Tabular Model SSAS
Tabular models are in-memory databases in Analysis Services. The xVelocity in-memory analytics engine (VertiPaq) delivers fast access to tabular model objects and data by reporting client applications such as Microsoft Excel and Microsoft Power View.
Tabular models support data access through two modes: Cached mode and DirectQuery mode.
In cached mode, you can integrate data from multiple sources including relational databases, data feeds, and flat text files.
In DirectQuery mode, you can bypass the in-memory model, allowing client applications to query data directly at the (SQL Server relational) source.