Question 1: How can I properly extract values from _StateBackedIterable when using an AsMultiMap side input? The most reliable way to extract values is to force materialization by converting the _StateBackedIterable to a list. While iterating can work, converting to a list ensures all data is processed and available.

lookup_table_iterable = ref_bsbcatname["104221"] or [] value_list = list(lookup_table_iterable) for value in value_list: logging.info(f"ref_bsbcatname Value 104221 : {value}")

Question 2: Is there a way to force materialization of the iterable when reading from the side input? Yes, as shown above, explicitly converting to a list (list(lookup_table_iterable)) forces materialization. This consumes the iterable, making subsequent iterations impossible without re-materializing.

Question 3: Could this issue be related to Apache Beam’s lazy evaluation model? How does Apache Beam manage periodic updates to a PCollection used as a side input? Yes, this issue is directly related to Apache Beam's lazy evaluation. The _StateBackedIterable is a consequence of this optimization. Apache Beam manages periodic updates by ensuring that when a transform using the side input executes, it receives the latest available version of the PCollection. The runner handles the update and data synchronization behind the scenes. The key is that the materialization happens at the point of use within the transform, not when the side input is created or updated.