[Iceberg CDC] Add Snapshot Changelog Scanner#38836
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Summary of ChangesHello, I'm Gemini Code Assist1! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed! This pull request adds a new Highlights
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Need to merge #38821 first |
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Code Review
This pull request introduces Iceberg CDC support by adding ChangelogDescriptor, ChangelogScanner, and corresponding unit tests. The ChangelogScanner optimizes data processing by routing changelog tasks into unidirectional, small bidirectional, or large bidirectional paths. The review feedback highlights a critical issue where instance variables in the DoFn could leak state or cause a NullPointerException, suggests an early return optimization when primary key fields are empty, and recommends removing unused helper methods in both the scanner and its test suite.
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| public void process(@Element Long snapshotId, MultiOutputReceiver out) throws IOException { | ||
| resetLocalMetrics(); | ||
| // not using getRefreshed because upstream Watch should have already refreshed the | ||
| // table to a state where this snapshot exists | ||
| this.table = SerializableTable.copyOf(TableCache.get(scanConfig.getTableIdentifier())); | ||
| this.snapshot = table.snapshot(snapshotId); | ||
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| // refresh on miss | ||
| if (this.snapshot == null) { | ||
| this.table = | ||
| SerializableTable.copyOf(TableCache.getRefreshed(scanConfig.getTableIdentifier())); | ||
| this.snapshot = | ||
| checkStateNotNull( | ||
| table.snapshot(snapshotId), "Could not retrieve table snapshot: %s", snapshotId); | ||
| } |
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The table and snapshot fields are instance variables of the DoFn. Since Beam runners can reuse DoFn instances across multiple elements, these fields can leak state from previous elements if they are not reset. Furthermore, if TableCache.get() returns null (e.g., on a cache miss), calling table.snapshot(snapshotId) directly will throw a NullPointerException before the 'refresh on miss' block is ever reached.
To fix both issues, reset the instance fields at the start of process and perform a null check on the cached table before attempting to retrieve the snapshot.
| public void process(@Element Long snapshotId, MultiOutputReceiver out) throws IOException { | |
| resetLocalMetrics(); | |
| // not using getRefreshed because upstream Watch should have already refreshed the | |
| // table to a state where this snapshot exists | |
| this.table = SerializableTable.copyOf(TableCache.get(scanConfig.getTableIdentifier())); | |
| this.snapshot = table.snapshot(snapshotId); | |
| // refresh on miss | |
| if (this.snapshot == null) { | |
| this.table = | |
| SerializableTable.copyOf(TableCache.getRefreshed(scanConfig.getTableIdentifier())); | |
| this.snapshot = | |
| checkStateNotNull( | |
| table.snapshot(snapshotId), "Could not retrieve table snapshot: %s", snapshotId); | |
| } | |
| @ProcessElement | |
| public void process(@Element Long snapshotId, MultiOutputReceiver out) throws IOException { | |
| resetLocalMetrics(); | |
| this.table = null; | |
| this.snapshot = null; | |
| Table cachedTable = TableCache.get(scanConfig.getTableIdentifier()); | |
| if (cachedTable != null) { | |
| this.table = SerializableTable.copyOf(cachedTable); | |
| this.snapshot = this.table.snapshot(snapshotId); | |
| } | |
| // refresh on miss | |
| if (this.snapshot == null) { | |
| this.table = | |
| SerializableTable.copyOf(TableCache.getRefreshed(scanConfig.getTableIdentifier())); | |
| this.snapshot = | |
| checkStateNotNull( | |
| table.snapshot(snapshotId), "Could not retrieve table snapshot: %s", snapshotId); | |
| } |
| boolean metricsAvailable = true; | ||
| MetricsConfig metricsConfig = MetricsConfig.forTable(table); | ||
| Collection<String> pkFields = table.schema().identifierFieldNames(); | ||
| for (String field : pkFields) { | ||
| MetricsModes.MetricsMode mode = metricsConfig.columnMode(field); | ||
| if (!(mode instanceof MetricsModes.Full) && !(mode instanceof MetricsModes.Truncate)) { | ||
| metricsAvailable = false; | ||
| break; | ||
| } | ||
| } | ||
| if (metricsAvailable) { | ||
| scan = scan.includeColumnStats(pkFields); | ||
| } |
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If pkFields is empty, metricsAvailable remains true, which leads to calling scan.includeColumnStats(Collections.emptyList()). If there are no primary keys, we cannot perform file-level overlap analysis anyway. Returning early when pkFields is empty avoids unnecessary overhead.
Collection<String> pkFields = table.schema().identifierFieldNames();
if (pkFields.isEmpty()) {
return scan;
}
boolean metricsAvailable = true;
MetricsConfig metricsConfig = MetricsConfig.forTable(table);
for (String field : pkFields) {
MetricsModes.MetricsMode mode = metricsConfig.columnMode(field);
if (!(mode instanceof MetricsModes.Full) && !(mode instanceof MetricsModes.Truncate)) {
metricsAvailable = false;
break;
}
}
if (metricsAvailable) {
scan = scan.includeColumnStats(pkFields);
}| static String name(String path) { | ||
| return Iterables.getLast(Splitter.on("-").split(path)); | ||
| } |
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The name helper method is unused in this class. Removing it keeps the codebase clean and avoids an unused dependency on Splitter (which can also be removed from the imports if this is the only usage).
| private static StructLike partition(long id) { | ||
| PartitionKey partitionKey = new PartitionKey(IDENTITY_ID_SPEC, SINGLE_PK_SCHEMA); | ||
| GenericRecord record = GenericRecord.create(SINGLE_PK_SCHEMA); | ||
| record.setField("id", id); | ||
| record.setField("data", "partition-" + id); | ||
| partitionKey.partition(record); | ||
| return partitionKey; | ||
| } |
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The partition helper method is unused in the test class. Removing it keeps the test file clean.
1 participant
Adds a DoFn that scans each incoming snapshot and plans batches of changelog scan tasks. Each batch is routed to the appropriate downstream path depending on its contents and size. There's a lot of details in the java doc, but to summarize:
We perform a few optimizations to limit the amount of data we need to resolve:
Note: "update resolution" is logic that compares inserts and deletes to check if an update has occurred.
Part of #38831
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