Re: Difference between using a global variable and broadcasting a variable

Posted by Stephan Ewen on
URL: http://deprecated-apache-flink-user-mailing-list-archive.369.s1.nabble.com/Difference-between-using-a-global-variable-and-broadcasting-a-variable-tp1128p1144.html

Hi!

I put a quick summary into the wiki. For future reference.

https://cwiki.apache.org/confluence/display/FLINK/Variables+Closures+vs.+Broadcast+Variables

Greetings,

Stephan

On Mon, Apr 27, 2015 at 11:10 AM, Stephan Ewen <[hidden email]> wrote:

Adding to Fabian's and Sebastian's answer:

Variable in Closure (global variable)

------------------------------------------------------

 - Happens when you reference some variable in the program from a function. The variable becomes part of the Function's closure.

 - The variable is distributed with the CODE. It is part of the function object and is distributed with by the TaskDeployment messages.

 - Data needs to be available in the driver program (cannot be a Flink DataSet, which lives distributedly)

 - Should be used for constants or config parameters or simple scalar values.

Summary: Small data that is available on the client (driver program)

Broadcast set

------------------------------------------------------

 - Refers to data that is produced by a Flink operation (DataSet) and that lives in the cluster, rather than on the client (or in the driver program)

 - Data distribution is part of the distributed data flow and happens through the Flink network stack

 - Can be much larger than the closure variables.

 - Should be used when you want to make an intermediate result of a Flink computation accessible to all functions.

Greetings,

Stephan

On Mon, Apr 27, 2015 at 10:23 AM, Fabian Hueske <[hidden email]> wrote:

You should also be aware that the value of a static variable is only accessible within the same JVM.

Flink is a distributed system and runs in multiple JVMs. So if you set a value in one JVM it is not visible in another JVM (on a different node).

In general, I would avoid to use static variables in Flink programs.

Best, Fabian

2015-04-26 9:54 GMT+02:00 Sebastian <[hidden email]>:

Hi Hung,

A broadcast variable can also refer to an intermediate result of a Flink computation.

Best,
Sebastian

On <a href="tel:25.04.2015%2021" value="[phone]" target="_blank">25.04.2015 21:10, HungChang wrote:

Hi,

What would be the difference between using global variable and broadcasting
it?

A toy example:

// Using global
{{...
private static int num = 10;
}

public class DivByTen implements FlatMapFunction<Tuple1&lt;Double>,
Tuple1<Double>> {
   @Override
   public void flatMap(Tuple1<Double>value, Collector<Tuple1&lt;Double>> out)
{
      out.collect(new Tuple1<Double>(value/ num));
   }
}}

// Using broadcasting :
{...
public static class DivByTen extends
                        RichGMapFunction<Tuple1&lt;Double>, Tuple1<Double>>{

                private long num;

                @Override
                public void open(Configuration parameters) throws Exception {
                        super.open(parameters);
                        num = getRuntimeContext().<Integer> getBroadcastVariable(
                                        "num").get(0);
                }

                @Override
                public void map(Tuple1<Double>value, Collector<Tuple1&lt;Double>> out))
throws Exception{                       
                        out.collect(new Tuple1<Double>(value/num));
                }
        }
}

Best regards,

Hung



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