Understanding Database Caching and When to Modify Cache Settings

Caching is a crucial component in optimizing performance and reducing database load. However, modifying cache configurations without understanding their impact may lead to unintended consequences. This article explains how database caching works, when adjustments are necessary, and how to interpret cache hit rates correctly.

How Database Caching Works

Our system employs internal caching mechanisms to store frequently accessed data in memory, reducing the need for repetitive database queries. One key aspect of this is the roles-to-permissions mapping, which is cached to enhance access control performance.

Official Caching Rules:

The execution of SQL queries incurs significant overhead, so ESE caches information retrieved from databases. The predefined cache durations are:

• Authentication data: Cached for one minute.

• User permissions: Cached for five minutes.

• Role permissions: Cached for one full day.

• Authentication failures: Cached for one minute.

As cache size is limited, evictions may occur before the expiration time. These caching rules also apply to the corresponding REST API data.

Common Misconceptions About Cache Modifications

Customers sometimes request changes to cache settings, assuming it will reduce database load. However, increasing cache hit rates is an indicator that caching is working efficiently. Reducing cache duration may actually increase database queries, causing additional strain.

Key Considerations Before Modifying Cache Settings:

1. Understand Cache Hit Rate:

   • A high cache hit rate means data is efficiently retrieved from cache instead of the database.

   • A low cache hit rate may indicate frequent evictions, requiring investigation.

2. Impact of Lowering Cache Duration:

   • Reducing cache lifespan forces more frequent database queries, increasing load instead of decreasing it.

   • Instead, consider adjusting the data update strategy if permissions are changing frequently.

3. Alternative Cache Properties:

   • Beyond roles-to-permissions caching, our system provides additional cache properties that may be more relevant to adjust.

   • Consult documentation or support before making changes.

Metrics to Watch

To properly evaluate caching efficiency and avoid unnecessary modifications, consider monitoring the following metrics:

• Authentication Cache Hit Rate:

  • com.hivemq.extensions.ese.sql.authentication.cache.hitrate.<realm_name>

  • Compare this with com.hivemq.extensions.ese.sql.user.time.<realm_name> to determine how many requests bypass the cache and hit the database.

• Permission Caching Efficiency:

  • com.hivemq.extensions.ese.sql.permission.time.<realm_name>

  • sql.permission.role.cache.hitrate.<realm_name> and sql.permission.user.cache.hitrate.<realm_name>. Helps analyze how often permissions-related queries hit the database instead of being served from cache.

If a customer is concerned about database load, retrieving these metrics will provide clarity on whether caching is effectively reducing the number of database queries.

Recommended Approach

Before adjusting cache configurations, evaluate the actual database load and analyze cache behavior. If the goal is to reduce database failures, changing cache settings may not be the best approach. Instead:

• Monitor database performance and identify bottlenecks.

• Review existing caching strategies to ensure proper usage.

• Retrieve relevant cache metrics to gain insight into current performance.

• Contact support for guidance on cache tuning.

Conclusion

Database caching is designed to optimize performance and reduce load. Misinterpreting cache usage and modifying settings without a clear strategy may lead to increased database queries and performance degradation. Always analyze cache hit rates and overall system behavior before making changes. Monitoring key metrics can help ensure the best approach for maintaining system stability and efficiency.

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