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[FLINK-39412][cdc-common] Skip duplicate columns in AddColumnEvent to ensure idempotency #4370

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[FLINK-39412][cdc-common] Skip duplicate columns in AddColumnEvent to ensure idempotency #4370

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10 changes: 10 additions & 0 deletions flink-cdc-common/src/main/java/org/apache/flink/cdc/common/utils/SchemaUtils.java
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Original file line number Diff line number Diff line change
Expand Up @@ -121,7 +121,16 @@ public static Schema applySchemaChangeEvent(Schema schema, SchemaChangeEvent eve

private static Schema applyAddColumnEvent(AddColumnEvent event, Schema oldSchema) {
LinkedList<Column> columns = new LinkedList<>(oldSchema.getColumns());
Set<String> existingColumnNames =
columns.stream()
.map(Column::getName)
.collect(Collectors.toCollection(HashSet::new));
for (AddColumnEvent.ColumnWithPosition columnWithPosition : event.getAddedColumns()) {
// Skip adding the column if it already exists in the schema to ensure idempotency.
// This can happen when schema change events are replayed after a failover recovery.
if (existingColumnNames.contains(columnWithPosition.getAddColumn().getName())) {
continue;
}
Comment on lines +124 to +133
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Copilot AI Apr 9, 2026

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Skipping duplicates purely by name can hide a real schema conflict (e.g., replay vs. a different upstream AddColumnEvent that reuses the same column name but with a different type/nullable/default/comment). To keep idempotency while avoiding silent corruption, consider: if the name exists, look up the existing Column and compare with columnWithPosition.getAddColumn(); only skip when they are equivalent, otherwise throw an informative exception.

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switch (columnWithPosition.getPosition()) {
case FIRST:
{
Expand Down Expand Up @@ -170,6 +179,7 @@ private static Schema applyAddColumnEvent(AddColumnEvent event, Schema oldSchema
break;
}
}
existingColumnNames.add(columnWithPosition.getAddColumn().getName());
}
return oldSchema.copy(columns);
}
Expand Down
40 changes: 40 additions & 0 deletions flink-cdc-common/src/test/java/org/apache/flink/cdc/common/utils/SchemaUtilsTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -160,6 +160,46 @@ void testApplyColumnSchemaChangeEvent() {
"AFTER type AddColumnEvent error: Column %s does not exist in table %s",
"col10", tableId));

// add duplicate column should be ignored (idempotency)
addedColumns = new ArrayList<>();
addedColumns.add(
new AddColumnEvent.ColumnWithPosition(
Column.physicalColumn("col3", DataTypes.STRING()),
AddColumnEvent.ColumnPosition.LAST,
null));
addColumnEvent = new AddColumnEvent(tableId, addedColumns);
schema = SchemaUtils.applySchemaChangeEvent(schema, addColumnEvent);
Comment on lines +163 to +171
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Copilot AI Apr 9, 2026

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Current tests cover replay of a single-column AddColumnEvent. Since the implementation now keeps existingColumnNames across iterations, add a unit test where a single AddColumnEvent contains the same column name twice (e.g., two ColumnWithPosition entries for col3), and assert only one column is added. This directly validates the new intra-event dedup behavior.

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Assertions.assertThat(schema)
.isEqualTo(
Schema.newBuilder()
.physicalColumn("col0", DataTypes.STRING())
.physicalColumn("col1", DataTypes.STRING())
.physicalColumn("col2", DataTypes.STRING())
.physicalColumn("col4", DataTypes.STRING())
.physicalColumn("col5", DataTypes.STRING())
.physicalColumn("col3", DataTypes.STRING())
.build());

// add duplicate column in FIRST position should be ignored
addedColumns = new ArrayList<>();
addedColumns.add(
new AddColumnEvent.ColumnWithPosition(
Column.physicalColumn("col0", DataTypes.STRING()),
AddColumnEvent.ColumnPosition.FIRST,
null));
addColumnEvent = new AddColumnEvent(tableId, addedColumns);
schema = SchemaUtils.applySchemaChangeEvent(schema, addColumnEvent);
Assertions.assertThat(schema)
.isEqualTo(
Schema.newBuilder()
.physicalColumn("col0", DataTypes.STRING())
.physicalColumn("col1", DataTypes.STRING())
.physicalColumn("col2", DataTypes.STRING())
.physicalColumn("col4", DataTypes.STRING())
.physicalColumn("col5", DataTypes.STRING())
.physicalColumn("col3", DataTypes.STRING())
.build());

// drop columns
DropColumnEvent dropColumnEvent =
new DropColumnEvent(tableId, Arrays.asList("col3", "col5"));
Expand Down
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