Keyed Watermarks: A Fine-grained Watermark Generation for Apache Flink

Abstract

Big Data Stream processing engines, exemplified by tools like Apache Flink, employ windowing techniques to manage unbounded streams of events. Aggregating relevant data within Windows is important for event-time windowing due to its impact on result accuracy. A pivotal role in this process is attributed to watermarks, unique timestamps signifying event progression in time. Nonetheless, the existing watermark generation method within Apache Flink, operating at the input stream level, exhibits a bias towards faster sub-streams, causing the omission of events from slower counterparts. Our analysis determined that Apache Flink's standard watermark generation approach results in an approximate 33% data loss when 50% of median-proximate keys experience delays. Furthermore, this loss exceeds 37% in cases where 50% of randomly selected keys encounter delays. In this paper, we introduce a pioneering approach termed keyed watermarks to address data loss concerns and enhance data processing precision to a minimum of 99% in most scenarios. Our strategy facilitates distinct progress monitoring by creating individualized watermarks for each logical sub-stream (key). Within our investigation, we delineate the essential architectural and API modifications requisite for integrating keyed watermarks while also highlighting our experience in navigating the expansion of Apache Flink's extensive codebase. Moreover, we conduct a comparative evaluation between the efficacy of our approach and the conventional watermark generation technique concerning the accuracy of event-time tracking, the latency of watermark processing, and the growth of Flink's maintained state.