I Built a Streaming Window Aggregator in Pure Python and Finally Understood How Flink Handles Late Data

📰 Dev.to · Haji Rufai

Learn how to handle late data in streaming aggregations using Python and understand Flink's watermark concept

intermediate Published 12 Jun 2026
Action Steps
  1. Build a simple streaming window aggregator in Python to understand the basics of event-time processing
  2. Run a simulation with late-arriving data to see how it affects the aggregation results
  3. Configure a watermark strategy in your Python aggregator to handle late data
  4. Test the aggregator with different watermark settings to observe the effects on result accuracy
  5. Apply the learned concepts to a Flink job to improve its handling of late data
Who Needs to Know This

Data engineers and stream processing developers can benefit from this knowledge to improve their handling of late-arriving data in real-time aggregations

Key Insight

💡 Watermarks are essential for handling late-arriving data in streaming aggregations, and understanding their role is crucial for building accurate and reliable stream processing applications

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💡 Handle late data in streaming aggregations with Python and understand Flink's watermark concept!

Key Takeaways

Learn how to handle late data in streaming aggregations using Python and understand Flink's watermark concept

Full Article

I spent two years writing Flink jobs before I understood what a watermark actually does. Not what...
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