Updates the Delta Lake source to support reading bounded change data by chamikaramj · Pull Request #39426 · apache/beam

chamikaramj · 2026-07-22T05:53:42Z

Please see here for a short design description.

This fixes #39492.

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gemini-code-assist · 2026-07-22T05:53:44Z

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github-actions · 2026-07-25T19:29:55Z

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github-actions · 2026-07-25T23:30:25Z

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Abacn · 2026-07-27T20:25:04Z

+
+  @Override
+  public String toString() {
+    return "DeltaCDCReadTask{"


nit: prefer String.format

Abacn · 2026-07-27T22:01:48Z

+      }
+    }
+
+    return ProcessContinuation.stop();


This is the only place where DoFn exits. if there is error throws, will it retry the whole element? In other words, would bounded SDF just resume from a claimed restriction even though previously crashed in the middle of a DoFn.process invocation?

I wonder if we need ProcessContinuation here at all, since we never call resume() ?

Abacn · 2026-07-27T22:03:09Z

+
+  private static StructType appendCDFColumns(StructType schema) {
+    return schema
+        .add("_change_type", StringType.STRING, false)


nit: consider delcare these field name as constants. There are used in multiple places

Abacn · 2026-07-27T22:07:10Z

+
+        while (logicalBatches.hasNext()) {
+          FilteredColumnarBatch batch = logicalBatches.next();
+          ColumnarBatch logicalBatch = batch.getData();


(based on AI comment): batch.getData() return raw data, ignoring selectionVector and causing filtered-out or deleted rows to be iterated over and emitted into the output PCollection.

Suggestion, call batch. getRows which handles selectionVector

while (logicalBatches.hasNext()) { FilteredColumnarBatch batch = logicalBatches.next(); if (!task.isCDC()) { ColumnarBatch logicalBatch = appendConstantCDFColumns( currentEngine, batch.getData(), task.getVersion(), task.getTimestamp()); batch = new FilteredColumnarBatch(logicalBatch, batch.getSelectionVector()); } try (CloseableIterator<io.delta.kernel.data.Row> logicalRows = batch.getRows()) { // <-- Uses FilteredColumnarBatch.getRows() while (logicalRows.hasNext()) { io.delta.kernel.data.Row deltaRow = logicalRows.next(); ... } } }

Abacn · 2026-07-27T22:15:16Z


    writePipeline.run().waitUntilFinish();

    System.out.println("FILES IN TABLE DIR:");


Not related to the change. But running the test realize there are System.out.println and System.err.println debugging leftovers. Consider removing them or using slf4j LOGGER.

Abacn · 2026-07-27T22:16:33Z

+    }
+  }
+
+  private byte[] writeParquetFile(File file, Schema schema, java.util.List<Row> rows)


There is already a writeParquetFile

https://github.com/chamikaramj/beam/blob/82ab11edcafe9251d75af1185daf33f247ca261c/sdks/java/io/delta/src/test/java/org/apache/beam/sdk/io/delta/DeltaIOTest.java#L379

ahmedabu98 · 2026-07-27T22:46:25Z

+      if (getStartVersion() == null && getStartTimestamp() == null) {
+        throw new IllegalArgumentException("Either startVersion or startTimestamp must be set.");
+      }


If there's no start version specified, we can have a default starting strategy. Either start at the very beginning, or at the very latest change.

Not blocking though, can add this in a future change

The IcebergIO default is we read from the beginning for batch and from the very end for streaming

ahmedabu98 · 2026-07-27T22:47:08Z

+      if (getEndVersion() != null && getEndTimestamp() != null) {
+        throw new IllegalArgumentException("Cannot set both endVersion and endTimestamp.");
+      }


Same as above. If no end is specified, we can just stop at the very latest change

I think your logic in the Create DoFn already handles this

ahmedabu98 · 2026-07-27T23:14:32Z

+        boolean hasCDC = !info.cdcRows.isEmpty();
+
+        List<Row> rowsToProcess = hasCDC ? info.cdcRows : info.addRows;
+        boolean isCDC = hasCDC;


nit: unnecessary assignment to another variable?

ahmedabu98 · 2026-07-27T23:19:38Z

+    final List<Row> cdcRows = new ArrayList<>();
+    final List<Row> addRows = new ArrayList<>();


Naming is throwing me off a little. So these aren't data rows right? Each row is metadata about a particular CDC or INSERT task?

ahmedabu98 · 2026-07-27T23:30:11Z

+ * files, converting rows to Beam Rows.
+ */
+@DoFn.BoundedPerElement
+class DeltaCDCSourceDoFn extends DoFn<DeltaCDCReadTask, Row> {


This DoFn parallelizes at the file level right? I think this should be fine for batch, but note that it may not be scalable in DF streaming mode because the runner opens too many threads per VM, each thread processing a file and competing for memory leading to OOM.

ahmedabu98 · 2026-07-27T23:33:45Z

+        .add("_change_type", StringType.STRING, false)
+        .add("_commit_version", LongType.LONG, false)
+        .add("_commit_timestamp", TimestampType.TIMESTAMP, false);


Can we have these field names be final static variables at the top?

What values can _change_type take?

Also is _commit_version increasingly monotonic?

hmmm I see that delta uses update_preimage and update_postimage for UPDATE_BEFORE/AFTER. This might not play too well with Iceberg CDC sink but I think should be okay because we can rely on the ValueKind metadata instead

ahmedabu98 · 2026-07-27T23:49:55Z

+      }
+    }
+
+    return ProcessContinuation.stop();


I wonder if we need ProcessContinuation here at all, since we never call resume() ?

ahmedabu98 · 2026-07-27T23:51:33Z

+  private static Row projectRow(Row row, Schema targetSchema) {
+    Row.Builder builder = Row.withSchema(targetSchema);
+    for (Schema.Field field : targetSchema.getFields()) {
+      builder.addValue(row.getValue(field.getName()));
+    }
+    return builder.build();
+  }


If schemas are equal we can just return the original row

ahmedabu98 · 2026-07-27T23:54:21Z

+  private void writeCommit(
+      File logDir,
+      long version,
+      long timestamp,
+      @Nullable String addPath,
+      long addSize,
+      @Nullable String removePath,
+      @Nullable String cdcPath,
+      long cdcSize,
+      boolean writeMetadata)
+      throws IOException {


Can we commit using the DeltaLake API? to make sure it works against the actual API and get notified if a library upgrade breaks things

ahmedabu98 · 2026-07-28T00:01:59Z

+    // Test 1: Read changes between start version 0 and end version 2
+    PCollection<Row> outputVersions =
+        readPipeline.apply(
+            "Read Changes Version Range",
+            DeltaIO.readChanges()
+                .from(tableDir.getAbsolutePath())
+                .withStartVersion(0L)
+                .withEndVersion(2L));


This test reads the whole range, so it can still pass if the range handling logic happened to be incorrect. We should have start version > 0 and/or end version < 2

github-actions Bot added java io delta labels Jul 22, 2026

chamikaramj marked this pull request as ready for review July 25, 2026 19:21

chamikaramj added 2 commits July 25, 2026 15:04

Updates the Delta Lake source to support reading bounded change data

2feaa26

Update and add a test

82ab11e

chamikaramj force-pushed the delta_lake_cdc branch from ed28244 to 82ab11e Compare July 25, 2026 22:05

github-actions Bot added the Next Action: Reviewers label Jul 25, 2026

Abacn reviewed Jul 27, 2026

View reviewed changes

ahmedabu98 reviewed Jul 28, 2026

View reviewed changes


		writePipeline.run().waitUntilFinish();

		System.out.println("FILES IN TABLE DIR:");

		final List<Row> cdcRows = new ArrayList<>();
		final List<Row> addRows = new ArrayList<>();

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chamikaramj commented Jul 22, 2026 •

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