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
> <https://github.com/apache/beam/blob/f66eb5fe23b2500b396e6f711cdf4aeef6b31ab8/sdks/java/core/src/main/java/org/apache/beam/sdk/schemas/Schema.java>
> 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
> <https://godoc.org/cloud.google.com/go/bigquery#hdr-Schemas> does
> something like that, using field tags. Similarly, the "encoding/json"
> <https://golang.org/doc/articles/json_and_go.html> 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
> <https://github.com/GoogleCloudPlatform/protoc-gen-bq-schema>, 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,
>>> <https://docs.google.com/document/d/1kQwx4Ah6PzG8z2ZMuNsNEXkGsLXm6gADOZaIO7reUOg/edit#>
>>> 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
>
