Watches and webhooks

// Chapter 10: watch an account's balance, and receive Blnk's webhooks
// safely.
package main

import (
    "bytes"
    "errors"
    "fmt"
    "log"
    "net/http"
    "net/http/httptest"
    "strconv"
    "time"

    "github.com/otyang/humanblnk"
    "github.com/otyang/humanblnk/blnk"
    "github.com/otyang/humanblnk/docs/examples/demo"
    "github.com/otyang/humanblnk/money"
)

// secret is Blnk's server.secret_key, which signs its webhooks.
const secret = "change-me"

// webhook is the handler you mount at the URL Blnk sends webhooks to.
func webhook(w http.ResponseWriter, r *http.Request) {
    ev, err := humanblnk.VerifyWebhook(secret, r, 5*time.Minute)
    switch {
    case errors.Is(err, blnk.ErrBadSignature), errors.Is(err, blnk.ErrStale):
        http.Error(w, "bad signature", http.StatusUnauthorized) // never act on its body
        fmt.Println("  rejected:", err)
        return
    case err != nil:
        // Signed, but not something to act on: say so, or Blnk sends it
        // again and again.
        w.WriteHeader(http.StatusNoContent)
        fmt.Println("  can't use it:", err)
        return
    }
    switch {
    case ev.Kind == humanblnk.EventBalanceMonitor:
        fmt.Println("  watch", ev.Watch, "fired on", ev.Account, "- e.g. text the customer 'low balance'")
    case ev.Entry != nil: // transaction.applied, transaction.rejected, …
        fmt.Println("  ", ev.Kind, ev.Entry.Ref, ev.Entry.Amount)
    }
    w.WriteHeader(http.StatusOK)
}

func main() {
    ctx, hb := demo.Setup()
    ada := demo.Customer(ctx, hb, "ada", demo.NGN)
    demo.Deposit(ctx, hb, ada, "5000")

    // 1. Tell Blnk to fire "balance.monitor" while Ada's balance is under ₦1,000.
    low, err := hb.Watches.Create(ctx, humanblnk.Watch{
        Account: humanblnk.AccountRef(ada.ID), Field: humanblnk.WatchBalance, Op: humanblnk.WatchBelow,
        Threshold: money.MustParse("1000", demo.NGN), Description: "low balance alert",
    })
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println("watch", low.ID, "on", low.Account, low.Field, low.Op, low.Threshold)

    // Move the line: Change makes the new watch before deleting the old one.
    low, err = hb.Watches.Change(ctx, low.ID, humanblnk.Watch{
        Account: humanblnk.AccountRef(ada.ID), Field: humanblnk.WatchBalance, Op: humanblnk.WatchBelow,
        Threshold: money.MustParse("2000", demo.NGN), Description: "low balance alert",
    })
    if err != nil {
        log.Fatal(err)
    }
    watches, _ := hb.Watches.List(ctx, humanblnk.AccountRef(ada.ID))
    fmt.Println("watches on Ada:", len(watches), "| threshold now", watches[0].Threshold)

    // 2. Receive webhooks. Here a test server stands in for your service,
    // and the requests are signed the way Blnk signs them.
    srv := httptest.NewServer(http.HandlerFunc(webhook))
    defer srv.Close()
    send := func(sign string, body string) {
        ts := strconv.FormatInt(time.Now().Unix(), 10)
        req, _ := http.NewRequest(http.MethodPost, srv.URL, bytes.NewBufferString(body))
        req.Header.Set(blnk.HeaderTimestamp, ts)
        req.Header.Set(blnk.HeaderSignature, blnk.Sign(sign, ts, []byte(body)))
        res, err := http.DefaultClient.Do(req)
        if err != nil {
            log.Fatal(err)
        }
        res.Body.Close()
        fmt.Println("  ->", res.Status)
    }
    body := `{"event":"balance.monitor","data":{"monitor_id":"` + string(low.ID) + `","balance_id":"` + ada.ID + `"}}`
    fmt.Println("a genuine webhook:")
    send(secret, body)
    fmt.Println("a forged one:")
    send("not-the-secret", body)

    // 3. Stop watching.
    if err := hb.Watches.Delete(ctx, low.ID); err != nil {
        log.Fatal(err)
    }
    fmt.Println("watch deleted")
}

watch mon_7a30c4bc-caaa-4da0-a4d0-f91cd0912cdf on bln_b8469f1d-e935-424c-8b27-ce4f4eea5ba8 balance < 1000.00
watches on Ada: 1 | threshold now 2000.00
a genuine webhook:
  watch mon_57366f82-1d37-4179-9434-bc1cd2481278 fired on bln_b8469f1d-e935-424c-8b27-ce4f4eea5ba8 - e.g. text the customer 'low balance'
  -> 200 OK
a forged one:
  rejected: blnk: webhook signature mismatch
  -> 401 Unauthorized
watch deleted