The insert_quorum setting in ClickHouse is a feature that ensures data consistency across replicated tables in a distributed environment. It specifies the minimum number of replicas that must successfully write data for an insert operation to be considered successful. This mechanism helps maintain data integrity and reliability in scenarios where multiple replicas are involved in storing and processing data.
Best Practices
Set an appropriate quorum value: Choose a quorum value that balances data consistency with system performance and availability.
Use with replicated tables: Insert quorum is most effective when used with replicated table engines like ReplicatedMergeTree.
Configure timeout settings: Adjust the insert_quorum_timeout setting to prevent long-running insert operations from blocking other queries.
Monitor quorum-related metrics: Keep track of insert_quorum_timeout exceptions and other related metrics to identify potential issues.
Combine with other consistency features: Use insert quorum alongside features like select_sequential_consistency for comprehensive data consistency.
Common Issues or Misuses
Setting too high quorum values: Overly strict quorum requirements can lead to frequent insert failures and reduced system availability.
Ignoring network partitions: Failing to account for network issues can result in unnecessary insert failures when replicas are temporarily unreachable.
Misunderstanding quorum behavior: Insert quorum ensures data is written to the specified number of replicas but doesn't guarantee immediate consistency across all replicas.
Overlooking performance impact: High quorum values can increase latency for insert operations, especially in geographically distributed clusters.
Neglecting proper error handling: Failing to handle insert_quorum_timeout exceptions appropriately can lead to data inconsistencies or application errors.
Additional Information
Insert quorum is configured using the following settings:
insert_quorum: Specifies the number of replicas required for a successful insert.insert_quorum_timeout: Sets the maximum time to wait for the quorum to be met.insert_quorum_parallel: Determines whether to wait for confirmation from replicas in parallel or sequentially.
These settings can be adjusted at the server, session, or query level to fine-tune the behavior of insert operations in different scenarios.
Frequently Asked Questions
Q: How does insert_quorum differ from replication factor?
A: While replication factor determines the total number of copies of data stored across replicas, insert_quorum specifies the minimum number of replicas that must acknowledge a successful write for an insert operation to be considered complete.
Q: Can insert_quorum be used with non-replicated tables?
A: No, insert_quorum is specifically designed for use with replicated table engines like ReplicatedMergeTree. It has no effect on non-replicated tables.
Q: What happens if the insert_quorum cannot be met within the specified timeout?
A: If the quorum is not met within the insert_quorum_timeout period, ClickHouse will throw an exception (TOO_FEW_LIVE_REPLICAS), and the insert operation will fail.
Q: Does insert_quorum guarantee that data is immediately readable from all replicas?
A: No, insert_quorum ensures that data is written to the specified number of replicas, but it doesn't guarantee immediate consistency across all replicas. There may be a delay before the data is visible on all replicas due to asynchronous replication.
Q: How can I monitor the effectiveness of my insert_quorum settings?
A: You can monitor system metrics related to insert operations, such as InsertedRows, InsertedBytes, and InsertQuery, along with exception counters like ExceptionCount for insert_quorum_timeout. Additionally, checking the replication queue and lag between replicas can provide insights into the overall health of your replicated tables.
