Eliaaazzz opened a new pull request, #39529:
URL: https://github.com/apache/beam/pull/39529
Adds a "Reading from an unbounded source" section to the Python I/O
connector development guide.
The guide currently documents only the bounded `BoundedSource` /
`RangeTracker` path and does not say how to build an unbounded (streaming)
source in Python. The Java guide covers this with its `UnboundedSource` /
`UnboundedReader` sections. The Python SDK has no `UnboundedSource` class; an
unbounded source is written as a Splittable DoFn, but the I/O connector guide,
which is where the Java unbounded documentation lives, had no Python
counterpart.
### What changes
- A note under "Implementing the Source interface" stating that the `Source`
interface is bounded only, linking to the new section.
- A new "Reading from an unbounded source" section with a minimal unbounded
Splittable DoFn example (`@beam.DoFn.unbounded_per_element()`,
`RestrictionParam`, `WatermarkEstimatorParam`, `try_claim`, `defer_remainder`).
- A mapping from each `UnboundedSource` / `UnboundedReader` concern in the
Java guide to its Splittable DoFn equivalent: initial and dynamic splitting,
checkpointing through residual restrictions, resuming through
`defer_remainder`, watermark estimators, deduplication, and drain through
`RestrictionProvider.truncate`.
- Pointers to the reusable building blocks: `PeriodicImpulse` /
`PeriodicSequence` and the experimental `Watch` transform added in #39023.
The example follows the same self-checkpoint pattern as the
`SDF_UserInitiatedCheckpoint` snippet in the programming guide and
`periodicsequence.py`: call `restriction_tracker.defer_remainder(...)` and
return, which is the mechanism the portable runners implement.
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