Elasticsearch Refresh Interval: Definition, Best Practices, and FAQs

What is a refresh interval?

Refresh interval in Elasticsearch is the time period between index refreshes. A refresh makes recent changes to an index visible to search. By default, Elasticsearch refreshes indices every second, which is why it's often described as providing near real-time search capabilities. The refresh interval can be configured to balance between search performance and indexing throughput.

The refresh operation is relatively lightweight but still has a cost. It creates a new segment in memory, which later gets merged and committed to disk. While a shorter refresh interval provides more up-to-date search results, it can impact indexing performance and increase segment counts.

Elasticsearch also provides the index.refresh_interval setting at the index level, allowing for fine-grained control over different indices based on their specific requirements.

Best practices

1. Adjust refresh interval based on your use case:

   • For write-heavy workloads, increase the refresh interval to reduce indexing overhead.
   • For search-heavy applications requiring up-to-date results, keep the interval short.

2. Use ?refresh=wait_for on indexing requests when you need immediate visibility of new documents.

3. Consider setting refresh_interval: -1 for bulk indexing operations to disable automatic refreshes.

4. Monitor refresh times using the _cat/indices API to ensure they're not impacting overall cluster performance.

5. Use time-based indices with different refresh intervals for hot and cold data.

Common issues or misuses

1. Setting refresh interval too low, causing unnecessary load on the cluster.

2. Forgetting to re-enable refreshes after bulk indexing operations.

3. Misunderstanding the relationship between refresh interval and transaction log flushes.

4. Expecting real-time consistency with default settings in distributed environments.

5. Overlooking the impact of refresh interval on segment merging and overall indexing performance.

Frequently Asked Questions

Q: How does refresh interval affect search latency?
A: A shorter refresh interval reduces search latency by making new documents visible more quickly, but it can increase overall system load due to more frequent refresh operations.

Q: Can I change the refresh interval dynamically?
A: Yes, you can update the refresh interval dynamically using the Update Index Settings API without closing the index.

Q: What's the difference between refresh and flush operations?
A: Refresh makes recent changes searchable without writing to disk, while flush writes the transaction log and in-memory segments to disk, clearing the transaction log.

Q: How does refresh interval impact indexing throughput?
A: A longer refresh interval generally improves indexing throughput by reducing the frequency of refresh operations, allowing more documents to be indexed between refreshes.

Q: Is it possible to trigger a manual refresh?
A: Yes, you can trigger a manual refresh using the Refresh API by sending a POST request to /_refresh or /{index}/_refresh for specific indices.