Cache management (query, request, field data) in Elasticsearch

Introduction

Cache helps Elasticsearch find data faster by saving recent searches and results. Managing cache well keeps your searches quick and your system smooth.

When you want to speed up repeated searches on the same data.

When you notice slow search responses and want to improve performance.

When you want to control memory use by clearing old or unused cached data.

When you update data and want to make sure searches use fresh information.

When you want to understand how Elasticsearch stores and reuses search results.

Syntax

Elasticsearch

POST /_cache/clear
{
  "query": true,
  "fielddata": true,
  "request": true
}

This example shows how to clear different caches in Elasticsearch.

You can clear query cache, field data cache, or request cache separately or all at once.

Examples

This clears only the query cache for the index named my_index.

Elasticsearch

POST /my_index/_cache/clear
{
  "query": true
}

This clears only the field data cache, which stores data for sorting and aggregations.

Elasticsearch

POST /my_index/_cache/clear
{
  "fielddata": true
}

This clears the request cache, which stores results of frequent searches.

Elasticsearch

POST /my_index/_cache/clear
{
  "request": true
}

This clears all caches on all indices.

Elasticsearch

POST /_cache/clear
{
  "query": true,
  "fielddata": true,
  "request": true
}

Sample Program

This example first runs a search with request cache enabled, then clears the request cache for my_index.

Elasticsearch

POST /my_index/_search?request_cache=true
{
  "query": {
    "match": { "message": "hello" }
  }
}

# Then clear the request cache

POST /my_index/_cache/clear
{
  "request": true
}

Important Notes

Clearing cache can slow down searches temporarily because Elasticsearch must rebuild the cache.

Use cache clearing carefully in busy systems to avoid performance drops.

Request cache works best for repeated identical queries.

Summary

Cache stores recent search data to speed up Elasticsearch queries.

You can clear query, field data, or request caches separately.

Managing cache helps keep search fast and system stable.

Practice

1. What is the primary purpose of cache management in Elasticsearch?

easy

A. To store recent search data and speed up query responses

B. To permanently save all search results for future use

C. To delete all data from the Elasticsearch index

D. To increase the size of the Elasticsearch cluster automatically

Solution

1. Step 1: Understand what cache does in Elasticsearch

   Cache temporarily stores recent search data to avoid repeating expensive operations.

2. Step 2: Identify the main benefit of caching

   By storing recent data, Elasticsearch can respond faster to repeated queries.

3. Final Answer:

   To store recent search data and speed up query responses -> Option A

4. Quick Check:

   Cache speeds up queries = A [OK]

Hint: Cache is for speed, not permanent storage [OK]

Common Mistakes:

• Thinking cache saves data permanently
• Confusing cache with index storage
• Assuming cache increases cluster size

2. Which of the following is the correct syntax to clear the query cache for an index named products using Elasticsearch REST API?

easy

A. POST /products/_cache/clear?query=true

B. POST /products/_cache/clear/query

C. POST /products/_cache/clear/fielddata

D. POST /products/_clear_cache/query

Solution

1. Step 1: Recall the correct REST API endpoint for clearing cache

   Elasticsearch uses _cache/clear with query parameters to specify cache types.

2. Step 2: Identify the correct parameter for query cache

   The query cache is cleared by adding ?query=true to the endpoint.

3. Final Answer:

   POST /products/_cache/clear?query=true -> Option A

4. Quick Check:

   Use query=true parameter to clear query cache [OK]

Hint: Use query=true parameter to clear query cache [OK]

Common Mistakes:

• Using wrong endpoint like _clear_cache
• Confusing fielddata cache with query cache
• Missing query parameter in URL

3. Given this Elasticsearch request to clear caches:

POST /myindex/_cache/clear
{
  "fielddata": true,
  "query": true
}

What caches will be cleared?

medium

A. Only the query cache

B. Only the fielddata cache

C. Request cache only

D. Both query and fielddata caches

Solution

1. Step 1: Analyze the JSON body parameters

   The request sets both fielddata and query to true, indicating both caches should be cleared.

2. Step 2: Understand cache clearing behavior

   When multiple cache types are true, Elasticsearch clears all specified caches.

3. Final Answer:

   Both query and fielddata caches -> Option D

4. Quick Check:

   fielddata=true + query=true clears both caches [OK]

Hint: True flags clear all specified caches together [OK]

Common Mistakes:

• Assuming only one cache clears at a time
• Confusing request cache with fielddata cache
• Ignoring JSON body parameters

4. You run this command to clear the request cache:

POST /logs/_cache/clear
{
  "request": true
}

But the cache is not cleared. What is the likely problem?

medium

A. The index name logs is invalid for cache clearing

B. The syntax is incorrect; request cache cannot be cleared this way

C. Request cache is disabled by default and must be enabled first

D. You must specify fielddata as true to clear request cache

Solution

1. Step 1: Understand request cache behavior

   Request cache is off by default and must be enabled in index settings to be used and cleared.

2. Step 2: Identify why clearing fails

   If request cache is disabled, clearing it has no effect, so the command appears to do nothing.

3. Final Answer:

   Request cache is disabled by default and must be enabled first -> Option C

4. Quick Check:

   Request cache off by default = no clearing effect [OK]

Hint: Enable request cache before clearing it [OK]

Common Mistakes:

• Assuming request cache is always enabled
• Using wrong syntax to clear caches
• Thinking fielddata cache affects request cache

5. You want to optimize Elasticsearch performance by managing caches. Which strategy correctly balances cache clearing and system stability?

hard

A. Clear all caches frequently to free memory, even during heavy query load

B. Clear query and request caches during low traffic, and monitor fielddata cache size

C. Never clear caches to keep all data in memory permanently

D. Disable all caches to avoid stale data and improve stability

Solution

1. Step 1: Understand cache clearing impact

   Clearing caches frees memory but can slow queries if done too often or during heavy load.

2. Step 2: Identify best practice for cache management

   Clearing query and request caches during low traffic avoids performance hits; monitoring fielddata cache prevents memory issues.

3. Final Answer:

   Clear query and request caches during low traffic, and monitor fielddata cache size -> Option B

4. Quick Check:

   Clear caches smartly during low load + monitor fielddata [OK]

Hint: Clear caches during low load, monitor fielddata size [OK]

Common Mistakes:

• Clearing caches too often during heavy load
• Disabling caches completely
• Ignoring fielddata cache memory use