Template priority and composition in Elasticsearch

Introduction

Templates help you reuse search or index settings easily. Priority and composition decide which templates apply when multiple templates match.

You want to apply different settings to indexes based on their names.

You have multiple templates and want to control which one is used first.

You want to combine settings from several templates for an index.

You want to update or add settings without rewriting all templates.

You want to manage templates in a clear order to avoid conflicts.

Syntax

Elasticsearch

PUT _index_template/template_name
{
  "index_patterns": ["pattern*"],
  "priority": number,
  "composed_of": ["template1", "template2"],
  "template": {
    "settings": { ... },
    "mappings": { ... },
    "aliases": { ... }
  }
}

priority is a number. Higher means this template applies first.

composed_of lets you combine other templates by name.

Examples

This template applies to indexes starting with 'logs-' and sets 1 shard.

Elasticsearch

PUT _index_template/base_template
{
  "index_patterns": ["logs-*"],
  "priority": 5,
  "template": {
    "settings": {
      "number_of_shards": 1
    }
  }
}

This template has higher priority and applies to 'logs-2024*' indexes, setting replicas to 2.

Elasticsearch

PUT _index_template/high_priority_template
{
  "index_patterns": ["logs-2024*"],
  "priority": 10,
  "template": {
    "settings": {
      "number_of_replicas": 2
    }
  }
}

This template combines two templates and adds a refresh interval setting.

Elasticsearch

PUT _index_template/composed_template
{
  "index_patterns": ["logs-2024*"],
  "priority": 15,
  "composed_of": ["base_template", "high_priority_template"],
  "template": {
    "settings": {
      "refresh_interval": "30s"
    }
  }
}

Sample Program

This example creates three templates. The last one combines the first two and adds a refresh interval. When you create an index named 'app-2024-01', Elasticsearch applies the composed template with the highest priority. The index will have 2 shards, 1 replica, a refresh interval of 1 second, and mappings for 'user' and 'timestamp'.

Elasticsearch

PUT _index_template/base_template
{
  "index_patterns": ["app-*"],
  "priority": 5,
  "template": {
    "settings": {
      "number_of_shards": 2
    },
    "mappings": {
      "properties": {
        "user": { "type": "keyword" }
      }
    }
  }
}

PUT _index_template/high_priority_template
{
  "index_patterns": ["app-2024*"],
  "priority": 10,
  "template": {
    "settings": {
      "number_of_replicas": 1
    },
    "mappings": {
      "properties": {
        "timestamp": { "type": "date" }
      }
    }
  }
}

PUT _index_template/composed_template
{
  "index_patterns": ["app-2024*"],
  "priority": 15,
  "composed_of": ["base_template", "high_priority_template"],
  "template": {
    "settings": {
      "refresh_interval": "1s"
    }
  }
}

GET app-2024-01/_settings
GET app-2024-01/_mapping

Important Notes

Templates with higher priority override settings from lower priority templates.

When using composed_of, the order of templates matters for merging.

Always test your templates by creating a new index and checking its settings and mappings.

Summary

Templates let you reuse index settings and mappings easily.

Priority controls which template applies first when multiple match.

Composition lets you combine templates for flexible configuration.

Practice

1. What does the priority setting control in Elasticsearch index templates?

easy

A. Which template applies first when multiple templates match an index

B. The order of fields in the index mapping

C. The number of shards for the index

D. The refresh interval of the index

Solution

1. Step 1: Understand the role of priority in templates

   The priority setting determines which template Elasticsearch applies first when multiple templates match the same index.

2. Step 2: Differentiate priority from other settings

   Priority does not affect shard count, field order, or refresh interval; it only controls template application order.

3. Final Answer:

   Which template applies first when multiple templates match an index -> Option A

4. Quick Check:

   Priority controls template application order = B [OK]

Hint: Priority decides template order when multiple match [OK]

Common Mistakes:

• Confusing priority with shard settings
• Thinking priority affects field order
• Assuming priority controls refresh interval

2. Which of the following is the correct way to define a template with priority 10 in Elasticsearch?

easy

A. { "index_patterns": ["logs-*"], "priority": true, "template": { "settings": {} } }

B. { "index_patterns": ["logs-*"], "priority": "high", "template": { "settings": {} } }

C. { "index_patterns": ["logs-*"], "priority": 10, "template": { "settings": {} } }

D. { "index_patterns": ["logs-*"], "priority": null, "template": { "settings": {} } }

Solution

1. Step 1: Check the priority value type

   Priority must be a numeric value, such as 10, not a string or boolean.

2. Step 2: Validate JSON structure

   { "index_patterns": ["logs-*"], "priority": 10, "template": { "settings": {} } } correctly uses an integer 10 for priority and valid JSON structure for template.

3. Final Answer:

   { "index_patterns": ["logs-*"], "priority": 10, "template": { "settings": {} } } -> Option C

4. Quick Check:

   Priority is a number, not string or boolean = C [OK]

Hint: Priority must be a number, not string or boolean [OK]

Common Mistakes:

• Using string like "high" instead of number
• Setting priority to true or null
• Incorrect JSON structure for template

3. Given two templates: Template A with priority 5 and Template B with priority 10, both matching index logs-2024. Which template settings will Elasticsearch apply first?

medium

A. Template B settings will apply first

B. Elasticsearch throws an error due to conflict

C. Both templates apply simultaneously with no order

D. Template A settings will apply first

Solution

1. Step 1: Understand priority order

   Elasticsearch sorts matching templates by priority in ascending order (lowest first) and merges their configurations, with later (higher priority) templates overriding earlier ones.

2. Step 2: Compare priorities of Template A and B

   Template A has priority 5, which is lower than Template B's 10, so Template A applies first.

3. Final Answer:

   Template A settings will apply first -> Option D

4. Quick Check:

   Lower priority number applies first = B [OK]

Hint: Lower priority number means earlier application [OK]

Common Mistakes:

• Thinking higher priority applies first
• Thinking templates apply simultaneously
• Expecting error on priority conflict

4. You have two templates: Template X with composed_of referencing Template Y, but Template Y is missing. What will happen when you try to create an index matching Template X?

medium

A. Template X applies without Template Y's settings

B. Index creation fails with an error about missing template

C. Elasticsearch ignores Template X and uses default settings

D. Template Y is automatically created as empty

Solution

1. Step 1: Understand template composition with composed_of

   Templates using composed_of require referenced templates to exist.

2. Step 2: Check behavior when referenced template is missing

   If Template Y is missing, Elasticsearch throws an error during index creation because it cannot compose Template X fully.

3. Final Answer:

   Index creation fails with an error about missing template -> Option B

4. Quick Check:

   Missing composed_of template causes error = A [OK]

Hint: All composed_of templates must exist or error occurs [OK]

Common Mistakes:

• Assuming missing templates are ignored
• Expecting automatic creation of missing templates
• Thinking index creation succeeds without all templates

5. You want to create a flexible index template setup where a base template defines common settings, and multiple specialized templates add mappings for different data types. How should you use priority and composition to achieve this?

hard

A. Create a base template with low priority and use composed_of in specialized templates with higher priority

B. Create specialized templates with low priority and base template with highest priority without composition

C. Use only one template with all settings combined, no priority or composition needed

D. Set all templates with the same priority and avoid composition

Solution

1. Step 1: Use base template for common settings with low priority

   The base template should have a lower priority so it is merged first in the composition order, providing common settings that higher priority templates can override.

2. Step 2: Use composed_of in specialized templates with higher priority

   Specialized templates compose the base template and have higher priority to add or override mappings.

3. Step 3: Combine priority and composition for flexible layering

   This setup allows reusable base settings and flexible specialized additions.

4. Final Answer:

   Create a base template with low priority and use composed_of in specialized templates with higher priority -> Option A

5. Quick Check:

   Base low priority + composed_of in higher priority = D [OK]

Hint: Base template low priority, specialized templates compose with higher priority [OK]

Common Mistakes:

• Setting base template with highest priority
• Not using composition for reusable settings
• Using same priority for all templates