Writing fails because of mutation size

The DataStax Apache Pulsar™ Connector collects records to write to the DataStax Enterprise (DSE) database in CQL BATCH commands. Data insertions and deletions in the records are known as mutations. The Connector is implemented to use single-partition batches that are submitted as one mutation operation.

For a given batch, if the total size of the mutation exceeds the maximum allowed by DSE (max_mutation_size_in_kb) the batch is rejected and the error message is written to the system.log, by default in var/log/cassandra.

For example:

Mutation of 28087887 bytes is too large for the maximum size of 16777216

Remediation

Before making any changes, understand the relationship between the max_mutation_size_in_kb and commitlog_segment_size_in_mb settings. The max_mutation_size_in_kb value is calculated as half of commitlog_segment_size_in_mb.

Defaults when not set explicitly in cassandra.yaml:

  • commitlog_segment_size_in_mb: 32 MB

  • max_mutation_size_in_kb: 16384 (16 MB)

If you set max_mutation_size_in_kb explicitly, also set commitlog_segment_size_in_mb to:

2 * max_mutation_size_in_kb / 1024

You can decrease the batch size by lowering the number of records collected in each batch (maxNumberOfRecordsInBatch). The default is 32.

If you cannot decrease the size of your batches, test whether increases to max_mutation_size_in_kb and commitlog_segment_size_in_mb result in batches completing successfully, without consuming too much RAM on the partition’s node. Also increase the DSE database batch threshold using batch_size_warn_threshold_in_kb.

Investigate why the mutations are larger than expected. Look for underlying issues with your client application, access patterns, and data model, because increasing the commitlog segment size is a limited fix.