Common Cassandra Errors

Reference guide for frequently encountered Cassandra errors, their causes, and solutions.

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Error Categories

Timeout Errors

Errors occurring when operations exceed configured time limits.

Error	Default Timeout	Common Cause
ReadTimeoutException	5000ms	Slow disk, large partitions, tombstones
WriteTimeoutException	2000ms	Overloaded nodes, disk issues
TruncateException	60000ms	Large table truncation

Legacy Exception

RangeSliceTimeoutException was a Thrift-era exception. In modern Cassandra (4.0+), range scan timeouts surface as ReadTimeoutException.

Availability Errors

Errors related to replica availability.

Error	Cause	Solution
UnavailableException	Insufficient replicas alive	Check node status, reduce consistency level
NoHostAvailableException	Cannot reach any coordinator	Check network, verify cluster is running
WriteFailureException	Replica write failed	Check failing node logs
ReadFailureException	Replica read failed	Check failing node logs

Data Errors

Errors related to data or schema.

Error	Cause	Solution
InvalidQueryException	CQL syntax or semantic error	Fix query syntax
InvalidRequestException	Invalid request parameters	Check request parameters
TombstoneOverwhelmingException	Too many tombstones	Fix data model, run compaction
SyntaxException	CQL syntax error	Check CQL syntax

Authentication/Authorization Errors

Security-related errors.

Error	Cause	Solution
AuthenticationException	Invalid credentials	Verify username/password
UnauthorizedException	Insufficient permissions	Grant required permissions

Lightweight Transaction Errors

Errors related to LWT (Paxos-based compare-and-set operations).

Error	Cause	Solution
CasWriteTimeoutException	Paxos consensus timeout	Check for contention, increase timeout
LWT/non-LWT mixing issues	Mixed clock domains	Use LWT consistently for all operations on same data
Silent operation failures	Paxos clock vs regular timestamp conflict	See LWT Troubleshooting

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Quick Diagnosis

Check Cluster Health

# Node status
nodetool status

# Thread pools (look for blocked/dropped)
nodetool tpstats

# Compaction status
nodetool compactionstats

Check Logs

# Recent errors
grep -i "error\|exception" /var/log/cassandra/system.log | tail -50

# Specific error
grep "TimeoutException" /var/log/cassandra/system.log | tail -20

Check Table Health

# Table statistics
nodetool tablestats my_keyspace.my_table

# Key metrics to check:
# - SSTable count (high = needs compaction)
# - Average tombstones per read (high = data model issue)
# - Partition size (large = data model issue)

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Error Resolution Workflow

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Detailed Error Guides

Timeout Errors

• ReadTimeoutException - Read operations timing out
• WriteTimeoutException - Write operations timing out

Lightweight Transaction Errors

• LWT Troubleshooting - Paxos contention, LWT/non-LWT mixing, timeout handling

Playbooks for Complex Issues

For issues requiring multi-step resolution, see the Troubleshooting Playbooks.

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Prevention

Monitoring

Set up alerts for early warning:

Metric	Warning Threshold	Critical Threshold
Read latency p99	> 100ms	> 500ms
Write latency p99	> 50ms	> 200ms
Pending compactions	> 20	> 100
Dropped messages	> 0	> 10/min
SSTable count	> 20 per table	> 50 per table

Best Practices

1. Monitor proactively - Don't wait for errors
2. Run repairs regularly - Weekly for most workloads
3. Keep compaction healthy - Monitor pending tasks
4. Size partitions correctly - Aim for < 100MB per partition
5. Avoid tombstone accumulation - Use TTLs and proper deletion patterns

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Related Documentation

• Troubleshooting Overview - General troubleshooting framework
• Diagnosis Guide - Systematic diagnosis procedures
• Log Analysis - Understanding Cassandra logs
• Playbooks - Step-by-step resolution guides