On Mon, Jun 3, 2019 at 10:04 PM Reuven Lax <re...@google.com> wrote:
>
>
> On Mon, Jun 3, 2019 at 12:27 PM Brian Hulette <bhule...@google.com> wrote:
>
>> > It has to go into the proto somewhere (since that's the only way the
>> SDK can get it), but I'm not sure they should be considered integral parts
>> of the type.
>> Are you just advocating for an approach where any SDK-specific
>> information is stored outside of the Schema message itself so that Schema
>> really does just represent the type? That seems reasonable to me, and
>> alleviates my concerns about how this applies to columnar encodings a bit
>> as well.
>>
>
> Yes, that's exactly what I'm advocating.
>
>
>>
>> We could lift all of the LogicalTypeConversion messages out of the Schema
>> and the LogicalType like this:
>>
>> message SchemaCoder {
>> Schema schema = 1;
>> LogicalTypeConversion root_conversion = 2;
>> map<string, LogicalTypeConversion> attribute_conversions = 3; // only
>> necessary for user type aliases, portable logical types by definition have
>> nothing SDK-specific
>> }
>>
>
> I'm not sure what the map is for? I think we have status quo wihtout it.
>
My intention was that the SDK-specific information (to/from functions) for
any nested fields that are themselves user type aliases would be stored in
this map. That was the motivation for my next question, if we don't allow
user types to be nested within other user types we may not need it.
I may be missing your meaning - but I think we currently only have status
quo without this map in the Java SDK because Schema.LogicalType is just an
interface that must be implemented. It's appropriate for just portable
logical types, not user-type aliases. Note I've adopted Kenn's terminology
where portable logical type is a type that can be identified by just a URN
and maybe some parameters, while a user type alias needs some SDK specific
information, like a class and to/from UDFs.
>
>> I think a critical question (that has implications for the above
>> proposal) is how/if the two different concepts Kenn mentioned are allowed
>> to nest. For example, you could argue it's redundant to have a user type
>> alias that has a Row representation with a field that is itself a user type
>> alias, because instead you could just have a single top-level type alias
>> with to/from functions that pack and unpack the entire hierarchy. On the
>> other hand, I think it does make sense for a user type alias or a truly
>> portable logical type to have a field that is itself a truly portable
>> logical type (e.g. a user type alias or portable type with a DateTime).
>>
>> I've been assuming that user-type aliases could be nested, but should we
>> disallow that? Or should we go the other way and require that logical types
>> define at most one "level"?
>>
>
> No I think it's useful to allow things to be nested (though of course the
> nesting must terminate).
>
>
>>
>> Brian
>>
>> On Mon, Jun 3, 2019 at 11:08 AM Kenneth Knowles <k...@apache.org> wrote:
>>
>>>
>>> On Mon, Jun 3, 2019 at 10:53 AM Reuven Lax <re...@google.com> wrote:
>>>
>>>> So I feel a bit leery about making the to/from functions a fundamental
>>>> part of the portability representation. In my mind, that is very tied to a
>>>> specific SDK/language. A SDK (say the Java SDK) wants to allow users to use
>>>> a wide variety of native types with schemas, and under the covers uses the
>>>> to/from functions to implement that. However from the portable Beam
>>>> perspective, the schema itself should be the real "type" of the
>>>> PCollection; the to/from methods are simply a way that a particular SDK
>>>> makes schemas easier to use. It has to go into the proto somewhere (since
>>>> that's the only way the SDK can get it), but I'm not sure they should be
>>>> considered integral parts of the type.
>>>>
>>>
>>> On the doc in a couple places this distinction was made:
>>>
>>> * For truly portable logical types, no instructions for the SDK are
>>> needed. Instead, they require:
>>> - URN: a standardized identifier any SDK can recognize
>>> - A spec: what is the universe of values in this type?
>>> - A representation: how is it represented in built-in types? This is
>>> how SDKs who do not know/care about the URN will process it
>>> - (optional): SDKs choose preferred SDK-specific types to embed the
>>> values in. SDKs have to know about the URN and choose for themselves.
>>>
>>> *For user-level type aliases, written as convenience by the user in
>>> their pipeline, what Java schemas have today:
>>> - to/from UDFs: the code is SDK-specific
>>> - some representation of the intended type (like java class): also
>>> SDK specific
>>> - a representation
>>> - any "id" is just like other ids in the pipeline, just avoiding
>>> duplicating the proto
>>> - Luke points out that nesting these can give multiple SDKs a hint
>>>
>>> In my mind the remaining complexity is whether or not we need to be able
>>> to move between the two. Composite PTransforms, for example, do have
>>> fluidity between being strictly user-defined versus portable URN+payload.
>>> But it requires lots of engineering, namely the current work on expansion
>>> service.
>>>
>>> Kenn
>>>
>>>
>>>> On Mon, Jun 3, 2019 at 10:23 AM Brian Hulette <bhule...@google.com>
>>>> wrote:
>>>>
>>>>> Ah I see, I didn't realize that. Then I suppose we'll need to/from
>>>>> functions somewhere in the logical type conversion to preserve the current
>>>>> behavior.
>>>>>
>>>>> I'm still a little hesitant to make these functions an explicit part
>>>>> of LogicalTypeConversion for another reason. Down the road, schemas could
>>>>> give us an avenue to use a batched columnar format (presumably arrow, but
>>>>> of course others are possible). By making to/from an explicit part of
>>>>> logical types we add some element-wise logic to a schema representation
>>>>> that's otherwise ambivalent to element-wise vs. batched encodings.
>>>>>
>>>>> I suppose you could make an argument that to/from are only for
>>>>> custom types. There will also be some set of well-known types identified
>>>>> only by URN and some parameters, which could easily be translated to a
>>>>> columnar format. We could just not support custom types fully if we add a
>>>>> columnar encoding, or maybe add optional toBatch/fromBatch functions
>>>>> when/if we get there.
>>>>>
>>>>> What about something like this that makes the two different types of
>>>>> logical types explicit?
>>>>>
>>>>> // Describes a logical type and how to convert between it and its
>>>>> representation (e.g. Row).
>>>>> message LogicalTypeConversion {
>>>>> oneof conversion {
>>>>> message Standard standard = 1;
>>>>> message Custom custom = 2;
>>>>> }
>>>>>
>>>>> message Standard {
>>>>> String urn = 1;
>>>>> repeated string args = 2; // could also be a map
>>>>> }
>>>>>
>>>>> message Custom {
>>>>> FunctionSpec(?) toRepresentation = 1;
>>>>> FunctionSpec(?) fromRepresentation = 2;
>>>>> bytes type = 3; // e.g. serialized class for Java
>>>>> }
>>>>> }
>>>>>
>>>>> And LogicalType and Schema become:
>>>>>
>>>>> message LogicalType {
>>>>> FieldType representation = 1;
>>>>> LogicalTypeConversion conversion = 2;
>>>>> }
>>>>>
>>>>> message Schema {
>>>>> ...
>>>>> repeated Field fields = 1;
>>>>> LogicalTypeConversion conversion = 2; // implied that representation
>>>>> is Row
>>>>> }
>>>>>
>>>>> Brian
>>>>>
>>>>> On Sat, Jun 1, 2019 at 10:44 AM Reuven Lax <re...@google.com> wrote:
>>>>>
>>>>>> Keep in mind that right now the SchemaRegistry is only assumed to
>>>>>> exist at graph-construction time, not at execution time; all information
>>>>>> in
>>>>>> the schema registry is embedded in the SchemaCoder, which is the only
>>>>>> thing
>>>>>> we keep around when the pipeline is actually running. We could look into
>>>>>> changing this, but it would potentially be a very big change, and I do
>>>>>> think we should start getting users actively using schemas soon.
>>>>>>
>>>>>> On Fri, May 31, 2019 at 3:40 PM Brian Hulette <bhule...@google.com>
>>>>>> wrote:
>>>>>>
>>>>>>> > Can you propose what the protos would look like in this case?
>>>>>>> Right now LogicalType does not contain the to/from conversion functions
>>>>>>> in
>>>>>>> the proto. Do you think we'll need to add these in?
>>>>>>>
>>>>>>> Maybe. Right now the proposed LogicalType message is pretty
>>>>>>> simple/generic:
>>>>>>> message LogicalType {
>>>>>>> FieldType representation = 1;
>>>>>>> string logical_urn = 2;
>>>>>>> bytes logical_payload = 3;
>>>>>>> }
>>>>>>>
>>>>>>> If we keep just logical_urn and logical_payload, the logical_payload
>>>>>>> could itself be a protobuf with attributes of 1) a serialized class and
>>>>>>> 2/3) to/from functions. Or, alternatively, we could have a
>>>>>>> generalization
>>>>>>> of the SchemaRegistry for logical types. Implementations for standard
>>>>>>> types
>>>>>>> and user-defined types would be registered by URN, and the SDK could
>>>>>>> look
>>>>>>> them up given just a URN. I put a brief section about this alternative
>>>>>>> in
>>>>>>> the doc last week [1]. What I suggested there included removing the
>>>>>>> logical_payload field, which is probably overkill. The critical piece is
>>>>>>> just relying on a registry in the SDK to look up types and to/from
>>>>>>> functions rather than storing them in the portable schema itself.
>>>>>>>
>>>>>>> I kind of like keeping the LogicalType message generic for now,
>>>>>>> since it gives us a way to try out these various approaches, but maybe
>>>>>>> that's just a cop out.
>>>>>>>
>>>>>>> [1]
>>>>>>> https://docs.google.com/document/d/1uu9pJktzT_O3DxGd1-Q2op4nRk4HekIZbzi-0oTAips/edit?ts=5cdf6a5b#heading=h.jlt5hdrolfy
>>>>>>>
>>>>>>> On Fri, May 31, 2019 at 12:36 PM Reuven Lax <re...@google.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On Tue, May 28, 2019 at 10:11 AM Brian Hulette <bhule...@google.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Sun, May 26, 2019 at 1:25 PM Reuven Lax <re...@google.com>
>>>>>>>>> wrote:
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> On Fri, May 24, 2019 at 11:42 AM Brian Hulette <
>>>>>>>>>> bhule...@google.com> wrote:
>>>>>>>>>>
>>>>>>>>>>> *tl;dr:* SchemaCoder represents a logical type with a base type
>>>>>>>>>>> of Row and we should think about that.
>>>>>>>>>>>
>>>>>>>>>>> I'm a little concerned that the current proposals for a portable
>>>>>>>>>>> representation don't actually fully represent Schemas. It seems to
>>>>>>>>>>> me that
>>>>>>>>>>> the current java-only Schemas are made up three concepts that are
>>>>>>>>>>> intertwined:
>>>>>>>>>>> (a) The Java SDK specific code for schema inference, type
>>>>>>>>>>> coercion, and "schema-aware" transforms.
>>>>>>>>>>> (b) A RowCoder[1] that encodes Rows[2] which have a particular
>>>>>>>>>>> Schema[3].
>>>>>>>>>>> (c) A SchemaCoder[4] that has a RowCoder for a
>>>>>>>>>>> particular schema, and functions for converting Rows with that
>>>>>>>>>>> schema
>>>>>>>>>>> to/from a Java type T. Those functions and the RowCoder are then
>>>>>>>>>>> composed
>>>>>>>>>>> to provider a Coder for the type T.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> RowCoder is currently just an internal implementation detail, it
>>>>>>>>>> can be eliminated. SchemaCoder is the only thing that determines a
>>>>>>>>>> schema
>>>>>>>>>> today.
>>>>>>>>>>
>>>>>>>>> Why not keep it around? I think it would make sense to have a
>>>>>>>>> RowCoder implementation in every SDK, as well as something like
>>>>>>>>> SchemaCoder
>>>>>>>>> that defines a conversion from that SDK's "Row" to the language type.
>>>>>>>>>
>>>>>>>>
>>>>>>>> The point is that from a programmer's perspective, there is nothing
>>>>>>>> much special about Row. Any type can have a schema, and the only
>>>>>>>> special
>>>>>>>> thing about Row is that it's always guaranteed to exist. From that
>>>>>>>> standpoint, Row is nearly an implementation detail. Today RowCoder is
>>>>>>>> never
>>>>>>>> set on _any_ PCollection, it's literally just used as a helper
>>>>>>>> library, so
>>>>>>>> there's no real need for it to exist as a "Coder."
>>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> We're not concerned with (a) at this time since that's specific
>>>>>>>>>>> to the SDK, not the interface between them. My understanding is we
>>>>>>>>>>> just
>>>>>>>>>>> want to define a portable representation for (b) and/or (c).
>>>>>>>>>>>
>>>>>>>>>>> What has been discussed so far is really just a portable
>>>>>>>>>>> representation for (b), the RowCoder, since the discussion is only
>>>>>>>>>>> around
>>>>>>>>>>> how to represent the schema itself and not the to/from functions.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Correct. The to/from functions are actually related to a). One of
>>>>>>>>>> the big goals of schemas was that users should not be forced to
>>>>>>>>>> operate on
>>>>>>>>>> rows to get schemas. A user can create PCollection<MyRandomType> and
>>>>>>>>>> as
>>>>>>>>>> long as the SDK can infer a schema from MyRandomType, the user never
>>>>>>>>>> needs
>>>>>>>>>> to even see a Row object. The to/fromRow functions are what make
>>>>>>>>>> this work
>>>>>>>>>> today.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> One of the points I'd like to make is that this type coercion is a
>>>>>>>>> useful concept on it's own, separate from schemas. It's especially
>>>>>>>>> useful
>>>>>>>>> for a type that has a schema and is encoded by RowCoder since that can
>>>>>>>>> represent many more types, but the type coercion doesn't have to be
>>>>>>>>> tied to
>>>>>>>>> just schemas and RowCoder. We could also do type coercion for types
>>>>>>>>> that
>>>>>>>>> are effectively wrappers around an integer or a string. It could just
>>>>>>>>> be a
>>>>>>>>> general way to map language types to base types (i.e. types that we
>>>>>>>>> have a
>>>>>>>>> coder for). Then it just becomes a general framework for extending
>>>>>>>>> coders
>>>>>>>>> to represent more language types.
>>>>>>>>>
>>>>>>>>
>>>>>>>> Let's not tie those conversations. Maybe a similar concept will
>>>>>>>> hold true for general coders (or we might decide to get rid of coders
>>>>>>>> in
>>>>>>>> favor of schemas, in which case that becomes moot), but I don't think
>>>>>>>> we
>>>>>>>> should prematurely generalize.
>>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> One of the outstanding questions for that schema representation
>>>>>>>>>>> is how to represent logical types, which may or may not have some
>>>>>>>>>>> language
>>>>>>>>>>> type in each SDK (the canonical example being a timsetamp type with
>>>>>>>>>>> seconds
>>>>>>>>>>> and nanos and java.time.Instant). I think this question is
>>>>>>>>>>> critically
>>>>>>>>>>> important, because (c), the SchemaCoder, is actually *defining a
>>>>>>>>>>> logical
>>>>>>>>>>> type* with a language type T in the Java SDK. This becomes clear
>>>>>>>>>>> when you
>>>>>>>>>>> compare SchemaCoder[4] to the Schema.LogicalType interface[5] - both
>>>>>>>>>>> essentially have three attributes: a base type, and two functions
>>>>>>>>>>> for
>>>>>>>>>>> converting to/from that base type. The only difference is for
>>>>>>>>>>> SchemaCoder
>>>>>>>>>>> that base type must be a Row so it can be represented by a Schema
>>>>>>>>>>> alone,
>>>>>>>>>>> while LogicalType can have any base type that can be represented by
>>>>>>>>>>> FieldType, including a Row.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> This is not true actually. SchemaCoder can have any base type,
>>>>>>>>>> that's why (in Java) it's SchemaCoder<T>. This is why PCollection<T>
>>>>>>>>>> can
>>>>>>>>>> have a schema, even if T is not Row.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>> I'm not sure I effectively communicated what I meant - When I said
>>>>>>>>> SchemaCoder's "base type" I wasn't referring to T, I was referring to
>>>>>>>>> the
>>>>>>>>> base FieldType, whose coder we use for this type. I meant "base type"
>>>>>>>>> to be
>>>>>>>>> analogous to LogicalType's `getBaseType`, or what Kenn is suggesting
>>>>>>>>> we
>>>>>>>>> call "representation" in the portable beam schemas doc. To define some
>>>>>>>>> terms from my original message:
>>>>>>>>> base type = an instance of FieldType, crucially this is something
>>>>>>>>> that we have a coder for (be it VarIntCoder, Utf8Coder, RowCoder, ...)
>>>>>>>>> language type (or "T", "type T", "logical type") = Some Java class
>>>>>>>>> (or something analogous in the other SDKs) that we may or may not
>>>>>>>>> have a
>>>>>>>>> coder for. It's possible to define functions for converting instances
>>>>>>>>> of
>>>>>>>>> the language type to/from the base type.
>>>>>>>>>
>>>>>>>>> I was just trying to make the case that SchemaCoder is really a
>>>>>>>>> special case of LogicalType, where `getBaseType` always returns a Row
>>>>>>>>> with
>>>>>>>>> the stored Schema.
>>>>>>>>>
>>>>>>>>
>>>>>>>> Yeah, I think I got that point.
>>>>>>>>
>>>>>>>> Can you propose what the protos would look like in this case? Right
>>>>>>>> now LogicalType does not contain the to/from conversion functions in
>>>>>>>> the
>>>>>>>> proto. Do you think we'll need to add these in?
>>>>>>>>
>>>>>>>>
>>>>>>>>> To make the point with code: SchemaCoder<T> can be made to
>>>>>>>>> implement Schema.LogicalType<T,Row> with trivial implementations of
>>>>>>>>> getBaseType, toBaseType, and toInputType (I'm not trying to say we
>>>>>>>>> should
>>>>>>>>> or shouldn't do this, just using it illustrate my point):
>>>>>>>>>
>>>>>>>>> class SchemaCoder extends CustomCoder<T> implements
>>>>>>>>> Schema.LogicalType<T, Row> {
>>>>>>>>> ...
>>>>>>>>>
>>>>>>>>> @Override
>>>>>>>>> FieldType getBaseType() {
>>>>>>>>> return FieldType.row(getSchema());
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> @Override
>>>>>>>>> public Row toBaseType() {
>>>>>>>>> return this.toRowFunction.apply(input);
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> @Override
>>>>>>>>> public T toInputType(Row base) {
>>>>>>>>> return this.fromRowFunction.apply(base);
>>>>>>>>> }
>>>>>>>>> ...
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>>> I think it may make sense to fully embrace this duality, by
>>>>>>>>>>> letting SchemaCoder have a baseType other than just Row and
>>>>>>>>>>> renaming it to
>>>>>>>>>>> LogicalTypeCoder/LanguageTypeCoder. The current Java SDK
>>>>>>>>>>> schema-aware
>>>>>>>>>>> transforms (a) would operate only on LogicalTypeCoders with a Row
>>>>>>>>>>> base
>>>>>>>>>>> type. Perhaps some of the current schema logic could alsobe
>>>>>>>>>>> applied more
>>>>>>>>>>> generally to any logical type - for example, to provide type
>>>>>>>>>>> coercion for
>>>>>>>>>>> logical types with a base type other than Row, like int64 and a
>>>>>>>>>>> timestamp
>>>>>>>>>>> class backed by millis, or fixed size bytes and a UUID class. And
>>>>>>>>>>> having a
>>>>>>>>>>> portable representation that represents those (non Row backed)
>>>>>>>>>>> logical
>>>>>>>>>>> types with some URN would also allow us to pass them to other
>>>>>>>>>>> languages
>>>>>>>>>>> without unnecessarily wrapping them in a Row in order to use
>>>>>>>>>>> SchemaCoder.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> I think the actual overlap here is between the to/from functions
>>>>>>>>>> in SchemaCoder (which is what allows SchemaCoder<T> where T != Row)
>>>>>>>>>> and the
>>>>>>>>>> equivalent functionality in LogicalType. However making all of
>>>>>>>>>> schemas
>>>>>>>>>> simply just a logical type feels a bit awkward and circular to me.
>>>>>>>>>> Maybe we
>>>>>>>>>> should refactor that part out into a LogicalTypeConversion proto, and
>>>>>>>>>> reference that from both LogicalType and from SchemaCoder?
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> LogicalType is already potentially circular though. A schema can
>>>>>>>>> have a field with a logical type, and that logical type can have a
>>>>>>>>> base
>>>>>>>>> type of Row with a field with a logical type (and on and on...). To
>>>>>>>>> me it
>>>>>>>>> seems elegant, not awkward, to recognize that SchemaCoder is just a
>>>>>>>>> special
>>>>>>>>> case of this concept.
>>>>>>>>>
>>>>>>>>> Something like the LogicalTypeConversion proto would definitely be
>>>>>>>>> an improvement, but I would still prefer just using a top-level
>>>>>>>>> logical
>>>>>>>>> type :)
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> I've added a section to the doc [6] to propose this alternative
>>>>>>>>>>> in the context of the portable representation but I wanted to bring
>>>>>>>>>>> it up
>>>>>>>>>>> here as well to solicit feedback.
>>>>>>>>>>>
>>>>>>>>>>> [1]
>>>>>>>>>>> https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/coders/RowCoder.java#L41
>>>>>>>>>>> [2]
>>>>>>>>>>> https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/values/Row.java#L59
>>>>>>>>>>> [3]
>>>>>>>>>>> https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/schemas/Schema.java#L48
>>>>>>>>>>> [4]
>>>>>>>>>>> https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/schemas/SchemaCoder.java#L33
>>>>>>>>>>> [5]
>>>>>>>>>>> https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/schemas/Schema.java#L489
>>>>>>>>>>> [6]
>>>>>>>>>>> https://docs.google.com/document/d/1uu9pJktzT_O3DxGd1-Q2op4nRk4HekIZbzi-0oTAips/edit?ts=5cdf6a5b#heading=h.7570feur1qin
>>>>>>>>>>>
>>>>>>>>>>> On Fri, May 10, 2019 at 9:16 AM Brian Hulette <
>>>>>>>>>>> bhule...@google.com> wrote:
>>>>>>>>>>>
>>>>>>>>>>>> Ah thanks! I added some language there.
>>>>>>>>>>>>
>>>>>>>>>>>> *From: *Kenneth Knowles <k...@apache.org>
>>>>>>>>>>>> *Date: *Thu, May 9, 2019 at 5:31 PM
>>>>>>>>>>>> *To: *dev
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>> *From: *Brian Hulette <bhule...@google.com>
>>>>>>>>>>>>> *Date: *Thu, May 9, 2019 at 2:02 PM
>>>>>>>>>>>>> *To: * <dev@beam.apache.org>
>>>>>>>>>>>>>
>>>>>>>>>>>>> We briefly discussed using arrow schemas in place of beam
>>>>>>>>>>>>>> schemas entirely in an arrow thread [1]. The biggest reason not
>>>>>>>>>>>>>> to this was
>>>>>>>>>>>>>> that we wanted to have a type for large iterables in beam
>>>>>>>>>>>>>> schemas. But
>>>>>>>>>>>>>> given that large iterables aren't currently implemented, beam
>>>>>>>>>>>>>> schemas look
>>>>>>>>>>>>>> very similar to arrow schemas.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>> I think it makes sense to take inspiration from arrow schemas
>>>>>>>>>>>>>> where possible, and maybe even copy them outright. Arrow already
>>>>>>>>>>>>>> has a
>>>>>>>>>>>>>> portable (flatbuffers) schema representation [2], and
>>>>>>>>>>>>>> implementations for
>>>>>>>>>>>>>> it in many languages that we may be able to re-use as we bring
>>>>>>>>>>>>>> schemas to
>>>>>>>>>>>>>> more SDKs (the project has Python and Go implementations). There
>>>>>>>>>>>>>> are a
>>>>>>>>>>>>>> couple of concepts in Arrow schemas that are specific for the
>>>>>>>>>>>>>> format and
>>>>>>>>>>>>>> wouldn't make sense for us, (fields can indicate whether or not
>>>>>>>>>>>>>> they are
>>>>>>>>>>>>>> dictionary encoded, and the schema has an endianness field), but
>>>>>>>>>>>>>> if you
>>>>>>>>>>>>>> drop those concepts the arrow spec looks pretty similar to the
>>>>>>>>>>>>>> beam proto
>>>>>>>>>>>>>> spec.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> FWIW I left a blank section in the doc for filling out what
>>>>>>>>>>>>> the differences are and why, and conversely what the interop
>>>>>>>>>>>>> opportunities
>>>>>>>>>>>>> may be. Such sections are some of my favorite sections of design
>>>>>>>>>>>>> docs.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Kenn
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> Brian
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> [1]
>>>>>>>>>>>>>> https://lists.apache.org/thread.html/6be7715e13b71c2d161e4378c5ca1c76ac40cfc5988a03ba87f1c434@%3Cdev.beam.apache.org%3E
>>>>>>>>>>>>>> [2]
>>>>>>>>>>>>>> https://github.com/apache/arrow/blob/master/format/Schema.fbs#L194
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> *From: *Robert Bradshaw <rober...@google.com>
>>>>>>>>>>>>>> *Date: *Thu, May 9, 2019 at 1:38 PM
>>>>>>>>>>>>>> *To: *dev
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> From: Reuven Lax <re...@google.com>
>>>>>>>>>>>>>>> Date: Thu, May 9, 2019 at 7:29 PM
>>>>>>>>>>>>>>> To: dev
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> > Also in the future we might be able to do optimizations at
>>>>>>>>>>>>>>> the runner level if at the portability layer we understood
>>>>>>>>>>>>>>> schemes instead
>>>>>>>>>>>>>>> of just raw coders. This could be things like only parsing a
>>>>>>>>>>>>>>> subset of a
>>>>>>>>>>>>>>> row (if we know only a few fields are accessed) or using a
>>>>>>>>>>>>>>> columnar data
>>>>>>>>>>>>>>> structure like Arrow to encode batches of rows across
>>>>>>>>>>>>>>> portability. This
>>>>>>>>>>>>>>> doesn't affect data semantics of course, but having a richer,
>>>>>>>>>>>>>>> more-expressive type system opens up other opportunities.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> But we could do all of that with a RowCoder we understood to
>>>>>>>>>>>>>>> designate
>>>>>>>>>>>>>>> the type(s), right?
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> > On Thu, May 9, 2019 at 10:16 AM Robert Bradshaw <
>>>>>>>>>>>>>>> rober...@google.com> wrote:
>>>>>>>>>>>>>>> >>
>>>>>>>>>>>>>>> >> On the flip side, Schemas are equivalent to the space of
>>>>>>>>>>>>>>> Coders with
>>>>>>>>>>>>>>> >> the addition of a RowCoder and the ability to materialize
>>>>>>>>>>>>>>> to something
>>>>>>>>>>>>>>> >> other than bytes, right? (Perhaps I'm missing something
>>>>>>>>>>>>>>> big here...)
>>>>>>>>>>>>>>> >> This may make a backwards-compatible transition easier.
>>>>>>>>>>>>>>> (SDK-side, the
>>>>>>>>>>>>>>> >> ability to reason about and operate on such types is of
>>>>>>>>>>>>>>> course much
>>>>>>>>>>>>>>> >> richer than anything Coders offer right now.)
>>>>>>>>>>>>>>> >>
>>>>>>>>>>>>>>> >> From: Reuven Lax <re...@google.com>
>>>>>>>>>>>>>>> >> Date: Thu, May 9, 2019 at 4:52 PM
>>>>>>>>>>>>>>> >> To: dev
>>>>>>>>>>>>>>> >>
>>>>>>>>>>>>>>> >> > FYI I can imagine a world in which we have no coders.
>>>>>>>>>>>>>>> We could define the entire model on top of schemas. Today's
>>>>>>>>>>>>>>> "Coder" is
>>>>>>>>>>>>>>> completely equivalent to a single-field schema with a
>>>>>>>>>>>>>>> logical-type field
>>>>>>>>>>>>>>> (actually the latter is slightly more expressive as you aren't
>>>>>>>>>>>>>>> forced to
>>>>>>>>>>>>>>> serialize into bytes).
>>>>>>>>>>>>>>> >> >
>>>>>>>>>>>>>>> >> > Due to compatibility constraints and the effort that
>>>>>>>>>>>>>>> would be involved in such a change, I think the practical
>>>>>>>>>>>>>>> decision should
>>>>>>>>>>>>>>> be for schemas and coders to coexist for the time being.
>>>>>>>>>>>>>>> However when we
>>>>>>>>>>>>>>> start planning Beam 3.0, deprecating coders is something I
>>>>>>>>>>>>>>> would like to
>>>>>>>>>>>>>>> suggest.
>>>>>>>>>>>>>>> >> >
>>>>>>>>>>>>>>> >> > On Thu, May 9, 2019 at 7:48 AM Robert Bradshaw <
>>>>>>>>>>>>>>> rober...@google.com> wrote:
>>>>>>>>>>>>>>> >> >>
>>>>>>>>>>>>>>> >> >> From: Kenneth Knowles <k...@apache.org>
>>>>>>>>>>>>>>> >> >> Date: Thu, May 9, 2019 at 10:05 AM
>>>>>>>>>>>>>>> >> >> To: dev
>>>>>>>>>>>>>>> >> >>
>>>>>>>>>>>>>>> >> >> > This is a huge development. Top posting because I
>>>>>>>>>>>>>>> can be more compact.
>>>>>>>>>>>>>>> >> >> >
>>>>>>>>>>>>>>> >> >> > I really think after the initial idea converges this
>>>>>>>>>>>>>>> needs a design doc with goals and alternatives. It is an
>>>>>>>>>>>>>>> extraordinarily
>>>>>>>>>>>>>>> consequential model change. So in the spirit of doing the work
>>>>>>>>>>>>>>> / bias
>>>>>>>>>>>>>>> towards action, I created a quick draft at
>>>>>>>>>>>>>>> https://s.apache.org/beam-schemas and added everyone on
>>>>>>>>>>>>>>> this thread as editors. I am still in the process of writing
>>>>>>>>>>>>>>> this to match
>>>>>>>>>>>>>>> the thread.
>>>>>>>>>>>>>>> >> >>
>>>>>>>>>>>>>>> >> >> Thanks! Added some comments there.
>>>>>>>>>>>>>>> >> >>
>>>>>>>>>>>>>>> >> >> > *Multiple timestamp resolutions*: you can use
>>>>>>>>>>>>>>> logcial types to represent nanos the same way Java and proto do.
>>>>>>>>>>>>>>> >> >>
>>>>>>>>>>>>>>> >> >> As per the other discussion, I'm unsure the value in
>>>>>>>>>>>>>>> supporting
>>>>>>>>>>>>>>> >> >> multiple timestamp resolutions is high enough to
>>>>>>>>>>>>>>> outweigh the cost.
>>>>>>>>>>>>>>> >> >>
>>>>>>>>>>>>>>> >> >> > *Why multiple int types?* The domain of values for
>>>>>>>>>>>>>>> these types are different. For a language with one "int" or
>>>>>>>>>>>>>>> "number" type,
>>>>>>>>>>>>>>> that's another domain of values.
>>>>>>>>>>>>>>> >> >>
>>>>>>>>>>>>>>> >> >> What is the value in having different domains? If your
>>>>>>>>>>>>>>> data has a
>>>>>>>>>>>>>>> >> >> natural domain, chances are it doesn't line up exactly
>>>>>>>>>>>>>>> with one of
>>>>>>>>>>>>>>> >> >> these. I guess it's for languages whose types have
>>>>>>>>>>>>>>> specific domains?
>>>>>>>>>>>>>>> >> >> (There's also compactness in representation, encoded
>>>>>>>>>>>>>>> and in-memory,
>>>>>>>>>>>>>>> >> >> though I'm not sure that's high.)
>>>>>>>>>>>>>>> >> >>
>>>>>>>>>>>>>>> >> >> > *Columnar/Arrow*: making sure we unlock the ability
>>>>>>>>>>>>>>> to take this path is Paramount. So tying it directly to a
>>>>>>>>>>>>>>> row-oriented
>>>>>>>>>>>>>>> coder seems counterproductive.
>>>>>>>>>>>>>>> >> >>
>>>>>>>>>>>>>>> >> >> I don't think Coders are necessarily row-oriented.
>>>>>>>>>>>>>>> They are, however,
>>>>>>>>>>>>>>> >> >> bytes-oriented. (Perhaps they need not be.) There
>>>>>>>>>>>>>>> seems to be a lot of
>>>>>>>>>>>>>>> >> >> overlap between what Coders express in terms of
>>>>>>>>>>>>>>> element typing
>>>>>>>>>>>>>>> >> >> information and what Schemas express, and I'd rather
>>>>>>>>>>>>>>> have one concept
>>>>>>>>>>>>>>> >> >> if possible. Or have a clear division of
>>>>>>>>>>>>>>> responsibilities.
>>>>>>>>>>>>>>> >> >>
>>>>>>>>>>>>>>> >> >> > *Multimap*: what does it add over an array-valued
>>>>>>>>>>>>>>> map or large-iterable-valued map? (honest question, not
>>>>>>>>>>>>>>> rhetorical)
>>>>>>>>>>>>>>> >> >>
>>>>>>>>>>>>>>> >> >> Multimap has a different notion of what it means to
>>>>>>>>>>>>>>> contain a value,
>>>>>>>>>>>>>>> >> >> can handle (unordered) unions of non-disjoint keys,
>>>>>>>>>>>>>>> etc. Maybe this
>>>>>>>>>>>>>>> >> >> isn't worth a new primitive type.
>>>>>>>>>>>>>>> >> >>
>>>>>>>>>>>>>>> >> >> > *URN/enum for type names*: I see the case for both.
>>>>>>>>>>>>>>> The core types are fundamental enough they should never really
>>>>>>>>>>>>>>> change -
>>>>>>>>>>>>>>> after all, proto, thrift, avro, arrow, have addressed this (not
>>>>>>>>>>>>>>> to mention
>>>>>>>>>>>>>>> most programming languages). Maybe additions once every few
>>>>>>>>>>>>>>> years. I prefer
>>>>>>>>>>>>>>> the smallest intersection of these schema languages. A oneof is
>>>>>>>>>>>>>>> more clear,
>>>>>>>>>>>>>>> while URN emphasizes the similarity of built-in and logical
>>>>>>>>>>>>>>> types.
>>>>>>>>>>>>>>> >> >>
>>>>>>>>>>>>>>> >> >> Hmm... Do we have any examples of the multi-level
>>>>>>>>>>>>>>> primitive/logical
>>>>>>>>>>>>>>> >> >> type in any of these other systems? I have a bias
>>>>>>>>>>>>>>> towards all types
>>>>>>>>>>>>>>> >> >> being on the same footing unless there is compelling
>>>>>>>>>>>>>>> reason to divide
>>>>>>>>>>>>>>> >> >> things into primitive/use-defined ones.
>>>>>>>>>>>>>>> >> >>
>>>>>>>>>>>>>>> >> >> Here it seems like the most essential value of the
>>>>>>>>>>>>>>> primitive type set
>>>>>>>>>>>>>>> >> >> is to describe the underlying representation, for
>>>>>>>>>>>>>>> encoding elements in
>>>>>>>>>>>>>>> >> >> a variety of ways (notably columnar, but also
>>>>>>>>>>>>>>> interfacing with other
>>>>>>>>>>>>>>> >> >> external systems like IOs). Perhaps, rather than the
>>>>>>>>>>>>>>> previous
>>>>>>>>>>>>>>> >> >> suggestion of making everything a logical of bytes,
>>>>>>>>>>>>>>> this could be made
>>>>>>>>>>>>>>> >> >> clear by still making everything a logical type, but
>>>>>>>>>>>>>>> renaming
>>>>>>>>>>>>>>> >> >> "TypeName" to Representation. There would be URNs
>>>>>>>>>>>>>>> (typically with
>>>>>>>>>>>>>>> >> >> empty payloads) for the various primitive types (whose
>>>>>>>>>>>>>>> mapping to
>>>>>>>>>>>>>>> >> >> their representations would be the identity).
>>>>>>>>>>>>>>> >> >>
>>>>>>>>>>>>>>> >> >> - Robert
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
