Adding suppliers showing integration with S/4

suppliers/srv/external/data/API_BUSINESS_PARTNER-A_BusinessPartner.csv

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+BusinessPartner;BusinessPartnerFullName
+ACME;A Company Making Everything
+B4U;Books for You
+S&C;Shakespeare & Co.
+WSL;Waterstones
+TLD;Thalia
+PNG;Penguin Books

suppliers/srv/external/data/API_BUSINESS_PARTNER-Suppliers.csv

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+ID;name
+ACME;A Company Making Everything
+B4U;Books for You
+S&C;Shakespeare & Co.
+WSL;Waterstones

suppliers/srv/mashup.cds

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+/*
+  Optionally add projections to external entities, to capture what
+  you actually want to use from there.
+ */
+
+using { API_BUSINESS_PARTNER as S4 } from './external/API_BUSINESS_PARTNER.csn';
+extend service S4 with {
+  entity Suppliers as projection on S4.A_BusinessPartner {
+    key BusinessPartner as ID,
+    BusinessPartnerFullName as name,
+  }
+}
+
+
+/*
+  You can mashup entities from external services, or projections
+  thereof, with your project's own entities
+ */
+using { sap.capire.bookshop.Books } from '@capire/bookshop';
+extend Books with {
+  supplier : Association to S4.Suppliers;
+}
+
+
+/*
+  You can also expose external entities through your own services
+  For this to work, you need to delegate the respective calls
+  addressed to your services into calls to the external service.
+ */
+extend service AdminService with {
+  entity Suppliers as projection on S4.Suppliers;
+}
+
+/*
+  Optionally add a local persistence to keep replicas of external
+  entities to have data in fast access locally; much like a cache.
+ */
+annotate S4.Suppliers with @cds.persistence:{table,skip:false};
+
+/**
+  Having locally cached replicas also allows us to display supplier
+  data in lists of books, which otherwise would generate unwanted
+  traffic on S4 backends.
+ */
+extend projection CatalogService.ListOfBooks with {
+  supplier
+}

suppliers/srv/mashup.js

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+////////////////////////////////////////////////////////////////////////////
+//
+//    Mashing up provided and required services...
+//
+module.exports = async()=>{ // called by server.js
+
+  if (!cds.services.AdminService) return //> mocking S4 service only
+
+  // Connect to services we want to mashup below...
+  const S4bupa = await cds.connect.to('API_BUSINESS_PARTNER')   //> external S4 service
+  const admin = await cds.connect.to('AdminService')            //> local domain service
+  const db = await cds.connect.to('db')                         //> our primary database
+
+  // Reflect CDS definition of the Suppliers entity
+  const { Suppliers } = S4bupa.entities
+
+  admin.prepend (()=>{
+
+    // Delegate Value Help reads for Suppliers to S4 backend
+    admin.on ('READ', 'Suppliers', async req => {
+      console.log ('>> delegating to S4 service...')
+      return await S4bupa.run(req.query)
+    })
+
+
+    // Replicate Supplier data when Books are edited
+    admin.on (['CREATE','UPDATE'], 'Books', async (req,next) => {
+      let { supplier } = req.data
+      if (supplier) {
+        let cached = await db.exists (Suppliers, supplier)
+        if (!cached) await replicate (supplier,'initial')
+      }
+      return next()
+    })
+
+  })
+
+  // Subscribe to changes in the S4 origin of Suppliers data
+  S4bupa.on ('BusinessPartner/Changed', async msg => {
+    let cached = await SELECT('ID').from (Suppliers)
+      .where ('ID in', msg.businessPartner.KEYS)
+    for (let each of cached) replicate (each)
+  })
+
+  // Helper function to replicate Suppliers data
+  async function replicate (ID,_initial) {
+    let data = await S4bupa.read (Suppliers, ID)
+    if (_initial) return db.insert (data) .into (Suppliers)
+    else return db.update (Suppliers,ID) .with (data)
+  }
+
+}