Template priority and composition in Elasticsearch - Step-by-Step Execution

Concept Flow - Template priority and composition

Define Base Template

↓

Define Child Template

↓

Set Priorities

↓

Elasticsearch Merges Templates

↓

Final Composed Template Applied

Templates are defined with priorities; Elasticsearch merges them by priority to create the final template.

Execution Sample

Elasticsearch

{
  "index_patterns": ["logs-*"],
  "priority": 10,
  "template": { "settings": { "number_of_shards": 1 } }
}

// Another template with priority 20 overrides settings

Two templates with different priorities are merged; the higher priority template settings override the lower.

Execution Table

Step	Template Name	Priority	Action	Resulting Settings
1	base_template	10	Apply base template settings	{"number_of_shards": 1}
2	child_template	20	Apply child template settings, overrides base	{"number_of_shards": 3}
3	Final	-	Merged templates by priority	{"number_of_shards": 3}

💡 All templates merged; highest priority settings override lower priority ones.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
settings.number_of_shards	undefined	1	3	3
applied_templates	[]	[base_template]	[base_template, child_template]	[base_template, child_template]

Key Moments - 2 Insights

Why does the child template setting override the base template setting?

Because the child template has a higher priority (20) than the base template (10), Elasticsearch merges templates by priority, applying higher priority settings last (see execution_table step 2).

What happens if two templates have the same priority?

If priorities are equal, Elasticsearch merges templates in the order they are loaded, so the last loaded template's settings override earlier ones.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of number_of_shards after step 1?

A 3

B undefined

C 1

D 0

💡 Hint

Check the 'Resulting Settings' column at step 1 in the execution_table.

At which step does the number_of_shards setting change to 3?

A Step 1

B Step 2

C Step 3

D It never changes

💡 Hint

Look at the 'Action' and 'Resulting Settings' columns in execution_table rows.

If the child template had priority 5 instead of 20, what would be the final number_of_shards?

A 1

B 3

C undefined

D 5

💡 Hint

Refer to variable_tracker and remember higher priority templates override lower priority ones.

Concept Snapshot

Template priority and composition in Elasticsearch:
- Templates have priorities (higher number = higher priority).
- Elasticsearch merges templates by priority order.
- Higher priority templates override settings of lower priority ones.
- If priorities tie, last loaded template wins.
- Final template is the composition applied to matching indices.

Full Transcript

In Elasticsearch, index templates can be composed by defining multiple templates with different priorities. The system merges these templates by applying the settings from lower priority templates first, then overriding them with higher priority templates. For example, a base template with priority 10 sets number_of_shards to 1. A child template with priority 20 sets number_of_shards to 3. When merged, the final setting is 3 because the child template has higher priority. If two templates have the same priority, the last loaded template's settings override earlier ones. This priority and composition system allows flexible and layered index template management.

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