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| <html> |
| <h2>Event Time Triggers</h2> |
| <p> |
| When collecting and grouping data into windows, Beam uses triggers to determine when to emit the |
| aggregated results of each window (referred to as a pane). If you use Beam’s default windowing |
| configuration and default trigger, Beam outputs the aggregated result when it estimates all data |
| has arrived, and discards all subsequent data for that window. |
| </p> |
| <p> |
| You can set triggers for your PCollections to change this default behavior. Beam provides a |
| number of pre-built triggers that you can set: |
| </p> |
| <div> |
| <ul> |
| <li>Event time triggers</li> |
| <li>Processing time triggers</li> |
| <li>Data-driven triggers</li> |
| <li>Composite triggers</li> |
| </ul> |
| </div> |
| <p> |
| Event time triggers operate on the event time, as indicated by the timestamp on each data |
| element. Beam’s default trigger is event time-based. |
| </p> |
| <p> |
| The AfterWatermark trigger operates on event time. The AfterWatermark trigger emits the contents |
| of a window after the watermark passes the end of the window, based on the timestamps attached |
| to the data elements. The watermark is a global progress metric, and is Beam’s notion of input |
| completeness within your pipeline at any given point. AfterWatermark.pastEndOfWindow() only fires |
| when the watermark passes the end of the window. |
| </p> |
| <p> |
| <b>Kata:</b> Given that events are being generated every second, please implement a trigger that |
| emits the number of events count within a fixed window of 5-second duration. |
| </p> |
| <br> |
| <div class="hint"> |
| Use <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/windowing/FixedWindows.html"> |
| FixedWindows</a> with 5-second duration using |
| <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/windowing/AfterWatermark.html#pastEndOfWindow--"> |
| AfterWatermark.pastEndOfWindow()</a> trigger. |
| </div> |
| <div class="hint"> |
| Set the <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/windowing/Window.html#withAllowedLateness-org.joda.time.Duration-"> |
| allowed lateness</a> to 0 with |
| <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/windowing/Window.html#discardingFiredPanes--"> |
| discarding accumulation mode</a>. |
| </div> |
| <div class="hint"> |
| Use <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/Combine.html#globally-org.apache.beam.sdk.transforms.CombineFnBase.GlobalCombineFn-"> |
| Combine.globally</a> and |
| <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/Count.html#combineFn--"> |
| Count.combineFn</a> to calculate the count of events. |
| </div> |
| <div class="hint"> |
| Refer to the Beam Programming Guide |
| <a href="https://beam.apache.org/documentation/programming-guide/#event-time-triggers"> |
| "Event time triggers"</a> section for more information. |
| </div> |
| </html> |
