Semantic Layer Change Review

Purpose

Changes to the semantic layer directly impact dashboards and business reporting. A silent drift in a metric definition destroys trust. Review every semantic-layer change for mathematical soundness and backwards compatibility before approval.

When to Use

• A PR modifies metric or semantic model YAML
• A metric's expr, aggregation, or filters are changing
• New dimensions or entities are being added to an existing semantic model

Inputs

• The semantic-layer diff (semantic models, metrics, saved queries)
• The underlying model's grain and entity keys

Workflow

1. Enumerate what changed:

   git diff origin/main...HEAD -- '*.yml' '*.yaml'
   dbt ls --resource-type metric
   dbt ls --resource-type semantic_model
   

2. Identify the change type:

   • Addition — safe (adding a new metric or dimension).
   • Deprecation — requires communication (removing a metric).
   • Modification — high risk (changing the SQL expression, aggregation, or filters of an existing metric).

3. Evaluate mathematical soundness:

   • Are we averaging an average?
   • Are we summing a distinct count?
   • Does adding this dimension cause a fan-out that inflates the metric?

4. Check backwards compatibility. If an existing metric's logic is changed, you MUST flag it. The recommended path is dbt's metric versioning or a new metric (e.g., revenue_v2) rather than silently altering historical numbers. Find consumers of the metric before judging impact:

   grep -rn "<metric_name>" --include="*.yml" --include="*.yaml" .
   

5. Verify entity mapping. Ensure entities (primary/foreign keys) match the granularity of the underlying semantic model.

6. Confirm definitions still parse:

   dbt parse
   

Output

• Change classification (addition / deprecation / modification) per touched metric or semantic model
• Mathematical soundness findings
• Backwards-compatibility verdict, with consumers that need communication
• Entity/grain mismatches, if any

Verification

• Every touched metric and semantic model classified by change type
• Modifications to existing metrics explicitly flagged, never silently approved
• Consumers of modified metrics identified
• dbt parse passes on the changed project
• Entity keys checked against the underlying model's grain

Failure Modes

• Silent restatement — approving a change to a core metric's expr without explicitly confirming the business requested the restatement of historical data.
• Treating modification like addition — additions are safe; modifications are high risk and need versioning or a new metric.
• Fan-out blindness — a new dimension join can inflate a metric while every individual definition still looks correct.