Inject SDK-side flattens while handling input/output coder mismatch in flattens.#34641
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lostluck merged 5 commits intoapache:masterfrom Apr 16, 2025
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…the fix. The test is also included in the test suite of flink, samza and spark, but without transcoding until their corresponding FRs are resolved.
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lostluck
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Apr 16, 2025
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I like how clean this ended up. Great work.
| func (h *runner) handleFlatten(tid string, t *pipepb.PTransform, comps *pipepb.Components) prepareResult { | ||
| if !h.config.SDKFlatten { | ||
| t.EnvironmentId = "" // force the flatten to be a runner transform due to configuration. | ||
| if !h.config.SDKFlatten && !strings.HasPrefix(tid, "ft_") { |
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I'll note that there's no user serviceable way to do these configurations at the moment, and it really was a hard binary. It would be acceptable to remove the SDKFlatten option in favour of just a single approach that biases to runner flattens, but does the SDK flatten to get around these issues.
| stg.internalCols = internal | ||
| // Sort the keys of internal producers (from stageFacts.PcolProducers) | ||
| // to ensure deterministic order for stable tests. | ||
| sort.Strings(stg.internalCols) |
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Good find! I thought I had everything deterministic already.
This was referenced Apr 18, 2025
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Following the idea of identity transform (#32930 (comment)) and valuable discussions with @lostluck, I have developed a "simpler and more general fix" for Prism runner issues involving
Flatten. This approach works not only forFlatten->Flattenscenarios, but also for others likeGroupBy->Flatten.The key observations are:
Flattenonly passes data through from its input PCollections.Flattenoutput.Previous approaches (including #34602) attempted to fix this by by overwriting the upstream coders with the
Flattenoutput coder. This works in many scenarios, especially when the upstream transform is a SDK-side transform or anotherFlatten(where #34602 ensured coder propagation).However, modifying upstream PCollection coders can cause side effects.
groupby->flattenexample (org.apache.beam.sdk.transforms.FlattenTest.testFlattenWithDifferentInputAndOutputCoders2), changingGroupBy's coder has no effect on the actual encoded data, becauseGroupByalways generateK,Iterable<V>. Downstream transforms expecting data encoded with theFlatten's coder then fail during decoding.C = [A, B] | Flatten(), D = A | GroupByKey(). IfFlattenoverwrites the coder associated with PCollectionA, theGroupByKeyonAmay fail due to coder incompatibility.The proposed solution avoids the pitfalls of modifying upstream coders. Instead, it inserts an identity transform (specifically, an SDK-side Flatten) before the Runner
Flatten. This identity transform implicitly converts the input PCollection's elements to use the same output coder as the target Runner Flatten, ensuring the correctly encoded data emitted from it.fixes #32930
fixes #34643
The PR also fixes the flaky test of
Test_preprocessor_preProcessGraph/ignoreEmptyAndIdentityTransformunder https://github.com/apache/beam/actions/workflows/beam_PreCommit_Go.yml.