Building a Savings Application with Blnk • Blnk Developer Documentation

In this tutorial, you’ll learn how to implement, monitor, and manage scheduled savings deposits with the Blnk ledger. Here’s what we’ll do:

Create a customer savings wallet balance.
Initiate a scheduled deposit transaction. For one-time funding: Wallet Funding Tutorial →.
Update the savings frequency.

For this tutorial, we’ll use the Blnk TypeScript SDK and Blnk Go SDK for the implementation. If you prefer, you can also refer to the API reference for details on the available endpoints.

Designing your map

Before writing code, it’s crucial to design a money movement map that outlines how money moves in your system. This serves as the blueprint for your implementation. For our scheduled savings deposit workflow, here’s our map:

Explore the map yourself here Read more: Scheduling transactions →

Set up your implementation

Based on our map, we’ll implement the following steps:

Initiate a scheduled transaction to Savings Wallet
Monitor the date of the next scheduled deposit to ensure timely execution.
Set up a new scheduled transaction after the current one has been successfully processed.

Prerequisites

Before starting, ensure you have:

A running Blnk Core instance (e.g. at http://localhost:5001).
An API key for Blnk (replace YOUR_API_KEY in the code examples). Required for authenticated requests.
Optionally, you can connect your Blnk Core to your Blnk Cloud workspace to view your ledger data.
A customer savings ledger to organise the customers’ savings balances. Learn how: How to Create a Ledger →

Create customer savings balance

Create a balance with the ledger_id of your Customer Savings Ledger to represent your customer savings wallet:

curl -X POST "http://localhost:5001/balances" \
  -H "X-blnk-key: <api-key>" \
  -H "Content-Type: application/json" \
  -d '{
    "ledger_id": "<customer-savings-ledger-id>",
    "identity_id": "<customer-identity-id>",
    "currency": "USD",
    "meta_data": {
        "wallet_type": "savings",
        "schedule_savings": true,
        "savings_frequency": "monthly",
        "savings_amount": 10000,
        "next_scheduled_savings": ""
    }
  }'

async function createSavingsWallet(savingsLedgerId, identityId, currency) {
  
  
  const savingsWallet = await blnk.LedgerBalances.create({
    ledger_id: savingsLedgerId,
    identity_id: identityId,
    currency: currency,
    meta_data: {
      wallet_type: "savings",
      schedule_savings: true,
      savings_frequency: "monthly",
      savings_amount: 10000,
      next_scheduled_savings: ""
    }
  });
  
  console.log("Savings Wallet created:", savingsWallet.data.balance_id);
  return savingsWallet.data.balance_id;
}

func createSavingsWallet(savingsLedgerID, identityID, currency string) (string, error) {
    client := getClient()
    balance, _, err := client.LedgerBalance.Create(blnkgo.CreateLedgerBalanceRequest{
        LedgerID:   savingsLedgerID,
        IdentityID: identityID,
        Currency:   currency,
        MetaData: map[string]interface{}{
            "wallet_type":            "savings",
            "schedule_savings":       true,
            "savings_frequency":      "monthly",
            "savings_amount":         10000,
            "next_scheduled_savings": "",
        },
    })
    if err != nil {
        return "", err
    }
    fmt.Println("Savings Wallet created:", balance.BalanceID)
    return balance.BalanceID, nil
}

This creates a savings balance with a set of default metadata:

This customer balance is subscribed to scheduled deposits.
Schedule frequency is monthly.
The next scheduled savings date can be monitored on the customer balance

Initiate scheduled transaction

Schedule a transaction on the ledger with the scheduled_for parameter:

curl -X POST "http://localhost:5001/transactions" \
  -H "X-blnk-key: <api-key>" \
  -H "Content-Type: application/json" \
  -d '{
    "precise_amount": 100000,
    "precision": 100,
    "reference": "SAV_0001",
    "description": "Scheduled savings deposit",
    "currency": "USD",
    "source": "@WorldUSD",
    "destination": "<savings-balance-id>",
    "scheduled_for": "2025-12-25T02:42:19Z",
    "meta_data": {
      "transaction_type": "savings"
    }
  }'

async function createScheduledSavingsTransaction(savingsBalanceId, savingsAmount, uniqueReference, description, scheduledDate) {
  
  
  const scheduledSavings = await blnk.Transactions.create({
    precise_amount: savingsAmount * 100,
    precision: 100,
    reference: uniqueReference,
    description: description || "Scheduled savings deposit",
    currency: "USD",
    source: "@WorldUSD",      // Represents external funding source
    destination: savingsBalanceId, 
    scheduled_for: scheduledDate,
    meta_data: {
      transaction_type: "savings"
    }
  });
  
  console.log("Savings successfully scheduled:", scheduledSavings.data.transaction_id);
  return scheduledSavings.data.transaction_id;
}

func createScheduledSavingsTransaction(savingsBalanceID string, savingsAmount float64, uniqueReference, description string, scheduledDate time.Time) (string, error) {
    client := getClient()
    if description == "" {
        description = "Scheduled savings deposit"
    }
    scheduledSavings, _, err := client.Transaction.Create(blnkgo.CreateTransactionRequest{
        ParentTransaction: blnkgo.ParentTransaction{
            PreciseAmount: savingsAmount * 100,
            Precision:   100,
            Reference:   uniqueReference,
            Description: description,
            Currency:    "USD",
            Source:      "@WorldUSD",
            Destination: savingsBalanceID,
            MetaData: map[string]interface{}{
                "transaction_type": "savings",
            },
        },
        ScheduledFor: &scheduledDate,
    })
    if err != nil {
        return "", err
    }
    fmt.Println("Savings successfully scheduled:", scheduledSavings.TransactionID)
    return scheduledSavings.TransactionID, nil
}

A SCHEDULED transaction is created waiting to be applied on the specified scheduled date.

Set up new scheduled transaction

To schedule the next savings deposit, verify that scheduled_savings is still active on the customer’s balance and retrieve its frequency.

Blnk sends a webhook when a scheduled transaction gets applied.

Update balance metadata and create the next scheduled savings transaction:

# After a scheduled savings transaction is applied, update balance metadata:
curl -X POST "http://localhost:5001/balances/<savings-balance-id>/metadata" \
  -H "X-blnk-key: <api-key>" \
  -H "Content-Type: application/json" \
  -d '{
    "meta_data": {
      "next_scheduled_savings": "2026-01-25T02:42:19Z"
    }
  }'

# Then create the next scheduled transaction:
curl -X POST "http://localhost:5001/transactions" \
  -H "X-blnk-key: <api-key>" \
  -H "Content-Type: application/json" \
  -d '{
    "precise_amount": 1000000,
    "precision": 100,
    "reference": "sch-savings-1709741652384",
    "description": "Scheduled savings deposit",
    "currency": "USD",
    "source": "@WorldUSD",
    "destination": "<savings-balance-id>",
    "scheduled_for": "2026-01-25T02:42:19Z",
    "meta_data": {
      "transaction_type": "savings"
    }
  }'

import express from 'express';
const app = express();
app.use(express.json());
app.post('/webhooks/transactions', async (req, res) => {
  const { event, data } = req.body;
  
  if (event === 'transaction.applied') {
    const transaction = data;
    
    if (
      transaction.meta_data?.transaction_type === 'savings'
    ) {
      console.log('Scheduled savings completed:', transaction.transaction_id);
      
      
      
      const savingsBalanceId = transaction.destination;
      const savingsBalance = await blnk.LedgerBalances.get(savingsBalanceId);
      
      if (
        savingsBalance.meta_data?.schedule_savings === true
      ) {
        const frequency = savingsBalance.meta_data.savings_frequency || "monthly";
        const amount = savingsBalance.meta_data.savings_amount;
        
        const nextScheduledDate = calculateNextScheduledDate(frequency);
        
        // 1. Update metadata with the next scheduled date
        const baseUrl = process.env.BLNK_BASE_URL ?? 'http://localhost:5001';
        const apiKey = process.env.BLNK_API_KEY ?? '';
        const metaRes = await fetch(`${baseUrl}/${savingsBalanceId}/metadata`, {
          method: 'POST',
          headers: {
            'X-Blnk-Key': apiKey,
            'Content-Type': 'application/json',
          },
          body: JSON.stringify({
            meta_data: {
              next_scheduled_savings: nextScheduledDate.toISOString(),
            },
          }),
        });
        if (!metaRes.ok) throw new Error(await metaRes.text());
        
        // 2. Create a new scheduled transaction
        const uniqueReference = `sch-savings-${Date.now()}`;
        const scheduledSavings = await blnk.Transactions.create({
          precise_amount: amount * 100,
          precision: 100,
          reference: uniqueReference,
          description: "Scheduled savings deposit",
          currency: "USD",
          source: "@WorldUSD",
          destination: savingsBalanceId,
          scheduled_for: nextScheduledDate.toISOString(),
          meta_data: {
            transaction_type: "savings"
          }
        });
        
        console.log(`Next scheduled deposit created for ${nextScheduledDate.toISOString()}:`, scheduledSavings.data.transaction_id);
      } else {
        console.log(`Scheduled savings is no longer active for balance ${savingsBalanceId}`);
      }
    }
  }
  
  res.status(200).send('Webhook received');
});

function calculateNextScheduledDate(frequency) {
  const now = new Date();
  const nextDate = new Date(now);
  
  switch (frequency) {
    case 'daily':
      nextDate.setDate(now.getDate() + 1);
      break;
    case 'weekly':
      nextDate.setDate(now.getDate() + 7);
      break;
    case 'biweekly':
      nextDate.setDate(now.getDate() + 14);
      break;
    case 'monthly':
      nextDate.setMonth(now.getMonth() + 1);
      break;
    default:
      nextDate.setMonth(now.getMonth() + 1);
  }
  
  return nextDate;
}

const PORT = process.env.PORT || 3000;
app.listen(PORT, () => {
  console.log(`Webhook server listening on port ${PORT}`);
});

func handleTransactionWebhook(w http.ResponseWriter, r *http.Request) {
    var payload struct {
        Event string                 `json:"event"`
        Data  map[string]interface{} `json:"data"`
    }
    if err := json.NewDecoder(r.Body).Decode(&payload); err != nil {
        http.Error(w, "invalid payload", http.StatusBadRequest)
        return
    }

    if payload.Event != "transaction.applied" {
        w.WriteHeader(http.StatusOK)
        return
    }

    meta, _ := payload.Data["meta_data"].(map[string]interface{})
    if meta["transaction_type"] != "savings" {
        w.WriteHeader(http.StatusOK)
        return
    }

    client := getClient()
    savingsBalanceID, _ := payload.Data["destination"].(string)
    savingsBalance, _, err := client.LedgerBalance.Get(savingsBalanceID)
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    balanceMeta := savingsBalance.MetaData
    if scheduleSavings, _ := balanceMeta["schedule_savings"].(bool); !scheduleSavings {
        w.WriteHeader(http.StatusOK)
        return
    }

    frequency, _ := balanceMeta["savings_frequency"].(string)
    if frequency == "" {
        frequency = "monthly"
    }
    amount, _ := balanceMeta["savings_amount"].(float64)
    nextScheduledDate := calculateNextScheduledDate(frequency)

    _, _, err = client.Metadata.UpdateMetadata(savingsBalanceID, blnkgo.UpdateMetaDataRequest{
        MetaData: map[string]interface{}{
            "next_scheduled_savings": nextScheduledDate.Format(time.RFC3339),
        },
    })
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    uniqueReference := fmt.Sprintf("sch-savings-%d", time.Now().Unix())
    scheduledSavings, _, err := client.Transaction.Create(blnkgo.CreateTransactionRequest{
        ParentTransaction: blnkgo.ParentTransaction{
            PreciseAmount: amount * 100,
            Precision:   100,
            Reference:   uniqueReference,
            Description: "Scheduled savings deposit",
            Currency:    "USD",
            Source:      "@WorldUSD",
            Destination: savingsBalanceID,
            MetaData: map[string]interface{}{
                "transaction_type": "savings",
            },
        },
        ScheduledFor: &nextScheduledDate,
    })
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    fmt.Printf("Next scheduled deposit created for %s: %s\n", nextScheduledDate.Format(time.RFC3339), scheduledSavings.TransactionID)
    w.WriteHeader(http.StatusOK)
}

Change savings frequency

To change the savings frequency of a balance, use the Update Metadata endpoint: Update the savings frequency on the customer balance:

curl -X POST "http://localhost:5001/balances/<balance-id>/metadata" \
  -H "X-blnk-key: <api-key>" \
  -H "Content-Type: application/json" \
  -d '{
  "meta_data": {
    "savings_frequency": "daily"
  }
}'

async function updateSavingsFrequency(balanceId, newFrequency) {
  
  
  try {
    const baseUrl = process.env.BLNK_BASE_URL ?? 'http://localhost:5001';
    const apiKey = process.env.BLNK_API_KEY ?? '';
    const res = await fetch(`${baseUrl}/${balanceId}/metadata`, {
      method: 'POST',
      headers: {
        'X-Blnk-Key': apiKey,
        'Content-Type': 'application/json',
      },
      body: JSON.stringify({
        meta_data: { savings_frequency: newFrequency },
      }),
    });
    if (!res.ok) throw new Error(await res.text());
    const data = await res.json();
    console.log(`Successfully updated savings frequency to ${newFrequency} for balance ${balanceId}`);
    return data;
  } catch (error) {
    console.error(`Failed to update savings frequency for balance ${balanceId}:`, error);
    throw error;
  }
}

func updateSavingsFrequency(balanceID, newFrequency string) error {
    client := getClient()
    _, _, err := client.Metadata.UpdateMetadata(balanceID, blnkgo.UpdateMetaDataRequest{
        MetaData: map[string]interface{}{
            "savings_frequency": newFrequency,
        },
    })
    if err != nil {
        return err
    }
    fmt.Printf("Successfully updated savings frequency to %s for balance %s\n", newFrequency, balanceID)
    return nil
}

Conclusion

In this tutorial, we’ve built a scheduled savings workflow for a wallet app using Blnk. Here’s what we covered:

Setting up savings ledgers and customer savings accounts
Adding scheduled deposits
Automatically rescheduling deposits after they’re completed
Handling deposit frequencies and customer preferences
Using webhooks to detect completed transactions and schedule new deposits.

With these features, your app offers customers an easy, automated way to save regularly without extra effort. The webhook-based architecture ensures that your system remains resilient and responsive to transaction events.

​Designing your map

​Set up your implementation

​Prerequisites

​Create customer savings balance

​Initiate scheduled transaction

​Set up new scheduled transaction

​Change savings frequency

​Conclusion

Designing your map

Set up your implementation

Prerequisites

Create customer savings balance

Initiate scheduled transaction

Set up new scheduled transaction

Change savings frequency

Conclusion