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Side Input
----------

In addition to the main input PCollection, you can provide additional inputs to a ParDo transform
in the form of side inputs. A side input is an additional input that your DoFn can access each time
it processes an element in the input PCollection. When you specify a side input, you create a view
of some other data that can be read from within the ParDo transform’s DoFn while processing each
element.

Side inputs are useful if your ParDo needs to inject additional data when processing each element
in the input PCollection, but the additional data needs to be determined at runtime (and not
hard-coded). Such values might be determined by the input data, or depend on a different branch of
your pipeline.

**Kata:** Please enrich each Person with the country based on the city he/she lives in.

<div class="hint">
  Use <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/View.html">
  View</a> to create PCollectionView of citiesToCountries.
</div>

<div class="hint">
  Use <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/ParDo.html">
  ParDo</a> with <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/DoFn.html">
  DoFn</a> that accepts
  <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/ParDo.SingleOutput.html#withSideInputs-org.apache.beam.sdk.values.PCollectionView...-">
  side input</a>.
</div>

<div class="hint">
  Refer to the Beam Programming Guide
  <a href="https://beam.apache.org/documentation/programming-guide/#side-inputs">"Side inputs"</a>
  section for more information.
</div>
