demo -> done

demo/server.js

@@ -1,11 +0,0 @@
-const express = require('express')
-const cds = require('@sap/cds')
-
-cds.once('bootstrap',(app)=>{
-  const {dirname} = require('path')
-  // serving the vue.js app imported from @capire/bookshop
-  const vue_app = dirname (require.resolve('@capire/bookshop/app/vue/index.html'))
-  app.use ('/vue', express.static(vue_app))
-})
-
-module.exports = cds.server

demo/srv/services.cds

@@ -1,17 +0,0 @@
-using { sap.capire, sap.capire.bookshop.Books } from '@capire/bookshop';
-using { API_BUSINESS_PARTNER as external } from './external/API_BUSINESS_PARTNER.csn';
-
-extend Books with {
-  supplier : Association to Suppliers;
-}
-
-extend service CatalogService with {
-  entity MySuppliers as projection on Suppliers;
-}
-
-@cds.persistence:{table,skip:false}
-entity Suppliers as projection on external.A_BusinessPartner {
-  BusinessPartner as ID,
-  LastName,
-  AcademicTitle
-}

done/package.json

@@ -7,6 +7,9 @@
   "private": true,
   "dependencies": {
     "@capire/bookshop": "^1.0.0",
+    "@capire/common": "^1.0.0",
+    "@capire/orders": "^1.0.0",
+    "@capire/reviews": "^1.0.0",
     "@sap/cds": "^4",
     "express": "^4"
   },

done/server.js

@@ -0,0 +1,67 @@
+const cds = require('@sap/cds')
+module.exports = cds.server
+
+
+cds.once('bootstrap',(app)=>{
+  app.use('/vue',_from('@capire/bookshop/app/vue'))
+})
+
+
+
+cds.once('served', async ()=>{
+
+  // 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', req => {
+      console.log ('>> delegating to S4 service...')
+      return S4bupa.read(Suppliers) .where (req.query.where)
+    })
+
+
+    // Replicate Supplier data when Books are edited
+    admin.on (['CREATE','UPDATE'], 'Books', async req => {
+      const { supplier } = req.data
+      if (supplier) {
+        let cached = await db.read (Suppliers, supplier)
+        if (!cached) await replicate (supplier,'initial')
+      }
+    })
+
+  // })
+
+  // Subscribe to changes in the S4 origin of Suppliers data
+  S4bupa.on ('BusinessPartner/Changed', async msg => {
+    const ID = msg.businessPartner.KEYS
+    const cached = await db.read (Suppliers, sup => sup.ID) .where ({ ID })
+    for (let each of cached) replicate (each, 'update')
+  })
+
+  // 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)
+  }
+
+})
+
+
+
+
+
+
+
+// -----------------------------------------------------------------------
+// Helper for serving static content from npm-installed packages
+const {static} = require('express')
+const {dirname} = require('path')
+const _from = target => static (dirname (require.resolve(`${target}/index.html`)))

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

@@ -0,0 +1,5 @@
+ID;name;city
+ACME;A Company Making Everything;Massachusetts
+B4U;Books for You;Philadelphia
+S&C;Shakespeare & Co.;Paris
+WSL;Waterstones;London

done/srv/services.cds

@@ -0,0 +1,55 @@
+/*
+  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,
+    to_BusinessPartnerAddress.CityName as city
+  }
+}
+
+
+/*
+  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
+}
+
+/*
+  The following using directives activate imported reuse packages.
+ */
+using from '@capire/common';
+using from '@capire/orders';
+using from '@capire/reviews';