Re: How to use operator list state like a HashMap?

Posted by Tony Wei on
URL: http://deprecated-apache-flink-user-mailing-list-archive.369.s1.nabble.com/How-to-use-operator-list-state-like-a-HashMap-tp15701p15708.html

Hi Fabian,

This is a good advice, but I had already tried adding random value to my data and it seems not very useful.

The key set of my data is small, around 10 ~ 20. If the range of random number is small, the distribution might not be better, even worse. I think the reason is that KeyedStream uses murmur hash to partition key and it wouldn't guarantee the distribution is fair.

Of course if the range of random number is large enough, the probability of even distribution is higher. It means I need to cache more data in state because the data with the original key would be separated to a larger key set. I would prefer to avoid this situation.

Best Regards,

Tony Wei

2017-09-19 22:56 GMT+08:00 Fabian Hueske <[hidden email]>:

Hi Tony,

operator state can only be kept on the heap.

One thing you could try is to add a random value to you data and keyBy on a composite key that consists of your original key and the random value.

It is important though, that you actually add the random value to your data to ensure that the extracted key is always the same, i.e., deterministic with respect to the data.

This should evenly distribute your data and allow you to use keyed MapState.

Hope this helps,

Fabian

2017-09-19 15:58 GMT+02:00 Tony Wei <[hidden email]>:

Hi,

I have a basic streaming job that continuously persist data from Kafka to S3.

Those data would be grouped by some dimensions and a limited amount.

Originally, I used 'keyBy' and key state to fulfill the requirement.

However, because the data is extremely skewed, I turned to use map function to aggregate data for some partitions only, so that I can balance the amount of data in each sub tasks.

I used a HashMap to store data by different dimensions inner map function and convert it to operator list state when 'snapshot()' is called.

But, that makes another problem. Because I can't access operator list state directly like using key state in KeyedStream, I have to use heap space to store those state. It leads to the limitation of the amount that I can cache in map function.

I was wondering if there is any good suggestion to deal with this problem or how to use operator list state like this scenario with a better manner. Thank you.

Best Regards,

Tony Wei