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https://issues.apache.org/jira/browse/FLINK-40795?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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ASF GitHub Bot updated FLINK-40795:
-----------------------------------
Labels: pull-request-available (was: )
> Re-added table with drifted schema is silently skipped: downstream table not
> re-created and stale schema state causes write failures
> ------------------------------------------------------------------------------------------------------------------------------------
>
> Key: FLINK-40795
> URL: https://issues.apache.org/jira/browse/FLINK-40795
> Project: Flink
> Issue Type: Bug
> Components: Flink CDC
> Affects Versions: cdc-3.6.0
> Reporter: peiyu
> Priority: Major
> Labels: pull-request-available
>
> h2. *Symptom*
> In a pipeline with `scan.newly-added-table.enabled: true` (verified with
> MySQL -> Paimon),
> when a table is removed from the pipeline configuration and later added back,
> while its
> upstream schema drifted in between (e.g. a column was dropped while it was
> out of capture):
> 1. The downstream table is NOT re-created automatically, even though the
> source re-snapshots
> the table and emits a `CreateTableEvent` carrying the latest schema.
> 2. If the downstream table is created manually with the current upstream
> schema, the job
> fails with metadata/schema mismatch errors when writing data, because the
> pipeline keeps
> coercing records into the stale schema recorded in state.
> h2. *Steps to Reproduce*
> 1. Run a MySQL -> Paimon pipeline with `scan.newly-added-table.enabled: true`
> capturing
> table `t`; take a savepoint and stop.
> 2. Remove `t` from the pipeline's `tables` configuration, and drop the Paimon
> table `t`.
> 3. Restart from the savepoint, run a few checkpoints, stop with a savepoint
> again.
> 4. Drop a column from `t` in MySQL (while it is out of capture), then add `t`
> back to the
> pipeline configuration.
> 5. Restart from the savepoint.
> Actual: the source re-snapshots `t`, but the Paimon table is never created.
> The coordinator
> logs "Schema change event CreateTableEvent{...} is redundant for current
> schema ..., just
> skip it."
> Expected: `t` is re-created downstream with its current schema and data flows
> normally.
> The same reproduces at unit level on current master: restore a regular
> `SchemaCoordinator`
> from a checkpoint containing schema V1 of table `t`, then send
> `CreateTableEvent(t, V2)`
> with V2 != V1. `SchemaChangeResponse#getAppliedSchemaChangeEvents()` is empty
> and the
> `MetadataApplier` is never invoked.
> h2. *Root Cause*
> 1. The MySQL enumerator DOES clean up its own state when the config no longer
> matches `t`
> (`MySqlSnapshotSplitAssigner#captureNewlyAddedTables`, removal branch), so
> a re-added
> `t` is correctly detected as newly-added, re-snapshotted, and the source
> emits a
> `CreateTableEvent` carrying the current (drifted) schema. This part works
> as intended.
> 2. The `SchemaCoordinator`'s schema state, however, is never cleaned up:
> `SchemaManager#originalSchemas` / `#evolvedSchemas` (checkpointed) have no
> removal API
> at all, so the stale V1 schema of the removed table survives across
> savepoints.
> 3. `SchemaCoordinator#applySchemaChange` gates every event through
> `SchemaUtils#isSchemaChangeEventRedundant`, whose `CreateTableEvent`
> branch decides
> "redundant" based only on the presence of *some* recorded schema:
> createTableEvent -> {
> // It has been applied if such table already exists
> return latestSchema.isPresent();
> },
> The recorded schema is never compared with the incoming one, so the
> `CreateTableEvent`
> carrying the drifted schema is silently dropped:
> - `MetadataApplier#applySchemaChange` is never invoked -> the downstream
> table is not
> created (symptom 1);
> - neither the original nor the evolved schema state is updated, and the
> stale evolved
> schema is pushed back to `SchemaOperator` (`refreshedEvolvedSchemas`)
> and to sink
> writers (`DataSinkWriterOperator#emitLatestSchema`), so records get
> coerced into the
> stale shape and written against a table with a different schema ->
> metadata mismatch
> (symptom 2).
> The redundancy check was introduced by FLINK-36690 (restructured by
> FLINK-37278) to skip
> duplicated `CreateTableEvent`s emitted in the snapshot stage. Skipping an
> *identical*
> duplicate is correct; skipping one that carries a drifted schema against
> stale state is not.
> h2. *Scope*
> - Regular schema evolution topology (default for MySQL and most sources);
> 3.3.0-3.6.0 and
> master are affected. 3.2 and earlier do not contain the redundancy check.
> - Any flow that re-snapshots a known table can hit this, including an ALTER
> landing
> mid-snapshot so that duplicated CreateTableEvents carry different schemas.
> - The distributed topology `SchemaCoordinator` has a similar gap
> (`getLeastCommonSchema`
> based derivation never narrows, and a `CreateTableEvent` is only emitted
> when no evolved
> schema exists); it can be tracked separately.
> - Known limitation remaining after the proposed fix: if the schema did NOT
> drift (identical
> `CreateTableEvent`) but the downstream table was dropped externally, the
> framework cannot
> distinguish it from a snapshot-stage duplicate and will not re-create it.
> h2. *Proposed Fix*
> 1. `SchemaUtils#isSchemaChangeEventRedundant`: treat a `CreateTableEvent` as
> redundant only
> when the recorded schema is identical to the incoming event's schema.
> 2. `SchemaCoordinator#deduceEvolvedSchemaChanges` (regular): when a
> `CreateTableEvent`
> targets a sink table that already has an evolved schema, emit
> `CreateTableEvent(targetSchema)` (idempotent create-or-ignore, covering an
> externally
> dropped table) followed by alignment events from
> `SchemaMergingUtils.getSchemaDifference(currentEvolvedSchema,
> targetSchema)` instead of
> forwarding the raw event. `targetSchema` honors the configured behavior:
> the incoming
> schema for EVOLVE / TRY_EVOLVE / EXCEPTION; `getLeastCommonSchema` (never
> narrowing) for
> LENIENT; the frozen current schema for IGNORE. Only the resulting
> `CreateTableEvent` is
> propagated downstream; alignment events are applied to the external system
> only, and the
> evolved-schema state update is guarded by the same redundancy check so
> alignment events
> are not applied to the state twice.
> 3. Tests: coordinator-level scenarios (EVOLVE with dropped/added column,
> LENIENT, IGNORE,
> identical-duplicate skip) plus unit tests for the utility change.
> h2. *Workaround (before a fixed release)*
> Create the downstream table manually with the schema recorded in the job
> state - i.e. the
> OLD schema including the column(s) dropped upstream, as nullable - instead of
> the current
> upstream schema. The pipeline coerces records into that schema (filling
> dropped columns
> with null) and writes succeed.
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