dynamodb.js

var AWS = require("aws-sdk");
AWS.config.update({
  region: "us-east-1"
});

exports.handler = async (event) => {
    console.log('event = ' + JSON.stringify(event)); // otherwise won't show

    if (event.httpMethod == "POST" && event.headers["User-Agent"] == "Starling-Bank-Web-Hook") {
        console.log("We have received a notification from Starling bank");
        
       let webhookSecret = process.env.webhookSecret; // this must be set in Environment variables

        // inspiration from
        // https://stackoverflow.com/questions/56664705/nodejs-base-64-encoding-of-the-sha-512-digest/59738487#59738487
    
        const crypto = require('crypto');
    
      let hash = crypto.createHash('sha512');
      hash.update(webhookSecret+event.body);
    
      const sigCheck = hash.digest('base64');
      const valid = sigCheck==event.headers['X-Hook-Signature'];
      
      if (valid) {
          console.log("Received valid request from Starling bank");
          
          let eb = JSON.parse(event.body);
          
          let webhookNotificationUid = eb.webhookNotificationUid;
          console.log(webhookNotificationUid);
          
          let timestamp = eb.timestamp;
          console.log(timestamp);
          
          let c = eb.content;
          console.log(c);
          
          let cclass = c.class;
          console.log(cclass);
          
          let transactionUid = c.transactionUid;
          console.log(transactionUid);
          
          let amount = c.amount;
          console.log(amount);
          
          let sourceCurrency = c.sourceCurrency;
          console.log(sourceCurrency);
          
          let sourceAmount = c.sourceAmount;
          console.log(sourceAmount);
          
          let counterParty = c.counterParty;
          console.log(counterParty);
          
          let reference = c.reference;
          console.log(reference);
          
          let type = c.type;
          console.log(type);
          
          let forCustomer = c.forCustomer;
          console.log(forCustomer);
          
          let accountHolderUid = eb.accountHolderUid;
          console.log(accountHolderUid);
          
          let webhookType = eb.webhookType;
          console.log(webhookType);
          
          let uid = eb.uid;
          console.log(uid);
          
          let customerUid = eb.customerUid;
          console.log(customerUid);
          
          // use values here and do what you want
          
    var db = new AWS.DynamoDB.DocumentClient();
                
    const dbdata = {
        TableName: 'StarlingBank',
      Item : {
              "id": 0,
              "accountHolderUid": accountHolderUid,
              "amount": amount,
              "class": cclass,
              "counterParty": counterParty,
              "customerUid": customerUid,
              "forCustomer": forCustomer,
              "reference": reference,
              "sourceAmount": sourceAmount,
              "sourceCurrency": sourceCurrency,
              "timestamp": timestamp,
              "transactionUid": transactionUid,
              "type": type,
              "uid": uid,
              "webhookNotificationUid": webhookNotificationUid,
              "webhookType": webhookType,
              "notes": "none"
        }
    };

    await db.put(dbdata).promise();
    
    // update the central balance
    var updateStr;
    if ( amount.toString()[0] == "-" ) {
        updateStr = "set bankbalance = bankbalance - :a" ;
    } else {
        updateStr = "set bankbalance = bankbalance + :a" ;
    }

    var updateBalance = {
        TableName: "Balance",
        Key:{
            "currentBalance": 0 // we're using this as a key only so the value must always remain 0
        },
        UpdateExpression: updateStr,
        ExpressionAttributeValues:{
            ":a": Math.abs(amount)
        },
        ReturnValues:"UPDATED_NEW"
    };
    
    console.log("Updating balance");
    await db.update(updateBalance).promise();
    // end balance
          
            const response = {
                statusCode: 200,
                body: JSON.stringify('Thanks for the notification Starling!'),
            };
            return response;
      }
        
    }
    

    const response = {
        statusCode: 404,
        body: JSON.stringify('404 error self inflicted'),
    };
    return response;

};