[jira] [Created] (FLINK-5929) Allow Access to Per-Window State in ProcessWindowFunction

["Aljoscha Krettek (JIRA)" <ji...@apache.org>]

Aljoscha Krettek created FLINK-5929:
---------------------------------------

             Summary: Allow Access to Per-Window State in ProcessWindowFunction
                 Key: FLINK-5929
                 URL: https://issues.apache.org/jira/browse/FLINK-5929
             Project: Flink
          Issue Type: Improvement
          Components: DataStream API
            Reporter: Aljoscha Krettek


Right now, the state that a {{WindowFunction}} or {{ProcessWindowFunction}} can access is scoped to the key of the window but not the window itself. That is, state is global across all windows for a given key.

For some use cases it is beneficial to keep state scoped to a window. For example, if you expect to have several {{Trigger}} firings (due to early and late firings) a user can keep state per window to keep some information between those firings.

The per-window state has to be cleaned up in some way. For this I see two options:
 - Keep track of all state that a user uses and clean up when we reach the window GC horizon.
 - Add a method {{cleanup()}} to {{ProcessWindowFunction}} which is called when we reach the window GC horizon that users can/should use to clean up their state.

On the API side, we can add a method {{windowState()}} on {{ProcessWindowFunction.Context}} that retrieves the per-window state and {{globalState()}} that would allow access to the (already available) global state. The {{Context}} would then look like this:
{code}
/**
 * The context holding window metadata
 */
public abstract class Context {
    /**
     * @return The window that is being evaluated.
     */
    public abstract W window();

    /**
     * State accessor for per-key and per-window state.
     */
    KeyedStateStore windowState();

    /**
     * State accessor for per-key global state.
     */
    KeyedStateStore globalState();
}
{code}



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