Write to . partition …

Error message

Write to %s.%s partition %s: %s (WriteWarningsSnapshot.writeSizeWarnMessage(value))

What it means

CoordinatorWriteWarnings.processWarnings() reports per-table write-size warnings aggregated from replicas. When a written partition value exceeded the write-size warning threshold, a message 'Write to <ks>.<table> partition <key>: <detail>' is sent as a client warning, logged, and writeSizeWarnings metric is incremented. The write succeeded but was unusually large.

Solutions

Redesign the schema to split large partitions (bucketing keys, time buckets)
Cap row/collection sizes at the application layer before writing
Raise the write-size warning threshold in cassandra.yaml only deliberately
Inspect the warning's partition key to find and fix the offending writer

Example fix

// before: one partition per user grows forever
INSERT INTO events (user_id, ts, payload) VALUES (?, ?, ?);
// after: bucket by day
INSERT INTO events_by_day (user_id, day, ts, payload) VALUES (?, ?, ?, ?);

Defensive patterns

Strategy: validation

Validate before calling

// application-side partition size guard
if (estimatePartitionBytes(key) + pendingWriteBytes > writeSizeWarnThreshold)
    throw new IllegalStateException("write would exceed partition size warning threshold for " + key);

Try / catch

ResultSet rs = session.execute(write);
for (String w : rs.getExecutionInfo().getWarnings())
    if (w.startsWith("Write to ")) partitionSizer.alert(w);

Prevention

Bound partition sizes with bucketed keys
Avoid unbounded collections/rows per partition
Alert on writeSizeWarnings metric

When it happens

Trigger: Mutations whose partition value (data written to a single partition key) exceeded the write_size warning threshold; the coordinator's next processWarnings call formats the message and warns the client.

Common situations: Unbounded partition growth (e.g. time-series rows never partitioned); very large blobs/collections in one partition; batch writes inflating partition size.

Understand the failure class

Background: payload too large / request exceeds maximum size: why libraries cap bytes and how to fix oversize payloads — this error's family across 50 libraries.

Related errors

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/80ce4aa48ab81fd2. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/service/writes/thresholds/CoordinatorWriteWarnings.java:139

        for (Map.Entry<TableId, Long> entry : snapshot.writeSize.tableValues.entrySet())
        {
            TableId tableId = entry.getKey();
            ColumnFamilyStore cfs = Schema.instance.getColumnFamilyStoreInstance(tableId);
            if (cfs == null)
            {
                logger.warn("ColumnFamilyStore is null for table {}, skipping", tableId);
                continue;
            }

            TableMetadata metadata = cfs.metadata();
            String partitionKey = metadata.partitionKeyType.toCQLString(warnings.partitionKey.getKey());
            String msg = String.format("Write to %s.%s partition %s: %s",
                                       metadata.keyspace,
                                       metadata.name,
                                       partitionKey,
                                       WriteWarningsSnapshot.writeSizeWarnMessage(entry.getValue()));
            ClientWarn.instance.warn(msg);
            logger.warn(msg);
            cfs.metric.writeSizeWarnings.mark();
        }

        for (Map.Entry<TableId, Long> entry : snapshot.writeTombstone.tableValues.entrySet())
        {
            TableId tableId = entry.getKey();
            ColumnFamilyStore cfs = Schema.instance.getColumnFamilyStoreInstance(tableId);
            if (cfs == null)
            {
                logger.warn("ColumnFamilyStore is null for table {}, skipping", tableId);
                continue;
            }

            TableMetadata metadata = cfs.metadata();
            String partitionKey = metadata.partitionKeyType.toCQLString(warnings.partitionKey.getKey());
            String msg = String.format("Write to %s.%s partition %s: %s",
                                       metadata.keyspace,

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