DAX formula and storage engine

Power BI Datasets and SQL Server Analysis Services (SSAS) share the same database engine and architecture. Both tools support both Import and DirectQuery data models and both DAX and MDX client applications such as Power BI (DAX) and Excel (MDX). The DAX Query Engine is comprised of a formula and a storage engine for both Import and DirectQuery models. The formula engine produces query plans, requests data from the storage engine, and performs any remaining complex logic not supported by the storage engine against this data such as IF and SWITCH functions

In DirectQuery models, the data source database is the storage engine--it receives SQL queries from the formula engine and returns the results to the formula engine. For In-Memory models, the imported and compressed columnar memory cache is the storage engine. See Chapter 11, Enhancing and Optimizing Existing Power BI Solutions, for more details.