perf example

performance/srv/performance-service.js

@@ -0,0 +1,37 @@
+const cds = require ('@sap/cds')
+class OrdersService extends cds.ApplicationService {
+
+  /** register custom handlers */
+  init(){
+    const { 'Orders.Items':OrderItems } = this.entities
+
+    this.before ('UPDATE', 'Orders', async function(req) {
+      const { ID, Items } = req.data
+      if (Items) for (let { product_ID, quantity } of Items) {
+        const { quantity:before } = await cds.tx(req).run (
+          SELECT.one.from (OrderItems, oi => oi.quantity) .where ({up__ID:ID, product_ID})
+        )
+        if (quantity != before) await this.orderChanged (product_ID, quantity-before)
+      }
+    })
+
+    this.before ('DELETE', 'Orders', async function(req) {
+      const { ID } = req.data
+      const Items = await cds.tx(req).run (
+        SELECT.from (OrderItems, oi => { oi.product_ID, oi.quantity }) .where ({up__ID:ID})
+      )
+      if (Items) await Promise.all (Items.map(it => this.orderChanged (it.product_ID, -it.quantity)))
+    })
+
+    return super.init()
+  }
+
+  /** order changed -> broadcast event */
+  orderChanged (product, deltaQuantity) {
+    // Emit events to inform subscribers about changes in orders
+    console.log ('> emitting:', 'OrderChanged', { product, deltaQuantity })
+    return this.emit ('OrderChanged', { product, deltaQuantity })
+  }
+
+}
+module.exports = OrdersService