apache/cassandra · error · RuntimeException

Last written key %s >= current key %s, writing into %s

Error message

Last written key %s >= current key %s, writing into %s

What it means

SortedTableWriter.verifyPartition enforces the SSTable invariant that partitions must be written in strictly increasing decorated-key order. If the key about to be appended compares <= the last written key, it throws, because BigFormat requires sorted input and would otherwise produce an unreadable/invalid SSTable.

Source

Thrown at src/java/org/apache/cassandra/io/sstable/format/SortedTableWriter.java:198

        }
        catch (IOException e)
        {
            throw new FSWriteError(e, getFilename());
        }
    }

    private boolean verifyPartition(DecoratedKey key)
    {
        assert key != null : "Keys must not be null"; // empty keys ARE allowed b/c of indexed column values

        if (key.getKey().remaining() > FBUtilities.MAX_UNSIGNED_SHORT)
        {
            logger.error("Key size {} exceeds maximum of {}, skipping row", key.getKey().remaining(), FBUtilities.MAX_UNSIGNED_SHORT);
            return false;
        }

        if (lastWrittenKey != null && lastWrittenKey.compareTo(key) >= 0)
            throw new RuntimeException(String.format("Last written key %s >= current key %s, writing into %s", lastWrittenKey, key, getFilename()));

        return true;
    }

    private void startPartition(DecoratedKey key, DeletionTime partitionLevelDeletion) throws IOException
    {
        partitionWriter.start(key, partitionLevelDeletion);
        metadataCollector.updatePartitionDeletion(partitionLevelDeletion);

        onStartPartition(key);
    }

    private void addStaticRow(DecoratedKey key, Row row) throws IOException
    {
        guardCollectionSize(key, row);

        partitionWriter.addStaticRow(row);
        if (!row.isEmpty())

View on GitHub (pinned to 88fd0f6a0e)

Solutions

1. Sort partitions by DecoratedKey before appending (DecoratedKey.compareTo, which orders by token then key bytes).
2. For bulk loads, feed the writer from an ordered source or use tools that sort internally (e.g. BulkLoader/CQLSSTableWriter usage guidelines).
3. If writing an SSTable from unsorted data, buffer and externally merge-sort by token first.
4. Check custom compaction/streaming code for iteration order assumptions.

Example fix

// before: appending keys in arbitrary order
for (UnfilteredRowIterator part : unsortedPartitions)
    writer.append(part);
// after: sort by decorated key first
unsortedPartitions.sort(Comparator.comparing(p ->
    partitioner.decorateKey(StorageEngine.getPartitionKey(p))));
for (UnfilteredRowIterator part : unsortedPartitions)
    writer.append(part);

// Java: assert incoming partitions are sorted before writing
DecoratedKey prev = null;
for (UnfilteredRowIterator part : partitions) {
    DecoratedKey k = decorateKey(part.partitionKey());
    if (prev != null && prev.compareTo(k) >= 0)
        throw new IllegalStateException("Unsorted partitions at " + k);
    prev = k;
}

try {
    writer.append(partition);
} catch (RuntimeException e) {
    if (e.getMessage().contains("Last written key"))
        logger.error("Bulk loader emitted unsorted keys — re-sort source data", e);
    throw e;
}

When it happens

Trigger: Calling sstableWriter.append(partition, ...) with keys not sorted by DecoratedKey — typically a compaction/scrub/rebuild iterating sources out of order, a custom streaming or bulk-loader writing unsorted partitions, or merging sources that yield keys out of order.

Common situations: Custom bulk load tools (sstable writer APIs) feeding unsorted data; bugs in custom compaction strategies or external stream receivers; repairing data with mismatched sort order after reading from an unordered source.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• No holder claimed
• SSTable format name in %s cannot be null
• SSTable format name for %s must be non-empty, lower-case let
• Multiple sstable format implementations with the same name %
• Configuration contains options of unknown sstable formats: %

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/bba71811e1531a7e. Report an issue: GitHub.