Add go callbacks example

Author: gvdongen

go/patterns-use-cases/README.md

@@ -114,6 +114,22 @@ Have a look at the logs to see the cancellations of the flight and car booking i
 ```
 </details>
 
+## Durable Webhook Event Processing
+
+This example processes webhook callbacks from a payment provider.
+Restate handlers can be used as the target for webhook callbacks.
+This turns handlers into durable event processors that ensure the event is processed exactly once.
+You don't need to do anything special!
+
+## Scheduling Tasks
+This example processes failed payment events from a payment provider.
+The service reminds the customer for 3 days to update their payment details, and otherwise escalates to support.
+
+To schedule the reminders, the handler uses Restate's durable timers and delayed calls.
+The handler calls itself three times in a row after a delay of one day, and then stops the loop and calls another handler.
+
+Restate tracks the timer across failures, and triggers execution.
+
 ## Stateful Actors
 
 This example implements a State Machine with a Virtual Object.

go/patterns-use-cases/src/dataupload/client/client.go

@@ -20,7 +20,7 @@ const RESTATE_URL = "http://localhost:8080"
 // workflow to send the upload url via email instead.
 
 func upload(id string, email string) error {
-	slog.Info(fmt.Sprintf("Start upload for %s", id))
+	slog.Info("Start upload for " + id)
 
 	client := &http.Client{Timeout: 5 * time.Second}
 
@@ -50,7 +50,7 @@ func upload(id string, email string) error {
 	}
 
 	// ... process result directly ...
-	slog.Info(fmt.Sprintf("Fast upload: URL was %s", string(body)))
+	slog.Info("Fast upload: URL was " + string(body))
 	return nil
 }
 

go/patterns-use-cases/src/webhookcallbacks/callbackrouter.go

@@ -0,0 +1,19 @@
+package main
+
+import restate "github.com/restatedev/sdk-go"
+
+type WebhookCallbackRouter struct{}
+
+// Any handler can be a durable webhook processor that never loses events
+// You don't need to do anything special for this. Just point your webhook to the handler endpoint.
+
+func (WebhookCallbackRouter) OnStripeEvent(ctx restate.Context, event StripeEvent) error {
+	if event.Type == "invoice.payment_failed" {
+		restate.ObjectSend(ctx, "PaymentTracker", "onPaymentFailed", event.Data.Object.ID).
+			Send(event)
+	} else if event.Type == "invoice.payment_succeeded" {
+		restate.ObjectSend(ctx, "PaymentTracker", "onPaymentSuccess", event.Data.Object.ID).
+			Send(event)
+	}
+	return nil
+}

go/patterns-use-cases/src/webhookcallbacks/stubs.go

@@ -0,0 +1,23 @@
+package main
+
+import restate "github.com/restatedev/sdk-go"
+
+type StripeEvent struct {
+	ID      string `json:"id"`
+	Type    string `json:"type"`
+	Created int64  `json:"created"`
+	Data    struct {
+		Object struct {
+			ID       string `json:"id"`
+			Customer string `json:"customer"`
+		} `json:"object"`
+	} `json:"data"`
+}
+
+// Have a look at the scheduling tasks example (../schedulingtasks/paymentreminders.go)
+// to see a full implementation of this
+
+type PaymentTracker struct{}
+
+func (PaymentTracker) OnPaymentFailed(ctx restate.Context, event StripeEvent) error  { return nil }
+func (PaymentTracker) OnPaymentSuccess(ctx restate.Context, event StripeEvent) error { return nil }