DoFn thread constraint.

[John MacMillan]

I'm trying to understand how restrictive this sentence from 4.3.2 of the Programming Guide is for our runner:

 

Each instance of your function object is accessed by a single thread on a worker instance, unless you explicitly create your own threads.

 

Does this imply that a runner may not ever use a DoFn instance on more than one thread (ie. it is not allowed to move the instance from one thread to another but must create a separate instance for each thread), or only that an instance would only ever be used by a single thread at a time?

 

Or put another way, would the following DoFn (or something like it, if I've messed up) ever be allowed to throw on a properly implemented runner?

 

public class MotionDetector<T> extends DoFn<T,T> {

    private static transient ThreadLocal<Long> id;
    @ProcessElement
    public void processElement(ProcessContext c) {
        if (id == null) {
            id = new ThreadLocal<Long>() {
                    @Override
                    protected Long initialValue() {
                        return Thread.currentThread().getId();
                    }
                };
        } else if (!id.get().equals(Thread.currentThread().getId())) {
            throw new RuntimeException("Moved!");
        }
        c.output(c.element());
    }

}

 

Thanks,

John