[BAHIR-295] Added backpressure & ratelimit support

[iammehrabalam]

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[eskabetxe]

@iammehrabalam thanks for your contribution
@lresende could you check?

[lresende]

@iammehrabalam What is the backward compatibility story for this change? Also, should the sample be using the new capabilities to demonstrate the new functionality?

[iammehrabalam]

The behaviour will be exactly same what was earlier if below spark streaming config is not set.

• spark.streaming.backpressure.initialRate
• spark.streaming.receiver.maxRate
• spark.streaming.backpressure.pid.minRate

So it means backward compatible.

Added a test case which demonstrate rate and batch size.

@lresende

[iammehrabalam]

@lresende@eskabetxe reminder

[datasherlock]

The backpressure implementation isn't working as expected. My understanding is that the backpressure mechanism will control the input rate but never exceed the spark.streaming.receiver.maxRate. But this doesn't seem to be honoured since we're noticing that the receiver input rate breaches the spark.streaming.receiver.maxRate every now and then and tends to put a lot of pressure on the pipeline.

Context - I created a Spark Scala app with 900 receivers, spark.streaming.receiver.maxRate=1500 and batchInterval=60s. My understanding is that the total number of records per batch should not be greater than 900*1500*60 = 81,000,000 records. But I am noticing that some batches are going as high as 776,732,455 records where the processing time is >>> batchInterval

[datasherlock]

Based on https://spark.apache.org/docs/latest/streaming-custom-receivers.html#receiver-reliability, the rate control mechanism will have to be implemented by the receiver (if reliable). I do not see any logic that caps the input rates to the maxRate in the code. Could that be the reason why the backpressure limits are not honoured?

[LeonardMeyer]

Just stumbled upon this PR. For anyone interested, my guess is that the correct implementation should use Spark Streaming's BlockGenerator class. It would give the whole process spark.streaming.backpressure.enabled support for free since its RateLimiter implementation can be notified by Spark.

[iammehrabalam]

@LeonardMeyer you are right but the rate limit will only be applied when single data is written into the store (https://github.com/apache/spark/blob/595ad30e6259f7e4e4252dfee7704b73fd4760f7/streaming/src/main/scala/org/apache/spark/streaming/receiver/Receiver.scala#L118). In case of writing iterator (i.e block) directly rate limit will not be applied by default.

In Pubsub Receiver, the iterator store method is called where we added rate limit (i.e the same rate limit is generated based on backpressure )

[iammehrabalam]

@datasherlock Ideally it should work. If possible share spark configuration so I can help.
For rate limit logic you can check updateRateLimit & pushToStoreAndAck method in this PR.

[irajhedayati]

This change was suggested two years ago. Is there any plan to push it through?