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.