Di APK (Android), saat user pesan:

val db = Firebase.database.reference
val orderId = "KPBB-" + System.currentTimeMillis()
db.child("kpbb_db/orders/$orderId").setValue(mapOf(
  "id" to orderId,
  "customerId" to customerId,
  "plan" to "monthly",  // weekly/monthly/yearly
  "amount" to 199000,
  "status" to "pending",
  "licenseKey" to null,
  "createdAt" to ISO8601_NOW
))

Baca license key (polling/listener):

db.child("kpbb_db/orders/$orderId/licenseKey")
  .addValueEventListener(object : ValueEventListener {
    override fun onDataChange(snap: DataSnapshot) {
      val key = snap.getValue(String::class.java)
      if (key != null) activateLicense(key)
    }
    override fun onCancelled(e: DatabaseError) {}
  })

Validasi lisensi yang diinput user:

// Cari customer dengan licenseKey ini
db.child("kpbb_db/customers")
  .orderByChild("licenseKey")
  .equalTo(inputKey)
  .get().addOnSuccessListener { snap ->
    val cust = snap.children.firstOrNull()
    val status = cust?.child("status")?.value
    if (status == "active") { /* aktifkan */ }
  }