While PostgreSQL is known for its robustness, scalability, and reliability, data inconsistency can occur in PostgreSQL clusters, which can cause issues and impact the overall performance of the system. In this blog, we’ll define data inconsistency in PostgreSQL clusters, discuss the challenges it poses, its causes, and provide some tips on how to prevent and resolve it if it occurs.

What is data inconsistency in a PostgreSQL cluster?

Data inconsistency refers to situations where data in different nodes of a cluster becomes out of sync, leading to inconsistent results and potentially causing data corruption. This situation arises when there is a delay in synchronizing the data across different nodes in the cluster, which can cause inconsistencies, errors, and other issues.

For instance, if two different nodes in a PostgreSQL cluster store different values for the same record, a query that returns results from both nodes may result in two different values for the same record, causing confusion, errors, and other issues.

Challenges of data inconsistency in PostgreSQL clusters

Data inconsistency in PostgreSQL clusters poses several challenges, including:

1. Inaccurate results: Query results may vary depending on which node is queried, leading to inconsistencies in the application. This also makes it difficult to make informed decisions based on the data.
2. Data integrity issues: Inconsistent data can lead to data integrity issues, which can cause problems when trying to update or delete records for transactions that rely on consistent data across all nodes. 
3. Performance issues: Data inconsistency can also impact the performance of the system, making it slower and less responsive.
4. Difficult to debug: Debugging issues that arise due to data inconsistency can be challenging, especially when trying to identify the source of the problem.

Causes of data inconsistency in PostgreSQL clusters

Data inconsistency can occur in PostgreSQL clusters due to several reasons, including:

1. Network issues: Synchronization delays can arise due to network issues, such as high latency, packet loss, and bandwidth constraints.
2. Upgrades: Schemas changes and database upgrades can cause temporary inconsistencies in the cluster. 
3. Replication lag: Replication lag refers to the delay in synchronizing data across different nodes in the cluster due to replication processes, such as streaming replication and logical replication.
4. Hardware failure: Hardware failures such as disk or power failure can lead to data inconsistency across the cluster. 

Preventing data inconsistency in PostgreSQL clusters

Preventing data inconsistency in PostgreSQL clusters involves taking several measures, including:

1. Use synchronous replication: Synchronous replication ensures that data is synchronized across different nodes in the cluster before a transaction is committed, minimizing the risk of data inconsistency. This, however, comes with a performance overhead. 
2. Implement a load balancer: Implementing a load balancer ensures that queries are sent to the same node that holds the data, minimizing the risk of data inconsistency.
3. Use monitoring tools: Using monitoring tools can help with the early identification of issues that can lead to data inconsistency, allowing you to take corrective measures before they become serious.
4. Maintenance windows: Perform schema changes or database upgrades during maintenance windows to minimize disruptions. 
5. Regular testing: Regularly test the cluster and failover scenarios to ensure that replication and backups are functioning as expected. 

Resolving data inconsistency in PostgreSQL clusters

Resolving data inconsistency in PostgreSQL clusters involves taking several steps, including:

1. Resynchronization: Perform manual resynchronization between nodes to correct data inconsistencies. 
2. Rollback: Use tools like pg_rewind and pg_backrest to perform point-in-time recovery and restore data consistency
3. Monitor the system: Monitoring the system after resolving data inconsistency issues is essential to ensure the problem doesn’t recur and identify other potential issues.

Conclusion

Data inconsistency can be a serious issue for PostgreSQL clusters, leading to inconsistent results, data corruption, and potential data loss. By following best practices for prevention and resolution, PostgreSQL administrators can maintain a reliable and consistent database environment.

A more abstract view of highly available PostgreSQL and its challenges is available in my previous blog: High availability made easy: A 100,000 ft view of auto failover in PostgreSQL.