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From Patrick Brunmayr <...@kpibench.com>
Subject Re: Flink the right tool for the job ? Huge Data window lateness
Date Fri, 24 Feb 2017 17:37:59 GMT
Hi

Yes it is and would be nice to handle this with Flink :)

-
*Size of data point*
The size of a data point is basically just a simple case class with two
fields in a 64bit OS

case class MachineData(sensorId: String, eventTime:Long)

*- Last write wins*

We have cassandra as data warehouse but i was hoping i could solve that
issue in the state level rather than in the db level. The reason beeing is
one could send me the same events
over and over again and this will cause that state to blow up until out of
memory. Secondly by doing aggregations per sensor results will be wrong due
multiple events with the same
timestamp.

thx





2017-02-24 17:47 GMT+01:00 Robert Metzger <rmetzger@apache.org>:

> Hi,
> sounds like a cool project.
>
> What's the size of one data point?
> If one datapoint is 2 kb, you'll have 100 800 000 * 2048 bytes = 206
> gigabytes of state. That's something one or two machines (depending on the
> disk throughput) should be able to handle.
>
> If possible, I would recommend you to do an experiment using a prototype
> to see how many machines you need for your workload.
>
> On Fri, Feb 24, 2017 at 5:41 PM, Tzu-Li (Gordon) Tai <tzulitai@apache.org>
> wrote:
>
>> Hi Patrick,
>>
>> Thanks a lot for feedback on your use case! At a first glance, I would
>> say that Flink can definitely solve the issues you are evaluating.
>>
>> I’ll try to explain them, and point you to some docs / articles that can
>> further explain in detail:
>>
>> *- Lateness*
>>
>> The 7-day lateness shouldn’t be a problem. We definitely recommend
>> using RocksDB as the state backend for such a use case, as you
>> mentioned correctly, the state would be kept for a long time.
>> The heavy burst when your locally buffered data on machines are
>> sent to Kafka once they come back online shouldn’t be a problem either;
>> since Flink is a pure data streaming engine, it handles backpressure
>> naturally without any additional mechanisms (I would recommend
>> taking a look at http://data-artisans.com/how-flink-handles-backpressure/
>> ).
>>
>> *- Out of Order*
>>
>> That’s exactly what event time processing is for :-) As long as the event
>> comes in before the allowed lateness for windows, the event will still
>> fall
>> into its corresponding event time window. So, even with the heavy burst of
>> the your late machine data, they will still be aggregated in the correct
>> windows.
>> You can look into event time in Flink with more detail in the event time
>> docs:
>> https://ci.apache.org/projects/flink/flink-docs-release-1.3/
>> dev/event_time.html
>>
>> *- Last write wins*
>>
>> Your operators that does the aggregations simply need to be able to
>> reprocess
>> results if it sees an event with the same id come in. Now, if results are
>> sent out
>> of Flink and stored in an external db, if you can design the db writes to
>> be idempotent,
>> then it’ll effectively be a “last write wins”. It depends mostly on your
>> pipeline and
>> use case.
>>
>> *- Computations per minute*
>> I think you can simply do this by having two separate window operators.
>> One that works on your longer window, and another on a per-minute basis.
>>
>> Hope this helps!
>>
>> - Gordon
>>
>> On February 24, 2017 at 10:49:14 PM, Patrick Brunmayr (jay@kpibench.com)
>> wrote:
>>
>> Hello
>>
>> I've done my first steps with Flink and i am very impressed of its
>> capabilities. Thank you for that :) I want to use it for a project we are
>> currently working on. After reading some documentation
>> i am not sure if it's the right tool for the job. We have an IoT
>> application in which we are monitoring machines in production plants. The
>> machines have sensors attached and they are sending
>> their data to a broker ( Kafka, Azure Iot Hub ) currently on a per minute
>> basis.
>>
>> Following requirements must be fulfilled
>>
>>
>>    - Lateness
>>
>>    We have to allow lateness for 7 days because machines can have down
>>    time due network issues, maintenance or something else. If thats the case
>>    buffering of data happens localy on the machine and once they
>>    are online again all data will be sent to the broker. This can result
>>    in some relly heavy burst.
>>
>>
>>    - Out of order
>>
>>    Events come out of order due this lateness issues
>>
>>
>>    - Last write wins
>>
>>    Machines are not stateful and can not guarantee exactly once sending
>>    of their data. It can happen that sometimes events are sent twice. In that
>>    case the last event wins and should override the previous one.
>>    Events are unique due a sensor_id and a timestamp
>>
>>    - Computations per minute
>>
>>    We can not wait until the windows ends and have to do computations on
>>    a per minute basis. For example aggregating data per sensor and writing it
>>    to a db
>>
>>
>> My biggest concern in that case is the huge lateness. Keeping data for 7
>> days would result in 10080 data points for just one sensor! Multiplying
>> that by 10.000 sensors would result in 100800000 datapoints which Flink
>> would have to handle in its state. The number of sensors are constantly
>> growing so will the number of data points
>>
>> So my questions are
>>
>>
>>    - Is Flink the right tool for the Job ?
>>
>>    - Is that lateness an issue ?
>>
>>    - How can i implement the Last write wins ?
>>
>>    - How to tune flink to handle that growing load of sensors and data
>>    points ?
>>
>>    - Hardware requirements, storage and memory size ?
>>
>>
>>
>> I don't want to maintain two code base for batch and streaming because
>> the operations are all equal. The only difference is the time range! Thats
>> the reason i wanted to do all this with Flink Streaming.
>>
>> Hope you can guide me in the right direction
>>
>> Thx
>>
>>
>>
>>
>>
>>
>>
>>
>

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