Implementing retractions in google dataflow

Question

I read the "The Dataflow Model: A Practical Approach to Balancing Correctness, Latency, and Cost in MassiveScale, Unbounded, Out of Order Data Processing" paper. Alas, the SDK does not yet expose the accumulating & retracting triggering mode (section 2.3).

I was wondering if there was a workaround for getting similar semantics?

I have been reading the source and have figured out that StateTag or StateNamespace may be the way i can store the "last emitted value of the window" and hence can be used to calculate the retraction message down the pipeline. Is this the correct path or are there other classes/ways I can/should look at.

Answer 1

The upcoming state API is indeed your best bet for emulating retractions. Those classes you mentioned are part of the state API, but everything in the com.google.cloud.dataflow.sdk.util is for internal use only; we technically make no guarantees that the APIs won't change drastically, or even remain unreleased. That said, releasing that API is on our roadmap, and I'm hopeful we'll get it released relatively soon.

One thing to keep in mind: all the code downstream of your custom retractions will need to be able to differentiate them from normal records. This is something we'll do automatically for you once bonafide retraction support is ready, but in the mean time, you'll just need to make sure all the code you write that might receive a retraction knows how to recognize and handle it as such.