## 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.

Use View to create PCollectionView of citiesToCountries.
Use ParDo with DoFn that accepts side input.
Refer to the Beam Programming Guide "Side inputs" section for more information.