Definitions in one place.
WHAT IT IS
A layer holding metric and dimension definitions, used by every consuming tool.
WHAT PROBLEM IT SOLVES
The same metric calculated differently in each report.
WHAT IT DEFINES
Measures, and how they aggregate Dimensions available to group by Joins between entities Filters that apply by default
WHY DEFINING AGGREGATION MATTERS
It prevents summing values that must not be summed, such as ratios and balances.
WHAT IT PROVIDES CONSUMERS
Consistent numbers regardless of which tool asks.
WHAT IMPLEMENTATIONS EXIST
Features within reporting tools Standalone semantic layers Definitions within transformation frameworks
WHAT TO PREFER
Whatever every consuming tool can actually use.
WHY
A semantic layer only one tool honours defeats the purpose.
WHAT TO BUILD WITHOUT ONE
Shared models in the warehouse, with metrics computed there rather than in reports.
WHAT TO NEVER ALLOW
A metric redefined downstream.
WHAT TO TEST
That the same metric from two tools returns the same value.
WHAT TO REVIEW
Definitions, with whoever owns the metric, before changing them.
