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From Akshay Aggarwal <akshay.aggar...@flipkart.com>
Subject Re: Persisting inactive state outside Flink
Date Tue, 18 Feb 2020 16:57:24 GMT
Thanks Till, I really appreciate your response.

We are in fact considering RocksDB as our state backend. The scale we are
looking at is 1.2 Bn new devices every year, with a growth of ~30% YoY, the
state per device is not expected to grow beyond few 10s of KBs though. The
peak ingestion rates are around 100k events per second. Another
consideration here is that many devices will go dormant forever, and it
seems pointless to keep that in state.

I have few concerns because of which I wasn't completely convinced of using
RocksDB (only) -
1. Will the "time to checkpoint" and "time to recovery"  keep increasing
with the size of the state?
2. Will there be a slowdown in RocksDB operations as the number of keys
increase over time?
3. If we go to production with just RocksDB and no external state
persistence, is there a way for us to migrate to lazy loading if we hit
scale issues?

The iterative solution with AsyncIO seems complex but feasible, it
certainly needs more thought to handle edge cases. Also, our use case can
manage an occasional glitch that comes with at-least once processing since
the output will be used for analytical purposes. Thanks for the suggestion.

Cheers!
Akshay


On Tue, Feb 18, 2020 at 4:06 PM Till Rohrmann <trohrmann@apache.org> wrote:

> 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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*This email and any files transmitted with it are confidential and 
intended solely for the use of the individual or entity to whom they are 
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system manager. This message contains confidential information and is 
intended only for the individual named. If you are not the named addressee, 
you should not disseminate, distribute or copy this email. Please notify 
the sender immediately by email if you have received this email by mistake 
and delete this email from your system. If you are not the intended 
recipient, you are notified that disclosing, copying, distributing or 
taking any action in reliance on the contents of this information is 
strictly prohibited.*****

 ****

*Any views or opinions presented in this 
email are solely those of the author and do not necessarily represent those 
of the organization. Any information on shares, debentures or similar 
instruments, recommended product pricing, valuations and the like are for 
information purposes only. It is not meant to be an instruction or 
recommendation, as the case may be, to buy or to sell securities, products, 
services nor an offer to buy or sell securities, products or services 
unless specifically stated to be so on behalf of the Flipkart group. 
Employees of the Flipkart group of companies are expressly required not to 
make defamatory statements and not to infringe or authorise any 
infringement of copyright or any other legal right by email communications. 
Any such communication is contrary to organizational policy and outside the 
scope of the employment of the individual concerned. The organization will 
not accept any liability in respect of such communication, and the employee 
responsible will be personally liable for any damages or other liability 
arising.*****

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*Our organization accepts no liability for the 
content of this email, or for the consequences of any actions taken on the 
basis of the information *provided,* unless that information is 
subsequently confirmed in writing. If you are not the intended recipient, 
you are notified that disclosing, copying, distributing or taking any 
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