UPI Architecture in Depth: VPA, PSP Apps and the NPCI Switch

The Virtual Payment Address, or VPA, is the identifier that makes UPI easy to use, because it replaces the bank account number and IFSC with a simple handle such as debarghya@okhdfcbank. A VPA is mapped to a specific bank account in the customer bank, and the mapping is maintained by the bank and the payment service provider. When a customer shares a VPA, they are not sharing account details, and they can create multiple VPAs for the same account or change the VPA without changing the account. A BA must document the VPA format, the uniqueness rules, the mapping table, and the process for handling a VPA that is deactivated or transferred to another bank. The VPA is also the key that the UPI switch uses to route a payment to the correct bank. PSP apps are the customer facing apps that provide the UPI service, such as Google Pay, PhonePe, Paytm and the bank own UPI apps. A PSP, or Payment Service Provider, is the entity that builds the app, onboards the customer, and connects to the NPCI switch through a sponsor bank. The app handles the customer interface, the device binding, the UPI PIN entry, and the transaction history, while the actual movement of money happens between banks. A BA must distinguish the PSP role from the bank role. The PSP does not hold the customer funds, the bank does, and a failure in the PSP app should never be confused with a failure in the bank account. The requirements should state what the app shows when the bank is down versus when the network is down. The NPCI switch is the central hub that routes UPI messages between banks and PSPs, and it is the single most important component in the architecture. When a customer initiates a payment, the PSP sends the request to the switch, the switch validates the message, identifies the remitter and beneficiary banks from the VPAs, and forwards the request to the remitter bank. The remitter bank debits the customer account, sends a confirmation to the switch, the switch forwards a credit request to the beneficiary bank, and the beneficiary bank credits the account and confirms back through the switch. The whole round trip is designed to complete in a few seconds, and the switch maintains the transaction status so that a timed out transaction can be resolved as success or failure. A BA should document the message flow, the timeout values and the status reconciliation process. Bank CBS integration is where UPI meets the core banking system, and it is the layer where many UPI failures originate. The CBS, or Core Banking System, holds the account, the balance and the transaction history, and the UPI module must call the CBS to check the balance, debit the account, and post the entry, all within the UPI response time of a few seconds. If the CBS is slow or under maintenance, the UPI transaction times out, and the customer sees a failure even though the money may have been debited. That is why the bank needs a robust reversal and reconciliation process. Every debit without a confirmed credit must be reversed automatically, and every timed out transaction must be checked against the CBS before the customer is told. The BA must specify the CBS interface, the timeout, the retry policy and the reversal timelines. A worked example shows the timing clearly. A customer in Bengaluru pays 2,000 rupees to a friend using a VPA at 10 in the morning, and the PSP app sends the request to the NPCI switch in under half a second. The switch routes it to the remitter bank, say ICICI Bank, which checks the balance and debits the account by 2 seconds past 10 in the morning, and the switch then routes the credit to the beneficiary bank, say State Bank of India, which credits the account by 3 seconds past 10 in the morning. Both customers receive a confirmation by 4 seconds past 10 in the morning, and the transaction appears in both bank statements with the same UPI reference. If the beneficiary bank does not respond within the timeout, the switch marks the transaction as pending, checks the status with the beneficiary bank, and either confirms the credit or reverses the debit, and the customer must never be left in doubt about whether the money moved.