API-Led Connectivity & 3-Tier Architecture in MuleSoft

The 3-Tier API Architecture

graph TD
    subgraph Consumers["Digital Consumer Channels"]
        MobileApp["Mobile App iOS/Android"]
        WebPortal["Customer Web Portal"]
        PartnerEDI["B2B Partner API"]
    end

    subgraph ExpLayer["Experience API Layer (Channel-Tailored)"]
        ExpMobile["Mobile Experience API<br/>(Lightweight JSON, Minimized Fields)"]
        ExpWeb["Web Experience API<br/>(Rich Metadata, Pagination)"]
        ExpPartner["Partner Experience API<br/>(Standardized B2B Contract)"]
    end

    subgraph ProcLayer["Process API Layer (Business Orchestration)"]
        OrderProc["Order Fulfillment Process API<br/>(Orchestrates Cart, Payment, Stock)"]
        CustomerProc["Customer 360 Process API<br/>(Aggregates CRM & Billing)"]
    end

    subgraph SysLayer["System API Layer (Core Asset Unlocking)"]
        SAPSys["SAP ERP System API"]
        SFSys["Salesforce System API"]
        DBSys["Customer Postgres System API"]
    end

    MobileApp --> ExpMobile
    WebPortal --> ExpWeb
    PartnerEDI --> ExpPartner

    ExpMobile --> OrderProc
    ExpMobile --> CustomerProc
    ExpWeb --> OrderProc
    ExpWeb --> CustomerProc
    ExpPartner --> OrderProc

    OrderProc --> SAPSys
    OrderProc --> SFSys
    CustomerProc --> SFSys
    CustomerProc --> DBSys

Layer Responsibilities & Rules of Engagement

1. System APIs (Bottom Layer):

   • Role: Provides secure, direct access to underlying systems of record (SAP, Oracle, Mainframes, Salesforce, MySQL).
   • Design Rule: Insulates the enterprise from legacy protocols; exposes standard REST/JSON or OAS contracts. Zero business logic allowed here.
   • Lifecycle: Slow-moving, tightly governed by central IT/ERP owners.

2. Process APIs (Middle Layer):

   • Role: Implements core business logic, orchestrating calls across multiple System APIs, data enrichment, and workflow rules.
   • Design Rule: Channel-agnostic. For example,
     CODE / PROMPTCopy

     OrderFulfillmentProcessAPI

     calculates taxes and validates discount codes whether the request originates from a phone or an in-store POS terminal.
   • Lifecycle: Medium agility, governed by line-of-business integration teams.

3. Experience APIs (Top Layer):

   • Role: Tailors, shapes, and filters composite data for specific consumption devices and security contexts.
   • Design Rule: The mobile app requires 4 fields to render a card view; the desktop web app requires 30 fields with full pagination. Experience APIs format payloads without forcing changes onto backend Process APIs.
   • Lifecycle: Fast-moving, owned by frontend product teams.