How does state management work in Apache Beam?

In Apache Beam, state management is accomplished through the State API. This API allows Beam pipelines to maintain and update the state while processing elements. The state can be stored in memory or external storage, depending on the implementation of the Runner.

Beam’s state management consists of two types: Keyed State and Timely State. Keyed State is the state associated with a key, such as the state maintained in a GroupByKey operation. Timely State is the state associated with time, such as the state maintained in a Window operation.

Keyed State can be accessed and updated through the Stateful DoFn in the State API. This special type of ParDo allows for accessing and modifying Keyed State while processing each element. Timely State can be accessed and updated by using the State API within Window operations.

The Runner for Beam is responsible for hiding the implementation details of state management in the background, ensuring consistency and fault tolerance of the states. Different Runners may use different methods to manage states, such as storing them in memory or external storage. The State API in Beam provides a unified way to access and update states, allowing developers to focus on business logic without worrying about the details of state management.