Lambda and Kappa

1. The Incident

The store now has two pipelines; the nightly batch computes the authoritative reports; the streaming pipeline feeds the live dashboard. Both compute revenue. On Wednesday morning, finance notices that they disagree; the dashboard's Tuesday total reads $412,880; the batch report says $413,205.

Which one is right? Both, by their own rules; the streaming job dropped some late events; that was its watermark policy, working as designed