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Hosted Deposit and Withdrawal

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This guide is available on three different programming languages: Typescript, Kotlin and Flutter (Dart). You can change the shown version on each page via the buttons above.

The SEP-24 standard defines the standard way for anchors and wallets to interact on behalf of users. Wallets use this standard to facilitate exchanges between on-chain assets (such as stablecoins) and off-chain assets (such as fiat, or other network assets such as BTC).

During the flow, a wallet makes several requests to the anchor, and finally receives an interactive URL to open in iframe. This URL is used by the user to provide an input (such as KYC) directly to the anchor. Finally, the wallet can fetch transaction information using query endpoints.

Get Anchor Information​

Let's start with getting an instance of Sep24 class, responsible for all SEP-24 interactions:

const sep24 = await anchor.sep24();

First, let's get the information about the anchor's support for SEP-24. This request doesn't require authentication, and will return generic info, such as supported currencies, and features supported by the anchor. You can get a full list of returned fields in the SEP-24 specification.

const getAnchorServices = async (): Promise<AnchorServiceInfo> => {
  return await anchor.getServicesInfo();
};

Interactive Flows​

Before getting started, make sure you have connected to the anchor and received an authentication token, as described in the Stellar Authentication wallet guide. We will use the authToken object in the examples below as the SEP-10 authentication token, obtained earlier.

To initiate an operation, we need to know an asset. You may want to hard-code it, or get it dynamically from the anchor's info file, like shown below (for USDC):

import { IssuedAssetId } from "@stellar/typescript-wallet-sdk";

const assetCode = "USDC";

const info = await anchor.getInfo();

const currency = info.currencies.find(({ code }) => code === assetCode);

if (!currency?.code || !currency?.issuer) {
  throw new Error(
    `Anchor does not support ${assetCode} asset or is not correctly configured on TOML file`,
  );
}

const asset = new IssuedAssetId(currency.code, currency.issuer);
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Before starting with the deposit flow, make sure that the user account has established a trustline for the asset you are working with.

Basic Flow​

Let's start with a basic deposit:

const deposit = await sep24.deposit({
  assetCode: asset.code,
  authToken,
});

As a result, you will get an interactive response from the anchor.

Open the received URL in an iframe and save the transaction ID for future reference:

const url = deposit.url;
const id = deposit.id;

Similarly to the deposit flow, a basic withdrawal flow has the same method signature and response type:

const withdrawal = await sep24.withdraw({
  assetCode: asset.code,
  authToken,
});
const url = withdrawal.url;
const id = withdrawal.id;

Providing KYC Info​

To improve the user experience, the SEP-24 standard supports passing user KYC to the anchor via SEP-9. In turn, the anchor will pre-fill this information in the interactive popup.

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While SEP-9 supports passing binary data, the current version of the SDK doesn't offer such functionality.

note

At the time, accepted SEP-9 is not strictly typed yet. Improved typing will be offered in future versions.

const deposit = await sep24.deposit({
  assetCode: asset.code,
  authToken,
  extraFields: { email_address: "[email protected]" },
});

Changing Stellar Transfer Account​

By default, the Stellar transfer will be sent to the authenticated account (with a memo) that initiated the deposit.

While in most cases it's acceptable, some wallets may split their accounts. To do so, pass additional account (and optionally a memo):

import { Memo, MemoText } from "stellar-sdk";

const recipientAccount = "G...";
const depositDifferentAccount = async (): Promise<InteractiveFlowResponse> => {
  return await sep24.deposit({
    destinationAccount: recipientAccount,
    destinationMemo: new Memo(MemoText, "some memo"),
    assetCode: asset.code,
    authToken,
  });
};

Similarly, for a withdrawal, the origin account of the Stellar transaction could be changed:

const originAccount = "G...";
const withdrawal = await sep24.withdraw({
  withdrawalAccount: originAccount,
  assetCode: asset.code,
  authToken,
});

Getting Transaction Info​

On the typical flow, the wallet would get transaction data to notify users about status updates. This is done via the SEP-24 GET /transaction and GET /transactions endpoint.

Alternatively, some anchors support webhooks for notifications. Note that this feature is not widely adopted yet.

Tracking Transaction​

Let's look into how to use the wallet SDK to track transaction status changes. We will use Watcher class for this purpose. First, let's initialize watcher and start tracking a transaction.

const watcher = sep24.watcher();

const { stop, refresh } = watcher.watchOneTransaction({
  authToken,
  assetCode: asset.code,
  id: successfulTransaction.id,
  onMessage,
  onSuccess,
  onError,
});

Alternatively, we can track multiple transactions for the same asset.

const watcher = sep24.watcher();

const { stop, refresh } = watcher.watchAllTransactions({
  authToken,
  assetCode: asset.code,
  onMessage,
  onError,
});

Fetching Transaction​

While Watcher class offers powerful tracking capabilities, sometimes it's required to just fetch a transaction (or transactions) once. The Anchor class allows you to fetch a transaction by ID, Stellar transaction ID, or external transaction ID:

const transaction = await sep24.getTransactionBy({
  authToken,
  id: transactionId,
});

// "id" is the actual Anchor transaction id, all transactions should have it.
const transaction = await anchor.sep24().getTransactionBy({
  authToken,
  id: transactionId,
});

// "stellarTransactionId" (aka "stellar_transaction_id" on the SEP spec)
// is the hash of the Stellar network transaction payment related to this
// Anchor transaction.
// The "stellarTransactionId" has a SHA256 hash format like the below:
// - "a35135d8ed4b29b66d821444f6760f8ca1e77bea1fb49541bebeb2c3d844364a"
// E.g. we'll only have this transaction id field AFTER the wallet sends funds
// to Anchor on the withdrawal flow or receives funds from Anchor on the
// deposit flow.
const transaction = await anchor.sep24().getTransactionBy({
  authToken,
  stellarTransactionId,
});

// "externalTransactionId" (aka "external_transaction_id" on the SEP spec)
// could refer to some ID of transaction on external network.
// E.g. this could be some "reference number" displayed to the user on
// the last step of the Interactive Flow UI which the user could use in some
// other external place to complete the deposit or withdraw operation.
const transaction = await anchor.sep24().getTransactionBy({
  authToken,
  externalTransactionId,
});

It's also possible to fetch transaction by the asset

const transactions = await sep24.getTransactionsForAsset({
  authToken,
  assetCode: asset.code,
});

Submitting Withdrawal Transfer​

Previously, we took a look at starting the withdrawal flow. Now, let's take a look at a full example.

First, start the withdrawal:

const withdrawal = await sep24.withdraw({
  assetCode: asset.code,
  authToken,
});

Next, open an interactive url :

const url = withdrawal.url;
// open the url

After that we need to wait until the anchor is ready to receive funds. To do so, we will be waiting until transaction reaches pending_user_transfer_start status. This code uses a simple watching (polling) mechanism with no bail-out condition. The application’s code should be more robust.

const watcher = sep24.watcher();

const onMessage = (transaction) => {
  if (transaction.status === "pending_user_transfer_start") {
    // begin transfer code
  }
};

const onSuccess = (transaction) => {
  // transaction comes back as completed / refunded / expired
};

const onError = (transaction) => {
  // runtime error, or the transaction comes back as
  // no_market / too_small / too_large / error
};

// We can watch for a particular transaction.
const { refresh, stop } = watcher.watchOneTransaction({
  authToken,
  assetCode: asset.code,
  id: successfulTransaction.id,
  onMessage,
  onSuccess,
  onError,
});

// Or watch for ALL transactions of a particular asset.
const { refresh, stop } = watcher.watchAllTransactions({
  authToken,
  assetCode: asset.code,
  onMessage,
  onError,
});

Next, sign and submit the Stellar transfer:

// Import Horizon to get the result codes for error handling
import { Horizon } from "@stellar/stellar-sdk";

// This creates a transaction builder which we'll be using to assemble
// our transfer withdrawal transaction as shown below.
const txBuilder = await stellar.transaction({
  sourceAddress: keypair,
  baseFee: 10000, // this is 0.001 XLM
  timebounds: 180, // in seconds
});

// We can use the transaction object received on the onMessage callback from
// the watcher, or, we can also fetch the transaction object using either
// getTransactionBy or getTransactionsForAsset as illustrated in previous step.
onMessage: (transaction) => {
  if (transaction.status === "pending_user_transfer_start") {
    // Use the builder to assemble the transfer transaction. Behind the scenes
    // it extracts the Stellar account (withdraw_anchor_account), memo (withdraw_memo)
    // and amount (amount_in) to use in the Stellar payment transaction that will
    // be submitted to the Stellar network.
    const transferTransaction = txBuilder
      .transferWithdrawalTransaction(transaction, asset)
      .build();

    // Signs it with the account key pair
    transferTransaction.sign(keypair);

    // Finally submits it to the stellar network. This stellar.submitTransaction()
    // function handles '504' status codes (timeout) by keep retrying it until
    // submission succeeds or we get a different error.
    try {
      const response = await stellar.submitTransaction(transferTransaction);
      console.log("Stellar-generated transaction ID: ", response.id);
    } catch (error) {
      /*
        In case it's not a 504 (timeout) error, the application could try some
        resolution strategy based on the error kind.

        On Stellar docs you can find a page dedicated to error handling:
        https://developers.stellar.org/docs/learn/encyclopedia/errors-and-debugging/error-handling

        And status/result codes:
        https://developers.stellar.org/docs/data/horizon/api-reference/errors
      */

      // Let's illustrate here how we could handle an 'invalid sequence number' error.

      // We can access all possible result codes through Horizon's API.
      const sdkResultCodes = Horizon.HorizonApi.TransactionFailedResultCodes;

      // We can access error's response data to check for useful error details.
      const errorData = error.response?.data;
      /*
        Sample of errorData object returned by the Wallet SDK:
        {
          type: 'https://stellar.org/horizon-errors/transaction_failed',
          title: 'Transaction Failed',
          status: 400,
          detail: 'The transaction failed when submitted to the stellar network.
            The `extras.result_codes` field on this response contains further details.
            Descriptions of each code can be found at:
            https://developers.stellar.org/docs/data/horizon/api-reference/errors/http-status-codes/horizon-specific/transaction-failed',
          extras: {
            envelope_xdr: 'AAAAAgAAAADBjF7n9gfByOwlnyaJH...k4BRagf/////////8AAAAAAAAAAA==',
            result_codes: { transaction: 'tx_bad_seq' },
            result_xdr: 'AAAAAAAAAGT////6AAAAAA=='
          }
        }
      */

      /*
        Example scenario: invalid sequence numbers.

        These errors typically occur when you have an outdated view of an account.
        This could be because multiple devices are using this account, you have
        concurrent submissions happening, or other reasons. The solution is relatively
        simple: retrieve the account details and try again with an updated sequence number.
      */
      if (
        errorData?.status == 400 &&
        errorData?.extras?.result_codes?.transaction ===
          sdkResultCodes.TX_BAD_SEQ
      ) {
        // Creating a new transaction builder means retrieving an updated sequence number.
        const txBuilder2 = await stellar.transaction({
          sourceAddress: keypair,
          baseFee: 10000,
          timebounds: 180,
        });

        // ...

        // Repeat all the steps until submitting the transaction again.

        // ...

        const response2 = await stellar.submitTransaction(transferTransaction);
        console.log(
          "Stellar-generated transaction ID on retry: ",
          response2.id,
        );

        // The application should take care to not resubmit the same transaction
        // blindly with an updated sequence number as it could result in more than
        // one payment being made when only one was intended.
      }
    }
  }
};

Where keypair is the SEP-10 authenticated account. If you want to transfer funds from a different address, refer to Changing Stellar Transfer Account section.

Code for submitting transactions to Stellar should be developed thoughtfully. The SDF has a documentation page dedicated to submitting transactions and handling errors gracefully. Here are a few things you need to keep in mind:

  • Offer a high fee. Your fee should be as high as you would offer before deciding the transaction is no longer worth sending. Stellar will only charge you the minimum necessary to be included in the ledger -- you won't be charged the amount you offer unless everyone else is offering the same amount or greater. Otherwise, you’ll pay the smallest fee offered in the set of transactions included in the ledger.
  • Set a maximum timebound on the transaction. This ensures that if your transaction is not included in a ledger before the set time, you can reconstruct the transaction with a higher offered fee and submit it again with better chances of inclusion.
  • Resubmit the transaction when you get 504 status codes. 504 status codes are just telling you that your transaction is still pending -- not that it has been canceled or that your request was invalid. You should simply make the request again with the same transaction to get a final status (either included or expired).

Finally, let's track transaction status updates. In this example we simply check if the transaction has been completed:

const watcher = sep24.watcher();

const onSuccess = (transaction) => {
  // transaction came back as completed / refunded / expired
  console.log("Transaction is completed");
};

const onError = (transaction) => {
  // runtime error, or the transaction comes back as
  // no_market / too_small / too_large / error
};

const { refresh, stop } = watcher.watchOneTransaction({
  authToken,
  assetCode: asset.code,
  id: successfulTransaction.id,
  onMessage,
  onSuccess,
  onError,
});