---
layout: post
title:  "Timely (and Stateful) Processing with Apache Beam"
date:   2017-08-28 00:00:01 -0800
excerpt_separator: <!--more-->
categories: blog
authors:
  - klk
---
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In a [prior blog
post]({{ site.baseurl }}/blog/2017/02/13/stateful-processing.html), I
introduced the basics of stateful processing in Apache Beam, focusing on the
addition of state to per-element processing. So-called _timely_ processing
complements stateful processing in Beam by letting you set timers to request a
(stateful) callback at some point in the future.

What can you do with timers in Beam? Here are some examples:

 - You can output data buffered in state after some amount of processing time.
 - You can take special action when the watermark estimates that you have
   received all data up to a specified point in event time.
 - You can author workflows with timeouts that alter state and emit output in
   response to the absence of additional input for some period of time.

These are just a few possibilities. State and timers together form a powerful
programming paradigm for fine-grained control to express a huge variety of
workflows.  Stateful and timely processing in Beam is portable across data
processing engines and integrated with Beam's unified model of event time
windowing in both streaming and batch processing.

<!--more-->

## What is stateful and timely processing?

In my prior post, I developed an understanding of stateful processing largely
by contrast with associative, commutative combiners. In this post, I'll
emphasize a perspective that I had mentioned only briefly: that elementwise
processing with access to per-key-and-window state and timers represents a
fundamental pattern for "embarrassingly parallel" computation, distinct from
the others in Beam.

In fact, stateful and timely computation is the low-level computational pattern
that underlies the others. Precisely because it is lower level, it allows you
to really micromanage your computations to unlock new use cases and new
efficiencies. This incurs the complexity of manually managing your state and
timers - it isn't magic! Let's first look again at the two primary
computational patterns in Beam.

### Element-wise processing (ParDo, Map, etc)

The most elementary embarrassingly parallel pattern is just using a bunch of
computers to apply the same function to every input element of a massive
collection. In Beam, per-element processing like this is expressed as a basic
`ParDo` - analogous to "Map" from MapReduce - which is like an enhanced "map",
"flatMap", etc, from functional programming.

The following diagram illustrates per-element processing. Input elements are
squares, output elements are triangles. The colors of the elements represent
their key, which will matter later. Each input element maps to the
corresponding output element(s) completely independently. Processing may be
distributed across computers in any way, yielding essentially limitless
parallelism.

<img class="center-block"
    src="{{ site.baseurl }}/images/blog/timely-processing/ParDo.png"
    alt="ParDo offers limitless parallelism"
    width="600">

This pattern is obvious, exists in all data-parallel paradigms, and has
a simple stateless implementation. Every input element can be processed
independently or in arbitrary bundles. Balancing the work between computers is
actually the hard part, and can be addressed by splitting, progress estimation,
work-stealing, etc.

### Per-key (and window) aggregation (Combine, Reduce, GroupByKey, etc.)

The other embarassingly parallel design pattern at the heart of Beam is per-key
(and window) aggregation. Elements sharing a key are colocated and then
combined using some associative and commutative operator. In Beam this is
expressed as a `GroupByKey` or `Combine.perKey`, and corresponds to the shuffle
and "Reduce" from MapReduce.  It is sometimes helpful to think of per-key
`Combine` as the fundamental operation, and raw `GroupByKey` as a combiner that
just concatenates input elements. The communication pattern for the input
elements is the same, modulo some optimizations possible for `Combine`.

In the illustration here, recall that the color of each element represents the
key. So all of the red squares are routed to the same location where they are
aggregated and the red triangle is the output.  Likewise for the yellow and
green squares, etc. In a real application, you may have millions of keys, so
the parallelism is still massive.

<img class="center-block"
    src="{{ site.baseurl }}/images/blog/timely-processing/CombinePerKey.png"
    alt="Gathering elements per key then combining them"
    width="600">

The underlying data processing engine will, at some level of abstraction, use
state to perform this aggregation across all the elements arriving for a key.
In particular, in a streaming execution, the aggregation process may need to
wait for more data to arrive or for the watermark to estimate that all input
for an event time window is complete. This requires some way to store the
intermediate aggregation between input elements as well a way to a receive a
callback when it is time to emit the result. As a result, the _execution_ of
per key aggregation by a stream processing engine fundamentally involves state
and timers.

However, _your_ code is just a declarative expression of the aggregation
operator.  The runner can choose a variety of ways to execute your operator. 
I went over this in detail in [my prior post focused on state alone]({{
site.baseurl }}/blog/2017/02/13/stateful-processing.html). Since you do not
observe elements in any defined order, nor manipulate mutable state or timers
directly, I call this neither stateful nor timely processing.

### Per-key-and-window stateful, timely processing

Both `ParDo` and `Combine.perKey` are standard patterns for parallelism that go
back decades. When implementing these in a massive-scale distributed data
processing engine, we can highlight a few characteristics that are particularly
important.

Let us consider these characteristics of `ParDo`:

 - You write single-threaded code to process one element.
 - Elements are processed in arbitrary order with no dependencies
   or interaction between processing of elements.

And these characteristics for `Combine.perKey`:

 - Elements for a common key and window are gathered together.
 - A user-defined operator is applied to those elements.

Combining some of the characteristics of unrestricted parallel mapping and
per-key-and-window combination, we can discern a megaprimitive from which we
build stateful and timely processing:

 - Elements for a common key and window are gathered together.
 - Elements are processed in arbitrary order.
 - You write single-threaded code to process one element or timer, possibly
   accessing state or setting timers.

In the illustration below, the red squares are gathered and fed one by one to
the stateful, timely, `DoFn`. As each element is processed, the `DoFn` has
access to state (the color-partitioned cylinder on the right) and can set
timers to receive callbacks (the colorful clocks on the left).

<img class="center-block"
    src="{{ site.baseurl }}/images/blog/timely-processing/StateAndTimers.png"
    alt="Gathering elements per key then timely, stateful processing"
    width="600">

So that is the abstract notion of per-key-and-window stateful, timely
processing in Apache Beam. Now let's see what it looks like to write code that
accesses state, sets timers, and receives callbacks.

## Example: Batched RPC

To demonstrate stateful and timely processing, let's work through a concrete
example, with code.

Suppose you are writing a system to analyze events.  You have a ton of data
coming in and you need to enrich each event by RPC to an external system. You
can't just issue an RPC per event.  Not only would this be terrible for
performance, but it would also likely blow your quota with the external system.
So you'd like to gather a number of events, make one RPC for them all, and then
output all the enriched events.

### State

Let's set up the state we need to track batches of elements. As each element
comes in, we will write the element to a buffer while tracking the number of
elements we have buffered. Here are the state cells in code:

```java
new DoFn<Event, EnrichedEvent>() {

  @StateId("buffer")
  private final StateSpec<BagState<Event>> bufferedEvents = StateSpecs.bag();

  @StateId("count")
  private final StateSpec<ValueState<Integer>> countState = StateSpecs.value();

  … TBD … 
}
```

```py
# State and timers are not yet supported in Beam's Python SDK.
# Follow https://issues.apache.org/jira/browse/BEAM-2687 for updates.
```

Walking through the code, we have:

 - The state cell `"buffer"` is an unordered bag of buffered events.
 - The state cell `"count"` tracks how many events have been buffered.

Next, as a recap of reading and writing state, let's write our `@ProcessElement`
method. We will choose a limit on the size of the buffer, `MAX_BUFFER_SIZE`. If
our buffer reaches this size, we will perform a single RPC to enrich all the
events, and output.

```java
new DoFn<Event, EnrichedEvent>() {

  private static final int MAX_BUFFER_SIZE = 500;

  @StateId("buffer")
  private final StateSpec<BagState<Event>> bufferedEvents = StateSpecs.bag();

  @StateId("count")
  private final StateSpec<ValueState<Integer>> countState = StateSpecs.value();

  @ProcessElement
  public void process(
      ProcessContext context,
      @StateId("buffer") BagState<Event> bufferState,
      @StateId("count") ValueState<Integer> countState) {

    int count = firstNonNull(countState.read(), 0);
    count = count + 1;
    countState.write(count);
    bufferState.add(context.element());

    if (count > MAX_BUFFER_SIZE) {
      for (EnrichedEvent enrichedEvent : enrichEvents(bufferState.read())) {
        context.output(enrichedEvent);
      }
      bufferState.clear();
      countState.clear();
    }
  }

  … TBD … 
}
```

```py
# State and timers are not yet supported in Beam's Python SDK.
# Follow https://issues.apache.org/jira/browse/BEAM-2687 for updates.
```

Here is an illustration to accompany the code:

<img class="center-block"
    src="{{ site.baseurl }}/images/blog/timely-processing/BatchedRpcState.png"
    alt="Batching elements in state, then performing RPCs"
    width="600">

 - The blue box is the `DoFn`.
 - The yellow box within it is the `@ProcessElement` method.
 - Each input event is a red square - this diagram just shows the activity for
   a single key, represented by the color red. Your `DoFn` will run the same
   workflow in parallel for all keys which are perhaps user IDs.
 - Each input event is written to the buffer as a red triangle, representing
   the fact that you might actually buffer more than just the raw input, even
   though this code doesn't.
 - The external service is drawn as a cloud. When there are enough buffered
   events, the `@ProcessElement` method reads the events from state and issues
   a single RPC.
 - Each output enriched event is drawn as a red circle. To consumers of this
   output, it looks just like an element-wise operation.

So far, we have only used state, but not timers. You may have noticed that
there is a problem - there will usually be data left in the buffer. If no more
input arrives, that data will never be processed. In Beam, every window has
some point in event time when any further input for the window is considered
too late and is discarded. At this point, we say that the window has "expired".
Since no further input can arrive to access the state for that window, the
state is also discarded. For our example, we need to ensure that all leftover
events are output when the window expires. 

### Event Time Timers

An event time timer requests a call back when the watermark for an input
`PCollection` reaches some threshold. In other words, you can use an event time
timer to take action at a specific moment in event time - a particular point of
completeness for a `PCollection` - such as when a window expires.

For our example, let us add an event time timer so that when the window expires,
any events remaining in the buffer are processed.

```java
new DoFn<Event, EnrichedEvent>() {
  …

  @TimerId("expiry")
  private final TimerSpec expirySpec = TimerSpecs.timer(TimeDomain.EVENT_TIME);

  @ProcessElement
  public void process(
      ProcessContext context,
      BoundedWindow window,
      @StateId("buffer") BagState<Event> bufferState,
      @StateId("count") ValueState<Integer> countState,
      @TimerId("expiry") Timer expiryTimer) {

    expiryTimer.set(window.maxTimestamp().plus(allowedLateness));

    … same logic as above …
  }

  @OnTimer("expiry")
  public void onExpiry(
      OnTimerContext context,
      @StateId("buffer") BagState<Event> bufferState) {
    if (!bufferState.isEmpty().read()) {
      for (EnrichedEvent enrichedEvent : enrichEvents(bufferState.read())) {
        context.output(enrichedEvent);
      }
    }
  }
}
```

```py
# State and timers are not yet supported in Beam's Python SDK.
# Follow https://issues.apache.org/jira/browse/BEAM-2687 for updates.
```

Let's unpack the pieces of this snippet:

 - We declare an event time timer with `@TimerId("expiry")`. We will use the
   identifier `"expiry"` to identify the timer for setting the callback time as
   well as receiving the callback.

 - The variable `expiryTimer`, annotated with `@TimerId`, is set to the value
   `TimerSpecs.timer(TimeDomain.EVENT_TIME)`, indicating that we want a
   callback according to the event time watermark of the input elements.

 - In the `@ProcessElement` element we annotate a parameter `@TimerId("expiry")
   Timer`. The Beam runner automatically provides this `Timer` parameter by which
   we can set (and reset) the timer. It is inexpensive to reset a timer
   repeatedly, so we simply set it on every element.

 - We define the `onExpiry` method, annotated with `@OnTimer("expiry")`, that
   performs a final event enrichment RPC and outputs the result. The Beam runner
   delivers the callback to this method by matching its identifier.

Illustrating this logic, we have the diagram below:

<img class="center-block"
    src="{{ site.baseurl }}/images/blog/timely-processing/BatchedRpcExpiry.png"
    alt="Batched RPCs with window expiration"
    width="600">

Both the `@ProcessElement` and `@OnTimer("expiry")` methods perform the same
access to buffered state, perform the same batched RPC, and output enriched
elements.

Now, if we are executing this in a streaming real-time manner, we might still
have unbounded latency for particular buffered data. If the watermark is advancing
very slowly, or event time windows are chosen to be quite large, then a lot of
time might pass before output is emitted based either on enough elements or
window expiration. We can also use timers to limit the amount of wall-clock
time, aka processing time, before we process buffered elements. We can choose
some reasonable amount of time so that even though we are issuing RPCs that are
not as large as they might be, it is still few enough RPCs to avoid blowing our
quota with the external service.

### Processing Time Timers

A timer in processing time (time as it passes while your pipeline is executing)
is intuitively simple: you want to wait a certain amount of time and then
receive a call back.

To put the finishing touches on our example, we will set a processing time
timer as soon as any data is buffered. We track whether or not the timer has
been set so we don't continually reset it. When an element arrives, if the
timer has not been set, then we set it for the current moment plus
`MAX_BUFFER_DURATION`. After the allotted processing time has passed, a
callback will fire and enrich and emit any buffered elements.

```java
new DoFn<Event, EnrichedEvent>() {
  …

  private static final Duration MAX_BUFFER_DURATION = Duration.standardSeconds(1);

  @TimerId("stale")
  private final TimerSpec staleSpec = TimerSpecs.timer(TimeDomain.PROCESSING_TIME);

  @ProcessElement
  public void process(
      ProcessContext context,
      BoundedWindow window,
      @StateId("count") ValueState<Integer> countState,
      @StateId("buffer") BagState<Event> bufferState,
      @TimerId("stale") Timer staleTimer,
      @TimerId("expiry") Timer expiryTimer) {

    boolean staleTimerSet = firstNonNull(staleSetState.read(), false);
    if (firstNonNull(countState.read(), 0) == 0) {
      staleTimer.offset(MAX_BUFFER_DURATION).setRelative());
    }

    … same processing logic as above …
  }

  @OnTimer("stale")
  public void onStale(
      OnTimerContext context,
      @StateId("buffer") BagState<Event> bufferState,
      @StateId("count") ValueState<Integer> countState) {
    if (!bufferState.isEmpty().read()) {
      for (EnrichedEvent enrichedEvent : enrichEvents(bufferState.read())) {
        context.output(enrichedEvent);
      }
      bufferState.clear();
      countState.clear();
    }
  }

  … same expiry as above …
}
```

```py
# State and timers are not yet supported in Beam's Python SDK.
# Follow https://issues.apache.org/jira/browse/BEAM-2687 for updates.
```

Here is an illustration of the final code:

<img class="center-block"
    src="{{ site.baseurl }}/images/blog/timely-processing/BatchedRpcStale.png"
    alt="Batching elements in state, then performing RPCs"
    width="600">

Recapping the entirety of the logic:

 - As events arrive at `@ProcessElement` they are buffered in state.
 - If the size of the buffer exceeds a maximum, the events are enriched and output.
 - If the buffer fills too slowly and the events get stale before the maximum is reached,
   a timer causes a callback which enriches the buffered events and outputs. 
 - Finally, as any window is expiring, any events buffered in that window are
   processed and output prior to the state for that window being discarded.

In the end, we have a full example that uses state and timers to explicitly
manage the low-level details of a performance-sensitive transform in Beam. As
we added more and more features, our `DoFn` actually became pretty large. That
is a normal characteristic of stateful, timely processing. You are really
digging in and managing a lot of details that are handled automatically when
you express your logic using Beam's higher-level APIs. What you gain from this
extra effort is an ability to tackle use cases and achieve efficiencies that
may not have been possible otherwise.

## State and Timers in Beam's Unified Model

Beam's unified model for event time across streaming and batch processing has
novel implications for state and timers. Usually, you don't need to do anything
for your stateful and timely `DoFn` to work well in the Beam model. But it will
help to be aware of the considerations below, especially if you have used
similar features before outside of Beam.

### Event Time Windowing "Just Works"

One of the raisons d'etre for Beam is correct processing of out-of-order event
data, which is almost all event data. Beam's solution to out-of-order data is
event time windowing, where windows in event time yield correct results no
matter what windowing a user chooses or what order the events come in.

If you write a stateful, timely transform, it should work no matter how the
surrounding pipeline chooses to window event time. If the pipeline chooses
fixed windows of one hour (sometimes called tumbling windows) or windows of 30
minutes sliding by 10 minutes, the stateful, timely transform should
transparently work correctly.

<img class="center-block"
    src="{{ site.baseurl }}/images/blog/timely-processing/WindowingChoices.png"
    alt="Two windowing strategies for the same stateful and timely transform"
    width="600">

This works in Beam automatically, because state and timers are partitioned per
key and window. Within each key and window, the stateful, timely processing is
essentially independent.  As an added benefit, the passing of event time (aka
advancement of the watermark) allows automatic release of unreachable state
when a window expires, so you often don't have to worry about evicting old
state.

### Unified real-time and historical processing

A second tenet of Beam's semantic model is that processing must be unified
between batch and streaming. One important use case for this unification
is the ability to apply the same logic to a stream of events in real time and
to archived storage of the same events.

A common characteristic of archived data is that it may arrive radically out of
order. The sharding of archived files often results in a totally different
ordering for processing than events coming in near-real-time. The data will
also all be all available and hence delivered instantaneously from the point of
view of your pipeline. Whether running experiments on past data or reprocessing
past results to fix a data processing bug, it is critically important that your
processing logic be applicable to archived events just as easily as incoming
near-real-time data.

<img class="center-block"
    src="{{ site.baseurl }}/images/blog/timely-processing/UnifiedModel.png"
    alt="Unified stateful processing over streams and file archives"
    width="600">

It is (deliberately) possible to write a stateful and timely DoFn that delivers
results that depend on ordering or delivery timing, so in this sense there is
additional burden on you, the `DoFn` author, to ensure that this nondeterminism
falls within documented allowances.

## Go use it!

I'll end this post in the same way I ended the last. I hope you will go try out
Beam with stateful, timely processing. If it opens up new possibilities for
you, then great! If not, we want to hear about it. Since this is a new feature,
please check the [capability matrix]({{ site.baseurl
}}/documentation/runners/capability-matrix/) to see the level of support for
your preferred Beam backend(s).

And please do join the Beam community at
[user@beam.apache.org]({{ site.baseurl }}/get-started/support) and follow
[@ApacheBeam](https://twitter.com/ApacheBeam) on Twitter.
