What is a dbt Semantic Layer?

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May 13, 2024

What is a dbt Semantic Layer?

The dbt Semantic Layer is a unique approach to defining the edges of a graph. Unlike most semantic layers that require users to describe the left and right join keys, the dbt Semantic Layer uses Entities to infer these edges. This method reduces the amount of logic needed to maintain the graph and results in cleaner SQL. The semantic layer is made up of two fundamental constructs: semantic models and metrics. Semantic models include entities, measures, and dimensions, while metrics are used to measure and analyze data.

  • Semantic Models: These are the entities, measures, and dimensions that make up the dbt Semantic Layer. They are used to define the structure of the data.
  • Metrics: These are used to measure and analyze the data within the semantic layer. They provide the means to understand and interpret the data.
  • Entities: Entities are used in the dbt Semantic Layer to infer the edges of the graph, reducing the amount of logic needed and leading to cleaner SQL.

What are the advantages of using a dbt Semantic Layer?

The dbt Semantic Layer offers numerous advantages, including consistency in metric definitions, flexibility in data consumption, reusability of metrics, reduced cost and compute, governance and auditing, reduced data inequality, and seamless integration with dbt.

  • Consistency: The dbt Semantic Layer provides consistency in metric definitions across the organization, eliminating inconsistencies and errors caused by business users defining metrics on their own.
  • Flexibility: It is agnostic to where end users consume data, allowing multiple consumption endpoints without being locked into a specific BI vendor or tool.
  • Reusability: Metrics only need to be created and updated in one place, saving time and reducing the risk of errors and downstream impacts. This allows for easier migration to different data platforms or BI tools.
  • Reduced cost/compute: The dbt Semantic Layer avoids duplication of data slices and transformations, which can represent a large portion of a company's compute budget. It reduces duplication and saves on compute resources.

How can a dbt Semantic Layer be implemented?

Implementing a dbt Semantic Layer involves several steps, including building a semantic model, defining metrics, testing the metrics, and communicating the existence of the semantic model to stakeholders.

  • Building a semantic model: The semantic model is defined in YAML, including entities, dimensions, measures, and configurations.
  • Defining metrics: The metrics are defined in YAML, providing names and descriptions for what you are calculating and sharing with users.
  • Testing the metrics: The functionality of the semantic model and metrics is validated in integrations or with direct API requests.
  • Communicating the existence of the semantic model: Stakeholders are informed about the availability of the semantic model and potentially modify existing BI reports to use the newer metric definitions powered by dbt Semantic Layer.

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