Apache Beam Combine.perKey using a compound key

Question

I want to combine Measurements by type and device name.

public class Measurement implements Serializable {
    private String measurementType;
    private String device;
    private Long ts;
    private Double reading;
}

I am doing an average with Combine.perKey(...) successfully by the type. But I want to have basically a compound key of device and measurementType.

Right now my KvDoFn looks like this:

public class KvByMeasurementType extends DoFn<Measurement, KV<String, Measurement>> implements Serializable {
@DoFn.ProcessElement
    public void processElement(DoFn<Measurement, KV<String, Measurement>>.ProcessContext context) {
        Measurement measurement = context.element();
        context.output(KV.of(measurement.getMeasurementType(), measurement));
    }
}

How to extend it to create a compound key of two values?

Answer 1

You can simply create a new object and make that the key. For example,

public class MyKey implements Serializable {
    private String measurementType;
    private String device;
}

And then generate and output KV s of type MyKey from your KvByMeasurementType .

Also, define a Beam CombineFn that performs the combining based on this key. See here for more information on the Beam's Combine transform.