them such > that the Go SDK would use it. > >>>> > >>>> > >>>> On Thu, Jan 3, 2019 at 1:39 PM Reuven Lax <re...@google.com> wrote: > >>>>> > >>>>> I'll make a different suggestion. There's been some chatter that > schemas are a better tool than coders, and that in Beam 3.0 we should make > schemas the basic semantics instead of coders. Schemas provide everything a > coder provides, but also allows for far more readable code. We can't make > such a change in Beam Java 2.X for compatibility reasons, but maybe in Go > we're better off starting with schemas instead of coders? > >>>>> > >>>>> Reuven > >>>>> > >>>>> On Thu, Jan 3, 2019 at 8:45 PM Robert Burke <rob...@frantil.com> > wrote: > >>>>>> > >>>>>> One area that the Go SDK currently lacks: is the ability for users > to specify their own coders for types. > >>>>>> > >>>>>> I've written a proposal document, and while I'm confident about the > core, there are certainly some edge cases that require discussion before > getting on with the implementation. > >>>>>> > >>>>>> At presently, the SDK only permits primitive value types (all > numeric types but complex, strings, and []bytes) which are coded with beam > coders, and structs whose exported fields are of those type, which is then > encoded as JSON. Protocol buffer support is hacked in to avoid the type > anaiyzer, and presents the current work around this issue. > >>>>>> > >>>>>> The high level proposal is to catch up with Python and Java, and > have a coder registry. In addition, arrays, and maps should be permitted as > well. > >>>>>> > >>>>>> If you have alternatives, or other suggestions and opinions, I'd > love to hear them! Otherwise my intent is to get a PR ready by the end of > January. > >>>>>> > >>>>>> Thanks! > >>>>>> Robert Burke > >>>> > >>>> > >>>> > >>>> -- > >>>> http://go/where-is-rebo > >> > >> > >> > >> -- > >> Cheers, > >> Gleb >