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From Balaji Rajagopalan <balaji.rajagopa...@olacabs.com>
Subject Re: Confusion about multiple use of one ValueState
Date Fri, 13 May 2016 05:56:43 GMT
I don't think the valuestate defined in one map function is accessible in
other map function this is my understanding, also you need to be aware
there will be instance of map function created for each of your tuple in
your stream, I had a similar use case where I had to pass in some state
from one map function to another, I used redis for that.

On Fri, May 13, 2016 at 8:58 AM, Nirmalya Sengupta <
sengupta.nirmalya@gmail.com> wrote:

> Hello all,
>
> Let's say I want to hold some state value derived during one
> transformation, and then use that same state value in a subsequent
> transformation? For example:
>
> myStream
> .keyBy(fieldID) // Some field ID, may be 0
> .map(new MyStatefulMapper())
> .map(new MySubsequentMapper())
> ....
>
> Now, I define MyStatefulMapper in the usual fashion:
>
> public class MyStatefulMapper extends RichFlatMapFunction<Tuple2<Long, Long>,
Tuple2<Long, Long>> {
>
>     /**     * The ValueState handle. The first field is the count, the second field a
running sum.     */
>     private transient ValueState<Tuple2<Long, Long>> sum;
>
>     @Override
>     public void flatMap(Tuple2<Long, Long> input, Collector<Tuple2<Long,
Long>> out) throws Exception {
>
>        // logic of accessing and updating the ValueState 'sum' above
>     }
>
>     @Override
>     public void open(Configuration config) {
>         ValueStateDescriptor<Tuple2<Long, Long>> descriptor =
>                 new ValueStateDescriptor<>(
>                         "mySum", // the state name
>                         TypeInformation.of(new TypeHint<Tuple2<Long, Long>>()
{}), // type information
>                         Tuple2.of(0L, 0L)); // default value of the state, if nothing
was set
>         sum = getRuntimeContext().getState(descriptor);
>     }}
>
>
> So, by now, RuntimeContext has registered a State holder named 'mySum'.
>
> In the implementation of 'MySubsequentMapper', I need to access this State
> holder named 'mySum', perhaps thus (my thinking, I may be wrong):
>
> public class MySubsequentMapper extends RichFlatMapFunction<Tuple2<Long, Long>,
Tuple2<Long, Long>> {
>
>     /**     * The ValueState handle. The first field is the count, the second field a
running sum.     */
>     private transient ValueState<Tuple2<Long, Long>> aSubsequentSum;
>
>     private transient ValueState<Tuple2<Long, Long>> sum; // defined earlier
>
>
>     @Override
>     public void flatMap(Tuple2<Long, Long> input, Collector<Tuple2<Long,
Long>> out) throws Exception {
>
>        // logic of accessing and updating the ValueState 'aSubsequentSum' above
>
>        // but this logic depends on the current contents of ValueState 'sum' created
earlier
>     }
>
>     @Override
>     public void open(Configuration config) {
>         // Logic to create ValueDescriptor for 'aSubsequentSum' which is owned by this
operator
>
>         // ...
>
>         // Question: now, how do I prepare for accessing 'sum' which is a State holder,
but created inside an earlier operator?
>         sum = getRuntimeContext().getState(descriptor) // how can I pass the name 'mySum'
(used in StateDescriptor)?
>     }}
>
> I have two questions:
>
> 1) What I am trying to achieve: is that possible and even, advisable? If
> not, then what is the alternative?
> 2) Is there a guarantee that Flink will execute MyStatefulOperator.open()
> always before MySubsequentOperator.open() because of the lexical order of
> appearance in the source code?
>
> -- Nirmalya
>
>
>
>
> --
> Software Technologist
> http://www.linkedin.com/in/nirmalyasengupta
> "If you have built castles in the air, your work need not be lost. That is
> where they should be.
> Now put the foundation under them."
>

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