---
title:  "Powerful and modular IO connectors with Splittable DoFn in Apache Beam"
date:   2017-08-16 00:00:01 -0800
categories:
  - blog
aliases:
  - /blog/2017/08/16/splittable-do-fn.html
authors:
  - jkff
---
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One of the most important parts of the Apache Beam ecosystem is its quickly
growing set of connectors that allow Beam pipelines to read and write data to
various data storage systems ("IOs"). Currently, Beam ships [over 20 IO
connectors](/documentation/io/built-in/) with many more in
active development. As user demands for IO connectors grew, our work on
improving the related Beam APIs (in particular, the Source API) produced an
unexpected result: a generalization of Beam's most basic primitive, `DoFn`.

<!--more-->

## Connectors as mini-pipelines

One of the main reasons for this vibrant IO connector ecosystem is that
developing a basic IO is relatively straightforward: many connector
implementations are simply mini-pipelines (composite `PTransform`s) made of the
basic Beam `ParDo` and `GroupByKey` primitives. For example,
`ElasticsearchIO.write()`
[expands](https://github.com/apache/beam/blob/f7e8f886c91ea9d0b51e00331eeb4484e2f6e000/sdks/java/io/elasticsearch/src/main/java/org/apache/beam/sdk/io/elasticsearch/ElasticsearchIO.java#L783)
into a single `ParDo` with some batching for performance; `JdbcIO.read()`
[expands](https://github.com/apache/beam/blob/f7e8f886c91ea9d0b51e00331eeb4484e2f6e000/sdks/java/io/jdbc/src/main/java/org/apache/beam/sdk/io/jdbc/JdbcIO.java#L329)
into `Create.of(query)`, a reshuffle to [prevent
fusion](https://cloud.google.com/dataflow/service/dataflow-service-desc#preventing-fusion),
and `ParDo(execute sub-query)`.  Some IOs
[construct](https://github.com/apache/beam/blob/8503adbbc3a590cd0dc2939f6a45d335682a9442/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/BigQueryIO.java#L1139)
considerably more complicated pipelines.

<img class="center-block"
    src="/images/blog/splittable-do-fn/jdbcio-expansion.png"
    alt="Expansion of the JdbcIO.read() composite transform"
    width="600">

This "mini-pipeline" approach is flexible, modular, and generalizes to data
sources that read from a dynamically computed `PCollection` of locations, such
as
[`SpannerIO.readAll()`](https://github.com/apache/beam/blob/f7e8f886c91ea9d0b51e00331eeb4484e2f6e000/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/spanner/SpannerIO.java#L222)
which reads the results of a `PCollection` of queries from Cloud Spanner,
compared to
[`SpannerIO.read()`](https://github.com/apache/beam/blob/f7e8f886c91ea9d0b51e00331eeb4484e2f6e000/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/spanner/SpannerIO.java#L318)
which executes a single query. We believe such dynamic data sources are a very
useful capability, often overlooked by other data processing frameworks.

## When ParDo and GroupByKey are not enough

Despite the flexibility of `ParDo`, `GroupByKey` and their derivatives, in some
cases building an efficient IO connector requires extra capabilities.

For example, imagine reading files using the sequence `ParDo(filepattern →
expand into files)`, `ParDo(filename → read records)`, or reading a Kafka topic
using `ParDo(topic → list partitions)`, `ParDo(topic, partition → read
records)`. This approach has two big issues:

* In the file example, some files might be much larger than others, so the
second `ParDo` may have very long individual `@ProcessElement` calls. As a
result, the pipeline can suffer from poor performance due to stragglers.

* In the Kafka example, implementing the second `ParDo` is *simply impossible*
with a regular `DoFn`, because it would need to output an infinite number of
records per each input element `topic, partition` *([stateful processing](/blog/2017/02/13/stateful-processing.html) comes close, but it
has other limitations that make it insufficient for this task*).

## Beam Source API

Apache Beam historically provides a Source API
([BoundedSource](https://beam.apache.org/releases/javadoc/{{< param release_latest >}}/org/apache/beam/sdk/io/BoundedSource.html)
and
[UnboundedSource](https://beam.apache.org/releases/javadoc/{{< param release_latest >}}/org/apache/beam/sdk/io/UnboundedSource.html)) which does
not have these limitations and allows development of efficient data sources for
batch and streaming systems. Pipelines use this API via the
[`Read.from(Source)`](https://beam.apache.org/releases/javadoc/{{< param release_latest >}}/org/apache/beam/sdk/io/Read.html) built-in `PTransform`.

The Source API is largely similar to that of most other data processing
frameworks, and allows the system to read data in parallel using multiple
workers, as well as checkpoint and resume reading from an unbounded data source.
Additionally, the Beam
[`BoundedSource`](https://beam.apache.org/releases/javadoc/{{< param release_latest >}}/org/apache/beam/sdk/io/BoundedSource.html)
API provides advanced features such as progress reporting and [dynamic
rebalancing](/blog/2016/05/18/splitAtFraction-method.html)
(which together enable autoscaling), and
[`UnboundedSource`](https://beam.apache.org/releases/javadoc/{{< param release_latest >}}/org/apache/beam/sdk/io/UnboundedSource.html) supports
reporting the source's watermark and backlog *(until SDF, we believed that
"batch" and "streaming" data sources are fundamentally different and thus
require fundamentally different APIs)*. 

Unfortunately, these features come at a price. Coding against the Source API
involves a lot of boilerplate and is error-prone, and it does not compose well
with the rest of the Beam model because a `Source` can appear only at the root
of a pipeline. For example:

* Using the Source API, it is not possible to read a `PCollection` of
filepatterns.

* A `Source` can not read a side input, or wait on another pipeline step to
produce the data.

* A `Source` can not emit an additional output (for example, records that failed to
parse) and so on.

The Source API is not composable even with itself. For example, suppose Alice
implements an unbounded `Source` that watches a directory for new matching
files, and Bob implements an unbounded `Source` that tails a file. The Source
API does not let them simply chain the sources together and obtain a `Source`
that returns new records in new log files in a directory (a very common user
request). Instead, such a source would have to be developed mostly from
scratch, and our experience shows that a full-featured monolithic
implementation of such a `Source` is incredibly difficult and error-prone.

Another class of issues with the `Source` API comes from its strict
bounded/unbounded dichotomy:

* It is difficult or impossible to reuse code between seemingly very similar
bounded and unbounded sources, for example, the `BoundedSource` that generates
a sequence `[a, b)` and the `UnboundedSource` that generates a sequence `[a,
inf)` [don't share any
code](https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/io/CountingSource.java)
in the Beam Java SDK.

* It is not clear how to classify the ingestion of a very large and
continuously growing dataset. Ingesting its "already available" part seems to
require a `BoundedSource`: the runner could benefit from knowing its size, and
could perform dynamic rebalancing. However, ingesting the continuously arriving
new data seems to require an `UnboundedSource` for providing watermarks. From
this angle, the `Source` API has [the same issues as Lambda
Architecture](https://www.oreilly.com/ideas/the-world-beyond-batch-streaming-101).

About two years ago we began thinking about how to address the limitations of
the Source API, and ended up, surprisingly, addressing the limitations of
`DoFn` instead.

## Enter Splittable DoFn

[Splittable DoFn](https://s.apache.org/splittable-do-fn) (SDF) is a
generalization of `DoFn` that gives it the core capabilities of `Source` while
retaining `DoFn`'s syntax, flexibility, modularity, and ease of coding.  As a
result, it becomes possible to develop more powerful IO connectors than before,
with shorter, simpler, more reusable code.

Note that, unlike `Source`, SDF *does not* have distinct bounded/unbounded APIs,
just as regular `DoFn`s don't: there is only one API, which covers both of these
use cases and anything in between. Thus, SDF closes the final gap in the unified
batch/streaming programming model of Apache Beam.

When reading the explanation of SDF below, keep in mind the running example of a
`DoFn` that takes a filename as input and outputs the records in that file.
People familiar with the `Source` API may find it useful to think of SDF as a
way to read a `PCollection` of sources, treating the source itself as just
another piece of data in the pipeline *(this, in fact, was one of the early
design iterations among the work that led to creation of SDF)*.

The two aspects where `Source` has an advantage over a regular `DoFn` are:

* **Splittability:** applying a `DoFn` to a single element is *monolithic*, but
reading from a `Source` is *non-monolithic*. The whole `Source` doesn't have to
be read at once; rather, it is read in parts, called *bundles*. For example, a
large file is usually read in several bundles, each reading some sub-range of
offsets within the file. Likewise, a Kafka topic (which, of course, can never
be read "fully") is read over an infinite number of bundles, each reading some
finite number of elements.

* **Interaction with the runner:** runners apply a `DoFn` to a single element as
a "black box", but interact quite richly with `Source`. `Source` provides the
runner with information such as its estimated size (or its generalization,
"backlog"), progress through reading the bundle, watermarks etc. The runner
uses this information to tune the execution and control the breakdown of the
`Source` into bundles. For example, a slowly progressing large bundle of a file
may be [dynamically
split](https://cloud.google.com/blog/big-data/2016/05/no-shard-left-behind-dynamic-work-rebalancing-in-google-cloud-dataflow)
by a batch-focused runner before it becomes a straggler, and a latency-focused
streaming runner may control how many elements it reads from a source in each
bundle to optimize for latency vs. per-bundle overhead.

### Non-monolithic element processing with restrictions

Splittable `DoFn` supports `Source`-like features by allowing the processing of
a single element to be non-monolithic.

The processing of one element by an SDF is decomposed into a (potentially
infinite) number of *restrictions*, each describing some part of the work to be
done for the whole element. The input to an SDF's `@ProcessElement` call is a
pair of an element and a restriction (compared to a regular `DoFn`, which takes
just the element).

Processing of every element starts by creating an *initial restriction* that
describes the entire work, and the initial restriction is then split further
into sub-restrictions which must logically add up to the original. For example,
for a splittable `DoFn` called `ReadFn` that takes a filename and outputs
records in the file, the restriction may be a pair of starting and ending byte
offset, and `ReadFn` may interpret it as *read records whose starting offsets
are in the given range*.

<img class="center-block"
    src="/images/blog/splittable-do-fn/restrictions.png"
    alt="Specifying parts of work for an element using restrictions"
    width="600">

The idea of restrictions provides non-monolithic execution - the first
ingredient for parity with `Source`. The other ingredient is *interaction with
the runner*: the runner has access to the restriction of each active
`@ProcessElement` call of an SDF, can inquire about the progress of the call,
and most importantly, can *split* the restriction while it is being processed
(hence the name *Splittable DoFn*).

Splitting produces a *primary* and *residual* restriction that add up to the
original restriction being split: the current `@ProcessElement` call keeps
processing the primary, and the residual will be processed by another
`@ProcessElement` call. For example, a runner may schedule the residual to be
processed in parallel on another worker.

Splitting of a running `@ProcessElement` call has two critically important uses:

* **Supporting infinite work per element.** A restriction is, in general, not
required to describe a finite amount of work. For example, reading from a Kafka
topic starting from offset *100* can be represented by the
restriction *[100, inf)*. A `@ProcessElement` call processing this
entire restriction would, of course, never complete. However, while such a call
runs, a runner can split the restriction into a *finite* primary *[100, 150)*
(letting the current call complete this part) and an *infinite* residual *[150,
inf)* to be processed later, effectively checkpointing and resuming the call;
this can be repeated forever.

<img class="center-block" 
    src="/images/blog/splittable-do-fn/kafka-splitting.png" 
    alt="Splitting an infinite restriction into a finite primary and infinite residual"
    width="400">

* **Dynamic rebalancing.** When a (typically batch-focused) runner detects that
a `@ProcessElement` call is going to take too long and become a straggler, it
can split the restriction in some proportion so that the primary is short enough
to not be a straggler, and can schedule the residual in parallel on another
worker. For details, see [No Shard Left
Behind](https://cloud.google.com/blog/big-data/2016/05/no-shard-left-behind-dynamic-work-rebalancing-in-google-cloud-dataflow).

Logically, the execution of an SDF on an element works according to the
following diagram, where "magic" stands for the runner-specific ability to split
the restrictions and schedule processing of residuals.

<img class="center-block" 
    src="/images/blog/splittable-do-fn/transform-expansion.png" 
    alt="Execution of an SDF - pairing with a restriction, splitting
    restrictions, processing element/restriction pairs"
    width="600">

This diagram emphasizes that splittability is an implementation detail of the
particular `DoFn`: a splittable `DoFn` still looks like a `DoFn<A, B>` to its
user, and can be applied via a `ParDo` to a `PCollection<A>` producing a
`PCollection<B>`.

### Which DoFns need to be splittable

Note that decomposition of an element into element/restriction pairs is not
automatic or "magical": SDF is a new API for *authoring* a `DoFn`, rather than a
new way to *execute* an existing `DoFn`. When making a `DoFn` splittable, the
author needs to:

* Consider the structure of the work it does for every element.

* Come up with a scheme for describing parts of this work using restrictions.

* Write code for creating the initial restriction, splitting it, and executing
an element/restriction pair.

An overwhelming majority of `DoFn`s found in user pipelines do not need to be
made splittable: SDF is an advanced, powerful API, primarily targeting authors
of new IO connectors *(though it has interesting non-IO applications as well:
see [Non-IO examples](https://s.apache.org/splittable-do-fn#heading=h.5cep9s8k4fxv))*.

### Execution of a restriction and data consistency

One of the most important parts of the Splittable `DoFn` design is related to
how it achieves data consistency while splitting. For example, while the runner
is preparing to split the restriction of an active `@ProcessElement` call, how
can it be sure that the call has not concurrently progressed past the point of
splitting?

This is achieved by requiring the processing of a restriction to follow a
certain pattern. We think of a restriction as a sequence of *blocks* -
elementary indivisible units of work, identified by a *position*. A
`@ProcessElement` call processes the blocks one by one, first *claiming* the
block's position to atomically check if it's still within the range of the
restriction, until the whole restriction is processed.

The diagram below illustrates this for `ReadFn` (a splittable `DoFn` that reads
Avro files) processing the element `foo.avro` with restriction `[30, 70)`. This
`@ProcessElement` call scans the Avro file for [data
blocks](https://avro.apache.org/docs/current/spec.html#Object+Container+Files)
starting from offset `30` and claims the position of each block in this range.
If a block is claimed successfully, then the call outputs all records in this
data block, otherwise, it terminates.

<img class="center-block"
    src="/images/blog/splittable-do-fn/blocks.png"
    alt="Processing a restriction by claiming blocks inside it"
    width="400">

For more details, see [Restrictions, blocks and positions](https://s.apache.org/splittable-do-fn#heading=h.vjs7pzbb7kw) in the
design proposal document.

### Code example

Let us look at some examples of SDF code. The examples use the Beam Java SDK,
which [represents splittable
`DoFn`s](https://github.com/apache/beam/blob/f7e8f886c91ea9d0b51e00331eeb4484e2f6e000/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/DoFn.java#L527)
as part of the flexible [annotation-based
`DoFn`](https://s.apache.org/a-new-dofn) machinery, and the [proposed SDF syntax
for Python](https://s.apache.org/splittable-do-fn-python).

* A splittable `DoFn` is a `DoFn` - no new base class needed. Any SDF derives
from the `DoFn` class and has a `@ProcessElement` method.

* The `@ProcessElement` method takes an additional
[`RestrictionTracker`](https://github.com/apache/beam/blob/f7e8f886c91ea9d0b51e00331eeb4484e2f6e000/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/splittabledofn/RestrictionTracker.java)
parameter that gives access to the current restriction in addition to the
current element.

* An SDF needs to define a `@GetInitialRestriction` method that can create a
restriction describing the complete work for a given element.

* There are several less important optional methods, such as
`@SplitRestriction` for pre-splitting the initial restriction into several
smaller restrictions, and a few others.

The "Hello World" of SDF is a counter, which takes pairs *(x, N)* as input and
produces pairs *(x, 0), (x, 1), …, (x, N-1)* as output.

{{< highlight java >}}
class CountFn<T> extends DoFn<KV<T, Long>, KV<T, Long>> {
  @ProcessElement
  public void process(ProcessContext c, OffsetRangeTracker tracker) {
    for (long i = tracker.currentRestriction().getFrom(); tracker.tryClaim(i); ++i) {
      c.output(KV.of(c.element().getKey(), i));
    }
  }

  @GetInitialRestriction
  public OffsetRange getInitialRange(KV<T, Long> element) {
    return new OffsetRange(0L, element.getValue());
  }
}

PCollection<KV<String, Long>> input = …;
PCollection<KV<String, Long>> output = input.apply(
    ParDo.of(new CountFn<String>());
{{< /highlight >}}

{{< highlight py >}}
class CountFn(DoFn):
  def process(element, tracker=DoFn.RestrictionTrackerParam)
    for i in xrange(*tracker.current_restriction()):
      if not tracker.try_claim(i):
        return
      yield element[0], i
        
  def get_initial_restriction(element):
    return (0, element[1])
{{< /highlight >}}

This short `DoFn` subsumes the functionality of
[CountingSource](https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/io/CountingSource.java),
but is more flexible: `CountingSource` generates only one sequence specified at
pipeline construction time, while this `DoFn` can generate a dynamic family of
sequences, one per element in the input collection (it does not matter whether
the input collection is bounded or unbounded).

However, the `Source`-specific capabilities of `CountingSource` are still
available in `CountFn`. For example, if a sequence has a lot of elements, a
batch-focused runner can still apply dynamic rebalancing to it and generate
different subranges of the sequence in parallel by splitting the `OffsetRange`.
Likewise, a streaming-focused runner can use the same splitting logic to
checkpoint and resume the generation of the sequence even if it is, for
practical purposes, infinite (for example, when applied to a `KV(...,
Long.MAX_VALUE)`).

A slightly more complex example is the `ReadFn` considered above, which reads
data from Avro files and illustrates the idea of *blocks*: we provide pseudocode
to illustrate the approach.

{{< highlight java >}}
class ReadFn extends DoFn<String, AvroRecord> {
  @ProcessElement
  void process(ProcessContext c, OffsetRangeTracker tracker) {
    try (AvroReader reader = Avro.open(filename)) {
      // Seek to the first block starting at or after the start offset.
      reader.seek(tracker.currentRestriction().getFrom());
      while (reader.readNextBlock()) {
        // Claim the position of the current Avro block
        if (!tracker.tryClaim(reader.currentBlockOffset())) {
          // Out of range of the current restriction - we're done.
          return;
        }
        // Emit all records in this block
        for (AvroRecord record : reader.currentBlock()) {
          c.output(record);
        }
      }
    }
  }

  @GetInitialRestriction
  OffsetRange getInitialRestriction(String filename) {
    return new OffsetRange(0, new File(filename).getSize());
  }
}
{{< /highlight >}}

{{< highlight py >}}
class AvroReader(DoFn):
  def process(filename, tracker=DoFn.RestrictionTrackerParam)
    with fileio.ChannelFactory.open(filename) as file:
      start, stop = tracker.current_restriction()
      # Seek to the first block starting at or after the start offset.
      file.seek(start)
      block = AvroUtils.get_next_block(file)
      while block:
        # Claim the position of the current Avro block
        if not tracker.try_claim(block.start()):
          # Out of range of the current restriction - we're done.
          return
        # Emit all records in this block
        for record in block.records():
          yield record
        block = AvroUtils.get_next_block(file)
        
  def get_initial_restriction(self, filename):
    return (0, fileio.ChannelFactory.size_in_bytes(filename))
{{< /highlight >}}

This hypothetical `DoFn` reads records from a single Avro file. Notably missing
is the code for expanding a filepattern: it no longer needs to be part of this
`DoFn`! Instead, the SDK includes a
[FileIO.matchAll()](https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/io/FileIO.java)
transform for expanding a filepattern into a `PCollection` of filenames, and
different file format IOs can reuse the same transform, reading the files with
different `DoFn`s.

This example demonstrates the benefits of increased modularity allowed by SDF:
`FileIO.matchAll()` supports continuous ingestion of new files in streaming
pipelines using `.continuously()`, and this functionality becomes automatically
available to various file format IOs. For example,
`TextIO.read().watchForNewFiles()` [uses `FileIO.matchAll()` under the
hood)](https://github.com/apache/beam/blob/3bd68ecfd7d576d78e02deb0476e549f11e1b5ef/sdks/java/core/src/main/java/org/apache/beam/sdk/io/TextIO.java#L486).

## Current status

Splittable `DoFn` is a major new API, and its delivery and widespread adoption
involves a lot of work in different parts of the Apache Beam ecosystem.  Some
of that work is already complete and provides direct benefit to users via new
IO connectors. However, a large amount of work is in progress or planned.

As of August 2017, SDF is available for use in the Beam Java Direct runner and
Dataflow Streaming runner, and implementation is in progress in the Flink and
Apex runners; see [capability matrix](/documentation/runners/capability-matrix/) for the current status. Support
for SDF in the Python SDK is [in active
development](https://s.apache.org/splittable-do-fn-python).

Several SDF-based transforms and IO connectors are available for Beam users at
HEAD and will be included in Beam 2.2.0. `TextIO` and `AvroIO` finally provide
continuous ingestion of files (one of the most frequently requested features)
via `.watchForNewFiles()` which is backed by the utility transforms
`FileIO.matchAll().continuously()` and the more general
[`Watch.growthOf()`](https://github.com/apache/beam/blob/f7e8f886c91ea9d0b51e00331eeb4484e2f6e000/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/Watch.java).
These utility transforms are also independently useful for "power user" use
cases.

To enable more flexible use cases for IOs currently based on the Source API, we
will change them to use SDF. This transition is [pioneered by
TextIO](https://s.apache.org/textio-sdf) and involves temporarily [executing SDF
via the Source API](https://s.apache.org/sdf-via-source) to support runners
lacking the ability to run SDF directly.

In addition to enabling new IOs, work on SDF has influenced our thinking about
other parts of the Beam programming model:

* SDF unified the final remaining part of the Beam programming model that was
not batch/streaming agnostic (the `Source` API). This led us to consider use
cases that cannot be described as purely batch or streaming (for example,
ingesting a large amount of historical data and carrying on with more data
arriving in real time) and to develop a [unified notion of "progress" and
"backlog"](https://s.apache.org/beam-fn-api-progress-reporting).

* The [Fn API](https://s.apache.org/beam-fn-api) - the foundation of Beam's
future support for cross-language pipelines - uses SDF as *the only* concept
representing data ingestion.

* Implementation of SDF has lead to [formalizing pipeline termination
semantics](https://lists.apache.org/thread.html/86831496a08fe148e3b982cdb904f828f262c0b571543a9fed7b915d@%3Cdev.beam.apache.org%3E)
and making it consistent between runners.

* SDF set a new standard for how modular IO connectors can be, inspiring
creation of similar APIs for some non-SDF-based connectors (for example,
`SpannerIO.readAll()` and the
[planned](https://issues.apache.org/jira/browse/BEAM-2706) `JdbcIO.readAll()`).

## Call to action

Apache Beam thrives on having a large community of contributors. Here are some
ways you can get involved in the SDF effort and help make the Beam IO connector
ecosystem more modular:

* Use the currently available SDF-based IO connectors, provide feedback, file
bugs, and suggest or implement improvements.

* Propose or develop a new IO connector based on SDF.

* Implement or improve support for SDF in your favorite runner.

* Subscribe and contribute to the occasional SDF-related discussions on
[user@beam.apache.org](mailto:user@beam.apache.org) (mailing list for Beam
users) and [dev@beam.apache.org](mailto:dev@beam.apache.org) (mailing list for
Beam developers)!
