Well-structured CDS: the VDM layer that won't embarrass you in three years

Most performance and maintenance problems in CDS don’t come from the SQL. They come from not deciding, before writing the first view, what role each view plays in the model. SAP’s Virtual Data Model (VDM) solves exactly that.

The three VDM layers

• Basic (interface): expose a table or business object 1:1, with no logic. Named I_.... They are your only gateway to raw data.
• Composite (interface): combine several Basic views (joins, associations, reusable aggregations). Also I_....
• Consumption: the layer an app, an OData service or a report consumes. Named C_.... UI annotations, @Search, value helps live here.

The golden rule: a Consumption view never reads tables directly; it reads Basic or Composite. That way, when the physical model changes, you only touch the Basic layer.

A concrete example

A Basic view over sales order headers, typed and with explicit associations:

@AbapCatalog.viewEnhancementCategory: [#NONE]
@AccessControl.authorizationCheck: #CHECK
@EndUserText.label: 'Sales order header'
@Metadata.ignorePropagatedAnnotations: true
define view entity I_SalesOrderHeader
  as select from vbak
  association [0..*] to I_SalesOrderItem as _Item
    on $projection.SalesOrder = _Item.SalesOrder
{
  key vbeln           as SalesOrder,
      erdat           as CreationDate,
      @Semantics.amount.currencyCode: 'Currency'
      netwr           as NetAmount,
      waerk           as Currency,
      // expose the association for the data navigator
      _Item
}

Notice three decisions that pay off three years down the line:

1. @Semantics.amount.currencyCode binds the amount to its currency: any consumer knows how to format it without guessing.
2. The _Item association is declared but not materialized as a join: it resolves only when someone navigates it.
3. The name I_SalesOrderHeader shouts “I’m an interface, reuse me”, not “I’m the view for that one screen”.

The mistake you’ll see in every project

A Consumption view with fifteen joins, three subselects and UI annotations tangled with business logic. It works on demo day and is impossible to touch on change day. If your C_... view has a select from a transparent table, you’ve already broken the VDM.

Layering costs you half an hour more today. Not layering costs the project’s life on every evolution.

Next post: how associations replace joins and why that changes the execution plan in HANA.