Collection does not appear to be sharded, fallback to…

Error message

Collection {} does not appear to be sharded, fallback to SampleSplitter.

What it means

ShardedSplitStrategy.split first reads the collection's metadata from config.collections. If the metadata is missing or the collection is not marked sharded, it logs this warning and falls back to SampleBucketSplitStrategy (sampling-based splitting). Splits are still produced, just via sampling instead of chunk boundaries.

Solutions

Shard the collection (sh.shardCollection) if chunk-based splitting is desired.
Or explicitly configure the SampleSplitter/sampled split strategy for unsharded collections.
Verify collection is sharded: db.collection.getShardVersion() or sh.status() in mongo shell.
Ignore if fallback sampling is acceptable — this is only a warning.

Example fix

// config before (wrong for unsharded collection)
split.strategy = SHARDED
// after
split.strategy = SAMPLED

Defensive patterns

Strategy: validation

Validate before calling

// mongosh: confirm sharded before choosing SHARDED strategy
const md = db.getSiblingDB('config').collections.findOne({_id: 'mydb.mycoll'});
if (!md || !md.sharded) print('NOT SHARDED -> use SampleSplitter');

Prevention

Match split strategy to actual collection sharding state.
Run sh.status() during environment setup to inventory sharded namespaces.
Re-check sharding state after topology changes.

When it happens

Trigger: Calling split() on a collection whose config.collections entry is absent or whose 'sharded'/partitioned flag is false — i.e. an unsharded collection was assigned to the sharded split strategy.

Common situations: Configuring split.strategy=sharded (chunk-based) against a replica-set or unsharded collection; collection was unsharded after config was written; mongos routing to a non-sharded namespace.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

AI-assisted analysis of apache/seatunnel@cf67b549a7 (2026-09-10). Data as JSON: /api/errors/52a70a1003df8df7. Report an issue: GitHub.

Appendix: source

Thrown at seatunnel-connectors-v2/connector-cdc/connector-cdc-mongodb/src/main/java/org/apache/seatunnel/connectors/seatunnel/cdc/mongodb/source/splitters/ShardedSplitStrategy.java:62

@Slf4j
public class ShardedSplitStrategy implements SplitStrategy {

    public static final ShardedSplitStrategy INSTANCE = new ShardedSplitStrategy();

    private ShardedSplitStrategy() {}

    @Override
    public Collection<SnapshotSplit> split(@Nonnull SplitContext splitContext) {
        TableId collectionId = splitContext.getCollectionId();
        MongoClient mongoClient = splitContext.getMongoClient();

        List<BsonDocument> chunks;
        BsonDocument collectionMetadata;
        try {
            collectionMetadata = readCollectionMetadata(mongoClient, collectionId);
            if (!isValidShardedCollection(collectionMetadata)) {
                log.warn(
                        "Collection {} does not appear to be sharded, fallback to SampleSplitter.",
                        collectionId);
                return SampleBucketSplitStrategy.INSTANCE.split(splitContext);
            }
            chunks = readChunks(mongoClient, collectionMetadata);
        } catch (MongoQueryException e) {
            if (e.getErrorCode() == UNAUTHORIZED_ERROR) {
                log.warn(
                        "Unauthorized to read config.collections or config.chunks: {}, fallback to SampleSplitter.",
                        e.getErrorMessage());
            } else {
                log.warn(
                        "Read config.chunks collection failed: {}, fallback to SampleSplitter",
                        e.getErrorMessage());
            }
            return SampleBucketSplitStrategy.INSTANCE.split(splitContext);
        }

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