CODEWITHSUNDEEP
google-cloud-dataflowapache-beam
Sanjay
Sanjay

Reputation: 76

How to Access Data from _StateBackedIterable in Apache Beam?

I am working with Apache Beam and encountering an issue when trying to access data from a PCollection that appears to be wrapped in _StateBackedIterable.

I have a side input in the form of an AsMultiMap, and when I try to access values for a given key, I get an instance of _StateBackedIterable instead of a standard iterable.

Context & Setup:

The PCollection has data before attaching to the side input, so I expect to extract values using lazy loading. The PCollection is updated every 30 minutes using a periodic impulse, ensuring that fresh data is available.

This issue I am facing only DataFlow Runner, locally Direct Runner works fine it retunes a defaultdict.

Approach One:

logging.info(f"ref_bsbcatname: {ref_bsbcatname}")  # Out: <apache_beam.runners.worker.bundle_processor.StateBackedSideInputMap.__getitem__.<locals>.MultiMap object at 0x7c3b0e7a3490>

lookup_table_iterable = ref_bsbcatname["104221"] or []
logging.info(f"ref_bsbcatname Value 104221 : {lookup_table_iterable}") #Out <apache_beam.runners.worker.bundle_processor._StateBackedIterable object at 0x7c3b11a1d990>

for value in lookup_table_iterable: # Not going inside the loop
    logging.info(f"ref_bsbcatname Value 104221 : {value}")

Approach Two:



lookup_table_iterable1 = ref_bsbcatname["104224"] or [] # Out: <apache_beam.runners.worker.bundle_processor.StateBackedSideInputMap.__getitem__.<locals>.MultiMap object at 0x7c3b0e7a3490>

logging.info(f"ref_bsbcatname Value 104224 : {lookup_table_iterable1}") #Out <apache_beam.runners.worker.bundle_processor._StateBackedIterable object at 0x7c3b11a1d990>

first_value_check = next(iter(lookup_table_iterable1), None)

if first_value_check is None:
    logging.info("No values found for key '104224' in ref_bsbcatname") # Printing this always
else:
    logging.info(f"First value for '104224' is: {first_value_check}")

Approach Three:

lookup_table_iterable3 = ref_bsbcatname["104236"] or [] # Out: <apache_beam.runners.worker.bundle_processor.StateBackedSideInputMap.__getitem__.<locals>.MultiMap object at 0x7c3b0e7a3490>

logging.info(f"ref_bsbcatname Value 104236 : {lookup_table_iterable3}")
#Out <apache_beam.runners.worker.bundle_processor._StateBackedIterable object at 0x7c3b11a1d990>

value_list = list(lookup_table_iterable3)  # Convert to a Python list
logging.info(f"All values as list for key '104236': {value_list}") # Out `[]`

for value in value_list:  # Now you can iterate over the list
    logging.info(f"  List Value: {value}")

Issue:

Questions:

  1. How can I properly extract values from _StateBackedIterable when using an AsMultiMap side input?
  2. Is there a way to force materialization of the iterable when reading from the side input?
  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? Do

Upvotes: 0

Views: 22

Answers (1)

Yzza Hario
Yzza Hario

Reputation: 1

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.

Upvotes: 0

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