What is the application layer in SAP's 3-tier architecture?

The application layer is the middle tier, where the actual business logic runs — ABAP programs executing, business rules being applied, and data being processed before it's sent to the database or back to the user. This layer consists of one or more application servers, each capable of running many concurrent user sessions.

flowchart LR
    A[Presentation Layer request] --> B[Application Server: dispatcher]
    B --> C[Work processes execute ABAP logic]
    C --> D[Database Layer: read/write data]
    C --> A

A single SAP system can have multiple application servers working together (an "application server group"), which is exactly how SAP systems scale to support more concurrent users — adding another application server spreads the processing load, without touching the presentation or database layers at all.

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