> ## Documentation Index > Fetch the complete documentation index at: https://docs.y.uno/llms.txt > Use this file to discover all available pages before exploring further. # How a transaction flows > Who does what, in which order, when a payment or an enrollment runs through the Yuno SDK. Every integration runs the same sequence: your backend creates a session, your app starts a `Transaction`, the SDK takes the shopper to a final status and reports it to `onStatus`, and Yuno confirms the outcome to your backend. What changes between integrations is who creates the payment and which session you pass. The three flows below cover every case. ## The default flow: the SDK creates the payment Create the checkout session with `workflow: "SDK_SEAMLESS"` and do not override `createPayment`. The SDK does everything between `start` and `onStatus`. ```mermaid theme={"theme":{"light":"github-dark","dark":"github-dark-dimmed"}} sequenceDiagram autonumber participant Shopper participant App as Your app participant Backend as Your backend participant SDK as Yuno SDK participant Yuno as Yuno API App->>Backend: Start checkout Backend->>Yuno: POST /v1/checkout/sessions (workflow SDK_SEAMLESS) Yuno-->>Backend: checkout session Backend-->>App: checkout session App->>SDK: Transaction(config).start(paymentSelected) SDK->>Shopper: Form for the selected method Shopper-->>SDK: Payment details SDK->>Yuno: Tokenize (one-time token) SDK->>Yuno: Create the payment SDK->>Shopper: 3DS or payment action, if required SDK->>Yuno: Poll the payment status SDK-->>App: onStatus(status) Yuno-->>Backend: Payment webhook ``` 1. Your backend creates the checkout session and hands its ID to your app. 2. Your app initializes the SDK once, builds a `TransactionConfig` with the session and your controller, and calls `start` with the method the shopper picked. 3. The SDK renders the method's form, or skips it when you pass `transactionData`, and tokenizes the details into a one-time token. 4. The SDK creates the payment with that token, runs 3DS or any other action the method needs, and polls the status until it is final. 5. `onStatus` receives the final status. On iOS, asynchronous methods report `PENDING` first; see [`onStatus`](/docs/yuno-sdk/controllers/on-status). If `showStatusScreen` is `true`, the SDK also shows its result screen. 6. Yuno sends the payment webhook to your backend. Confirm the outcome there before fulfilling the order. ## When your backend creates the payment Override `createPayment` and keep the default session workflow (`SDK_CHECKOUT`). The SDK pauses after tokenization, hands you the token, and resumes when your handler returns. ```mermaid theme={"theme":{"light":"github-dark","dark":"github-dark-dimmed"}} sequenceDiagram autonumber participant Shopper participant App as Your app participant Backend as Your backend participant SDK as Yuno SDK participant Yuno as Yuno API App->>Backend: Start checkout Backend->>Yuno: POST /v1/checkout/sessions (default workflow) Yuno-->>Backend: checkout session Backend-->>App: checkout session App->>SDK: Transaction(config).start(paymentSelected) SDK->>Shopper: Form for the selected method Shopper-->>SDK: Payment details SDK->>Yuno: Tokenize (one-time token) SDK-->>App: createPayment(token) App->>Backend: one-time token Backend->>Yuno: POST /v1/payments Backend-->>App: payment created App-->>SDK: return from createPayment SDK->>Shopper: 3DS or payment action, if required SDK->>Yuno: Poll the payment status SDK-->>App: onStatus(status) Yuno-->>Backend: Payment webhook ``` The difference is steps 9 to 13: the SDK delivers the one-time token to `createPayment`, your backend creates the payment, and returning from the handler is what resumes the flow. If the handler throws, the flow stops and `onStatus` is not called. Set `autoContinuePayment` to `false` only if you need the SDK to stop right after the payment is created. On Web it applies only when you override `createPayment`; on iOS it applies to the SDK's own payment creation too. The SDK then runs no 3DS, polls no status, and reports nothing to `onStatus` for that attempt. On Web, PayPal ignores the flag and always continues. See [`autoContinuePayment`](/docs/yuno-sdk/reference/objects-you-pass#autocontinuepayment). The session `workflow` must match who creates the payment. A default-workflow session with no `createPayment` override ends in `INTERNAL_ERROR`: the backend rejects the SDK's payment creation. See [`createPayment`](/docs/yuno-sdk/controllers/create-payment). ## Enrollment: save a method without charging Pass a `customerSession` instead of a `checkoutSession`. There is no payment to create, so `createPayment` is never invoked. ```mermaid theme={"theme":{"light":"github-dark","dark":"github-dark-dimmed"}} sequenceDiagram autonumber participant Shopper participant App as Your app participant Backend as Your backend participant SDK as Yuno SDK participant Yuno as Yuno API Backend->>Yuno: POST /v1/customers (once per customer) Backend->>Yuno: POST /v1/customers/sessions Yuno-->>Backend: customer session Backend-->>App: customer session App->>SDK: Transaction(config).start(paymentSelected) SDK->>Shopper: Form for the selected method Shopper-->>SDK: Payment details SDK->>Yuno: Tokenize and enroll SDK->>Shopper: 3DS, if required SDK-->>App: onStatus(status) Yuno-->>Backend: Enrollment webhook Backend->>Yuno: List the customer's payment methods ``` The vaulted token is never delivered to the app. After `onStatus` reports success, your backend reads the enrolled method from Yuno and uses its vaulted token for future payments. ## Where each step is documented | Step | Page | | - | - | | Creating sessions and the first payment | [Get Started](/docs/yuno-sdk/get-started) | | Choosing who creates the payment, collects the data, renders the form | [Where do I start?](/docs/yuno-sdk/where-do-i-start) | | `createPayment` and the session workflow | [`createPayment`](/docs/yuno-sdk/controllers/create-payment) | | The final status and confirming it on your server | [`onStatus`](/docs/yuno-sdk/controllers/on-status) | | Enrollment with a customer session | [Enrollment](/docs/yuno-sdk/features/enrollment) | | Returning from an external app or page (iOS) | [Handle deeplinks](/docs/yuno-sdk/features/handle-deeplinks) | This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.