[GitHub] [beam] robertwb commented on a diff in pull request #25941: Add typescript docs to beam site.

["robertwb (via GitHub)" <gi...@apache.org>]

robertwb commented on code in PR #25941:
URL: https://github.com/apache/beam/pull/25941#discussion_r1147820257


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website/www/site/content/en/documentation/sdks/typescript.md:
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+---
+type: languages
+title: "Apache Beam Typescript SDK"
+---
+<!--
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-->
+# Apache Beam Typescript SDK
+
+The Typescript SDK for Apache Beam provides a simple, powerful API for building batch and streaming data processing pipelines.
+
+## Get started with the Typescript SDK
+
+Get started with the [Beam Typescript SDK quickstart](/get-started/quickstart/typescript)
+to set up your development environment, get the Beam SDK for Typescript, and run an example pipeline.
+Then, read through the [Beam programming guide](/documentation/programming-guide)
+to learn the basic concepts that apply to all SDKs in Beam.
+
+## Overview
+
+We generally try to apply the concepts from the Beam API in a TypeScript
+idiomatic way.
+In addition, some notable departures are taken from the traditional SDKs:
+
+* We take a "relational foundations" approach, where
+[schema'd data](https://docs.google.com/document/d/1tnG2DPHZYbsomvihIpXruUmQ12pHGK0QIvXS1FOTgRc/edit#heading=h.puuotbien1gf)
+is the primary way to interact with data, and we generally eschew the key-value
+requiring transforms in favor of a more flexible approach naming fields or
+expressions.  For example, we favor the more flexible
+[GroupBy](https://beam.apache.org/documentation/programming-guide/#groupbykey)
+PTransform over the traditional GroupByKey.
+JavaScript's native Object is used as the row type.
+
+* As part of being schema-first we also de-emphasize Coders as a first-class
+concept in the SDK, relegating it to an advanced feature used for interop.
+Though we can infer schemas from individual elements, it is still TBD to
+figure out if/how we can leverage the type system and/or function introspection
+to regularly infer schemas at construction time. A fallback coder using BSON
+encoding is used when we don't have sufficient type information.
+
+* We have added additional methods to the PCollection object, notably `map`
+and `flatmap`, [rather than only allowing apply](https://www.mail-archive.com/dev@beam.apache.org/msg06035.html).
+In addition, `apply` can accept a function argument `(PCollection) => ...` as
+well as a PTransform subclass, which treats this callable as if it were a
+PTransform's expand.
+
+* In the other direction, we have eliminated the
+[problematic Pipeline object](https://s.apache.org/no-beam-pipeline)
+from the API, instead providing a `Root` PValue on which pipelines are built,
+and invoking run() on a Runner.  We offer a less error-prone `Runner.run`
+which finishes only when the pipeline is completely finished as well as
+`Runner.runAsync` which returns a handle to the running pipeline.
+
+* Rather than introduce PCollectionTuple, PCollectionList, etc. we let PValue
+literally be an
+[array or object with PValue values](https://github.com/robertwb/beam-javascript/blob/de4390dd767f046903ac23fead5db333290462db/sdks/node-ts/src/apache_beam/pvalue.ts#L116)
+which transforms can consume or produce.
+These are applied by wrapping them with the `P` operator, e.g.
+`P([pc1, pc2, pc3]).apply(new Flatten())`.
+
+* Like Python, `flatMap` and `ParDo.process` return multiple elements by
+yielding them from a generator, rather than invoking a passed-in callback.
+There is currently an operation to split a PCollection into multiple
+PCollections based on the properties of the elements, and
+we may consider using a callback for side outputs.
+
+* The `map`, `flatMap`, and `ParDo.process` methods take an additional
+(optional) context argument, which is similar to the keyword arguments
+used in Python. These are javascript objects whose members may be constants
+(which are passed as is) or special DoFnParam objects which provide getters to
+element-specific information (such as the current timestamp, window,
+or side input) at runtime.
+
+* Rather than introduce multiple-output complexity into the map/do operations
+themselves, producing multiple outputs is done by following with a new
+`Split` primitive that takes a
+`PCollection<{a?: AType, b: BType, ... }>` and produces an object
+`{a: PCollection<AType>, b: PCollection<BType>, ...}`.
+
+* JavaScript supports (and encourages) an asynchronous programing model, with
+many libraries requiring use of the async/await paradigm.
+As there is no way (by design) to go from the asynchronous style back to
+the synchronous style, this needs to be taken into account
+when designing the API.
+We currently offer asynchronous variants of `PValue.apply(...)` (in addition
+to the synchronous ones, as they are easier to chain) as well as making
+`Runner.run` asynchronous. TBD to do this for all user callbacks as well.
+
+An example pipeline can be found at [wordcount.ts](https://github.com/apache/beam/blob/master/sdks/typescript/src/apache_beam/examples/wordcount.ts)
+and more documentation can be found in the [beam programming guide](/documentation/programming-guide/).
+
+
+## Pipeline I/O
+
+See the [Beam-provided I/O Transforms](/documentation/io/built-in/) page for a list of the currently available I/O transforms.
+
+
+## Supported Features
+
+The Typescript SDK is still under development but already supports many,
+but not all, features currently supported by the Beam model, including those required by streaming.

Review Comment:
   It actually does handle all the streaming stuff except state and timers. Reworded. (A different runner is required for some of the features though.)



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