lukecwik commented on a change in pull request #13160:
URL: https://github.com/apache/beam/pull/13160#discussion_r510399030
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File path: website/www/site/content/en/documentation/programming-guide.md
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@@ -5143,3 +5143,282 @@ perUser.apply(ParDo.of(new DoFn<KV<String, ValueT>,
OutputT>() {
}
}));
{{< /highlight >}}
+
+## 12 Splittable DoFns {#splittable-dofns}
+
+Splittable DoFns (SDFs) enable users to create modular components containing
I/Os (and some advanced
+[non I/O use
cases](https://s.apache.org/splittable-do-fn#heading=h.5cep9s8k4fxv)). Having
modular
+I/O components that can be connected to each other simplify typical patterns
that users want.
+For example, a popular use case is to read filenames from a message queue
followed by parsing those
+files. Traditionally users were required to either write a single I/O
connector that contained the
+logic for the message queue and the file reader (increased complexity) or
choose to reuse a message
+queue I/O followed by a regular DoFn that read the file (decreased
performance). With splittable DoFns,
+we bring the richness of Apache Beam’s I/O APIs to DoFns enabling modularity
while maintaining the
+performance of traditional I/O connectors.
+
+### 12.1 Splittable DoFn basics {#splittable-dofn-basics}
+
+At a high level, a splittable DoFn is responsible for processing element and
restriction pairs. A
+restriction represents a subset of work that would have been necessary to have
been done when
+processing the element.
+
+Executing a splittable DoFn follows the following steps:
+
+1. Each element is paired with a restriction (e.g. filename is paired with
offset range representing the whole file).
+2. Each element and restriction pair is split (e.g. offset ranges are broken
up into smaller pieces).
+3. The runner redistributes the element and restriction pairs to several
workers.
+4. Element and restriction pairs are processed in parallel (e.g. the file is
read).
+
+
+
+Within the last step, the element and restriction pair can pause its own
processing and/or be split into
+further element and restriction pairs. This last step is what enables I/O-like
capabilities for DoFns.
+
+
+#### 12.1.1 A basic splittable DoFn {#a-basic-splittable-dofn}
+
+A basic splittable DoFn is composed of three parts: a restriction, a
restriction provider, and a
+restriction tracker. The restriction is used to represent a subset of work for
a given element.
+The restriction provider lets SDF authors override default implementations for
splitting, sizing,
+watermark estimation, and so forth. In
[Java](https://github.com/apache/beam/blob/f4c2734261396858e388ebef2eef50e7d48231a8/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/DoFn.java#L92)
+and
[Go](https://github.com/apache/beam/blob/0f466e6bcd4ac8677c2bd9ecc8e6af3836b7f3b8/sdks/go/pkg/beam/pardo.go#L226),
+this is the DoFn.
[Python](https://github.com/apache/beam/blob/f4c2734261396858e388ebef2eef50e7d48231a8/sdks/python/apache_beam/transforms/core.py#L213)
+has a dedicated RestrictionProvider type. The restriction tracker is
responsible for tracking
+what subset of the restriction has been completed during processing.
+
+To define a splittable DoFn, you must choose whether the splittable DoFn is
bounded (default) or
+unbounded and define a way to initialize an initial restriction for an element.
+
+{{< highlight java >}}
+{{< code_sample
"examples/java/src/main/java/org/apache/beam/examples/snippets/Snippets.java"
SDF_BasicExample >}}
+{{< /highlight >}}
+
+{{< highlight py >}}
+{{< code_sample "sdks/python/apache_beam/examples/snippets/snippets.py"
SDF_BasicExample >}}
+{{< /highlight >}}
+
+{{< highlight go >}}
+func (fn *splittableDoFn) CreateInitialRestriction(filename string)
offsetrange.Restriction {
+ return offsetrange.Restriction{
+ Start: 0,
+ End: getFileLength(filename),
+ }
+}
+
+func (fn *splittableDoFn) CreateTracker(rest offsetrange.Restriction)
*sdf.LockRTracker {
+ return sdf.NewLockRTracker(offsetrange.NewTracker(rest))
+}
+
+func (fn *splittableDoFn) ProcessElement(rt *sdf.LockRTracker, filename
string, emit func(int)) error {
+ file, err := os.Open(filename)
+ if err != nil {
+ return err
+ }
+ offset, err := seekToNextRecordBoundaryInFile(file,
rt.GetRestriction().(offsetrange.Restriction).Start)
+
+ if err != nil {
+ return err
+ }
+ for rt.TryClaim(offset) {
+ record, newOffset := readNextRecord(file)
+ emit(record)
+ offset = newOffset
+ }
+ return nil
+}
+{{< /highlight >}}
+
+At this point, we have a splittable DoFn that supports [runner-initiated
splits](#runner-initiated-split)
Review comment:
I think we should deal with this in the follow-up about trySplit being
implemented correctly within a restriction tracker and adding that to the
runner initiated splits section then lumping it in now.
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