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# Function Interfaces and User-Defined Functions
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Type-safe interfaces for implementing custom processing logic including window functions, process functions, and rich functions with lifecycle management.
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## Window Function Interfaces
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```scala { .api }
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trait WindowFunction[IN, OUT, KEY, W <: Window] {
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  def apply(key: KEY, window: W, input: Iterable[IN], out: Collector[OUT]): Unit
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}
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abstract class ProcessWindowFunction[IN, OUT, KEY, W <: Window] {
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  def process(key: KEY, context: Context, elements: Iterable[IN], out: Collector[OUT]): Unit
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  def clear(context: Context): Unit
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}
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trait AllWindowFunction[IN, OUT, W <: Window] {
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  def apply(window: W, input: Iterable[IN], out: Collector[OUT]): Unit
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}
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```
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## Rich Function Interfaces
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```scala { .api }
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abstract class RichWindowFunction[IN, OUT, KEY, W <: Window] extends WindowFunction[IN, OUT, KEY, W]
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abstract class RichProcessWindowFunction[IN, OUT, KEY, W <: Window] extends ProcessWindowFunction[IN, OUT, KEY, W]
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abstract class RichAllWindowFunction[IN, OUT, W <: Window] extends AllWindowFunction[IN, OUT, W]
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```
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## Async Function Interface
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```scala { .api }
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trait AsyncFunction[IN, OUT] {
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  def asyncInvoke(input: IN, collector: AsyncCollector[OUT]): Unit
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}
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```
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Function interfaces provide type-safe contracts for implementing custom processing logic with access to runtime context, state, and timers.