brkyvz opened a new pull request #27380: [SPARK-30669][SS] Introduce AdmissionControl APIs for StructuredStreaming URL: https://github.com/apache/spark/pull/27380 ### What changes were proposed in this pull request? We propose to add a new interface `SupportsAdmissionControl` and `ReadLimit`. A ReadLimit defines how much data should be read in the next micro-batch. `SupportsAdmissionControl` specifies that a source can rate limit its ingest into the system. The source can tell the system what the user specified as a read limit, and the system can enforce this limit within each micro-batch or impose it's own limit if the Trigger is Trigger.Once() for example. ### Why are the changes needed? Sources currently have no information around execution semantics such as whether the stream is being executed in Trigger.Once() mode. This interface will pass this information into the sources as part of planning. With a trigger like Trigger.Once(), the semantics are to process all the data available to the datasource in a single micro-batch. However, this semantic can be broken when data source options such as `maxOffsetsPerTrigger` (in the Kafka source) rate limit the amount of data read for that micro-batch without this interface. ### Does this PR introduce any user-facing change? DataSource developers can extend this interface for their streaming sources to add admission control into their system and correctly support Trigger.Once(). ### How was this patch tested? Existing tests, as this API is mostly internal
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