The Apache Beam Python SDK now has an UnboundedSource API for writing custom unbounded sources and a Watch transform for repeatedly polling an input that keeps growing. I built both during my Google Summer of Code 2026 project with Apache Beam, mentored by Yi Hu. This post describes both APIs as of Beam 2.77.0.
UnboundedSource and the Watch Transform in the Apache Beam Python SDK
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September 10, 2026
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apache-beam