Saga

The saga design pattern is a way to manage data consistency across microservices in distributed transaction scenarios. A Saga is useful when you need to manage data in a consistent manner across services in distributed transaction scenarios. Or when you need to compose multiple actions with a compensation that needs to run in a transaction like style.

For example, let's say that we have the following domain types Order, Payment.

data class Order(val id: UUID, val amount: Long)
data class Payment(val id: UUID, val orderId: UUID)

The creation of an Order can only remain when a payment has been made. In SQL, you might run this inside a transaction, which can automatically roll back the creation of the Order when the creation of the Payment fails.

When you need to do this across distributed services, or a multiple atomic references, etc. You need to manually facilitate the rolling back of the performed actions, or compensating actions.

The Saga type, and saga DSL remove all the boilerplate of manually having to facilitate this with a convenient suspending DSL.

data class Order(val id: UUID, val amount: Long)
suspend fun createOrder(): Order = Order(UUID.randomUUID(), 100L)
suspend fun deleteOrder(order: Order): Unit = println("Deleting $order")

data class Payment(val id: UUID, val orderId: UUID)
suspend fun createPayment(order: Order): Payment = Payment(UUID.randomUUID(), order.id)
suspend fun deletePayment(payment: Payment): Unit = println("Deleting $payment")

suspend fun Payment.awaitSuccess(): Unit = throw RuntimeException("Payment Failed")

suspend fun main() {
  saga {
    val order = saga({ createOrder() }) { deleteOrder(it) }
    val payment = saga { createPayment(order) }, ::deletePayment)
    payment.awaitSuccess()
  }.transact()
}