Hi Akshay,

there is no easy out-of-the-box implementation for what you are asking.

Before drafting a potential solution I wanted to ask whether using the RocksDB state backend could already solve your problem. With this state backend Flink is able to spill state data to disk. Would this work for your use case or do you expect the device data per node to grow so big that it no longer fits onto disk?

If using the RocksDB state backend does not work for you and you really need to offload state data to an external storage system from where you can load it lazily it become significantly more complicated. One approach I could think of is the following: You have a primary operator (process) which is responsible for processing the incoming events and keeps the state of the non-dormant devices. Once a device becomes dormant, you could send the data to a secondary operator (offload+fetching) which uses AsyncIO to offload the state to Hbase, for example. If the process operator should encounters an event from a dormant device, it would need to ask the secondary operator to load it (via sending a fetch event downstream). The secondary operator would again use AsyncIO to load the requested data. Once it retrieves the data, you would need to send the data back to the primary operator via a feedback edge (iteration edge).

The problem with this approach is that Flink does not give you exactly-once processing guarantees when using iterations at the moment. The community is working on changing this, though.

Cheers,
Till

On Fri, Feb 14, 2020 at 6:05 PM Akshay Aggarwal <akshay.aggarwal@flipkart.com> wrote:
Hi,

We have a use case where we have to persist some state information about a device forever. Each new event will fetch the keyed state and update it. And this has to be applied in-order of events.

The problem is that the number of devices (keys) will keep growing infinitely. Usually a device comes online, stays active for a while (generates new events) and then goes into dormant mode. Is there a way we can persist the state outside of Flink (say HBase) when the device goes dormant and later fetch when it's activated?

I know we can do this in process function using timers. But here I'll have to make a synchronous call to the external store every time a new device comes live, or when an active device goes dormant, which will stall the task and become a scalability bottleneck. Using AsyncIO also doesn't seem to be an option.

Is there a way to achieve this without hacking into Flink code?

Thanks,
Akshay Aggarwal

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