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From Reuven Lax <re...@google.com>
Subject Re: [DISCUSS] Portability representation of schemas
Date Wed, 08 May 2019 19:46:08 GMT
On Wed, May 8, 2019 at 10:57 AM Rui Wang <ruwang@google.com> wrote:

> Regarding to DATETIME, I totally agree it should be removed as
> primitive type to avoid that each language has to find their time libraries
> (and if they could not find any, they will likely go to logical type and
> use int64 from Schema).
>
> I have two questions regarding to the representation:
>
> 1. There is nullable field for FieldType. I am not an expert of
> programming language. So does this field in proto means "null" is common in
> programming languages? Or this field is really optional, that if a language
> does not need "null", they can just ignore this field?
>

We could also call this optional instead. I used nullable because that
seemed consistent with what SQL does.

A programming language that supports optional is free to implement this
using optionals instead of null values. However all the current Beam
languages (Java, Python, Go) all support null.


>
> 2. How's time zone is dealt with?
>

DATETIME is poorly named. It's really a timestamp type, so it has no time
zone.


>
>
> -Rui
>
>
>
>
> *From: *Reuven Lax <relax@google.com>
> *Date: *Wed, May 8, 2019 at 9:54 AM
> *To: *dev
>
> Beam Java's support for schemas is just about done: we infer schemas from
>> a variety of types, we have a variety of utility transforms (join,
>> aggregate, etc.) for schemas, and schemas are integrated with the ParDo
>> machinery. The big remaining task I'm working on is writing documentation
>> and examples for all of this so that users are aware. If you're interested,
>> these slides
>> <https://docs.google.com/presentation/d/1kjgmbG2OkVldUM_aSHgho_C3rCftz_v66iBHSUb08P0/edit?usp=sharing>
from
>> the London Beam meetup show a bit more how schemas can be used and how they
>> simplify the API.
>>
>> I want to start integrating schemas into portability so that they can be
>> used from other languages such as Python (in particular this will also
>> allow BeamSQL to be invoked from other languages). In order to do this, the
>> Beam portability protos must have a way of representing schemas. Since this
>> has not been discussed before, I'm starting this discussion now on the list.
>>
>> As a reminder: a schema represents the type of a PCollection as a
>> collection of fields. Each field has a name, an id (position), and a field
>> type. A field type can be either a primitive type (int, long, string, byte
>> array, etc.), a nested row (itself with a schema), an array, or a map.
>>
>> We also support logical types. A logical type is a way for the user to
>> embed their own types in schema fields. A logical type is always backed by
>> a schema type, and contains a function for mapping the user's logical type
>> to the field type. You can think of this as a generalization of a coder:
>> while a coder always maps the user type to a byte array, a logical type can
>> map to an int, or a string, or any other schema field type (in fact any
>> coder can always be used as a logical type for mapping to byte-array field
>> types). Logical types are used extensively by Beam SQL to represent SQL
>> types that have no correspondence in Beam's field types (e.g. SQL has 4
>> different date/time types). Logical types for Beam schemas have a lot of
>> similarities to AVRO logical types.
>>
>> An initial proto representation for schemas is here
>> <https://github.com/apache/beam/blob/master/model/pipeline/src/main/proto/beam_runner_api.proto#L654>.
>> Before we go further with this, I would like community consensus on what
>> this representation should be. I can start by suggesting a few possible
>> changes to this representation (and hopefully others will suggest others):
>>
>>    - Kenn Knowles has suggested removing DATETIME as a primitive type,
>>    and instead making it a logical type backed by INT64 as this keeps our
>>    primitive types closer to "classical" PL primitive types. This also allows
>>    us to create multiple versions of this type - e.g. TIMESTAMP(millis),
>>    TIMESTAMP(micros), TIMESTAMP(nanos).
>>    - If we do the above, we can also consider removing DECIMAL and
>>    making that a logical type as well.
>>    - The id field is currently used for some performance optimizations
>>    only. If we formalized the idea of schema types having ids, then we might
>>    be able to use this to allow self-recursive schemas (self-recursive types
>>    are not currently allowed).
>>    - Beam Schemas currently have an ARRAY type. However Beam supports
>>    "large iterables" (iterables that don't fit in memory that the runner can
>>    page in), and this doesn't match well to arrays. I think we need to add an
>>    ITERABLE type as well to support things like GroupByKey results.
>>
>> It would also be interesting to explore allowing well-known metadata tags
>> on fields that Beam interprets. e.g. key and value, to allow Beam to
>> interpret any two-field schema as a KV, or window and timestamp to allow
>> automatically filling those out. However this would be an extension to the
>> current schema concept and deserves a separate discussion thread IMO.
>>
>> I ask that we please limit this discussion to the proto representation of
>> schemas. If people want to discuss (or rediscuss) other things around Beam
>> schemas, I'll be happy to create separate threads for those discussions.
>>
>> Thank you!
>>
>> Reuven
>>
>

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