Why it matters
Payments are where banking meets the outside world in real time. A customer who taps “pay” expects an answer in seconds, and a merchant expects the money to be there. Behind that answer, several independently run systems must agree on what happened, even when a network link drops halfway through. Getting that agreement right, every time, is the core problem of payment architecture.
The model explained
Participants, switch and settlement
A payment system has participants, usually banks, and an operator that runs a central switch. The switch receives a payment message from the payer’s bank, routes it to the payee’s bank, collects the answer and passes it back. Smaller banks and non-bank firms often join through a sponsor bank that is a direct member and settles on their behalf.
Moving the message is only half the job. Settlement moves the money between banks, in the accounts they hold at the central bank. Some systems settle each payment on its own; others settle the net amount each bank owes at intervals.
From payment messages to settlement
Text description
Banks exchange payment messages through the switch. The switch works out each bank's net position for the cycle, and those net amounts are settled in the banks' accounts at RBI.
- Bank A and Switch: routes messages and works out net positions
- Bank B and Switch: routes messages and works out net positions: messages
- Bank C and Switch: routes messages and works out net positions
- Switch: routes messages and works out net positions to Banks' settlement accounts at RBI: net amounts per cycle
Message standards
Messages follow published formats so that every participant can read them. Card and ATM networks have long used ISO 8583ISO 8583. An international standard for card and similar transaction messages, used by card networks and ATM switches., a compact format built for authorisations. ISO 20022ISO 20022. An international standard for richly structured financial messages, increasingly used for high-value and cross-border payments. is a richer, structured format increasingly used for high-value and cross-border payments, because it carries more information about the payer, the payee and the purpose. UPIUnified Payments Interface. NPCI's real-time system for paying from a bank account with a UPI ID, QR code or phone number, through apps on a phone. defines its own interface specifications, which banks and apps must implement and certify against.
Every payment is a state machine
A payment moves through a small number of states and must end in exactly one final state. The difficult cases are the ones in the middle: the payer has been debited but no answer has come back from the payee’s bank.
States of a push payment
Text description
A payment is initiated. It is either declined, or the payer is debited. After the debit, the payee is credited and the payment completes, or no answer arrives and it becomes pending. A pending payment either completes when the credit is confirmed or is reversed back to the payer.
- Initiated to Declined: PIN or funds fail
- Initiated to Payer debited
- Payer debited to Payee credited
- Payer debited to Pending (no answer yet): time-out
- Payee credited to Completed
- Pending (no answer yet) to Completed: credit confirmed (optional or asynchronous)
- Pending (no answer yet) to Reversed: no credit
Idempotency: the same request twice must do nothing new
Networks fail, so senders retry. If a retry is processed as a new payment, the customer pays twice. Every payment therefore carries a unique reference created by the sender, and every system in the chain records references it has already handled. A repeat with a known reference returns the original answer instead of moving money again. The same reference lets any party ask later, “what happened to this payment?”, and get one consistent answer.
Reversals, disputes and reconciliation
A pending payment is resolved either by confirming the credit or by reversing the debit. At the end of each cycle, every bank compares its own records with the operator’s settlement report. Any mismatch becomes an exception to investigate. Disputes, such as a customer saying goods never arrived, follow a separate, slower process with its own rules and deadlines.
How it plays out in Indian banking
India runs both kinds of settlement side by side. RTGS settles each transaction in real time on a one-to-one gross basis.1Institute for Development and Research in Banking Technology. About IDRBT. As of 1 January 2025. When NEFT moved to 24x7 in December 2019, RBI set 48 half-hourly settlement batches a day.2Reserve Bank of India. Availability of National Electronic Funds Transfer (NEFT) System on 24x7 basis (RBI/2019-20/111). As of 6 December 2019. Cheques now settle on a fixed hourly rhythm: Under continuous clearing, CTS settles every hour from 11:00, based on positive confirmations and deemed approvals from drawee banks.5Reserve Bank of India. Introduction of Continuous Clearing and Settlement on Realisation in Cheque Truncation System (RBI/2025-26/73). As of 13 August 2025.
Scale turns rare edge cases into daily events. At about 791 million a day4Business Standard. UPI sets new record at 24.51 bn transactions in Aug, value at ₹29.82 trn. As of 31 August 2026., even a tiny share of time-outs means many stuck payments every day, so the rules for resolving them matter. RBI's framework for failed transactions, in force since 15 October 2019, sets outer time limits for reversal and requires compensation to be paid automatically, without a complaint.3Reserve Bank of India. Harmonisation of Turn Around Time (TAT) and customer compensation for failed transactions using authorised Payment Systems (RBI/2019-20/67). As of 20 September 2019. If a UPI transfer debits the payer but does not credit the beneficiary, the beneficiary bank must reverse it by T+1 day; after that, ₹100 a day is payable.3Reserve Bank of India. Harmonisation of Turn Around Time (TAT) and customer compensation for failed transactions using authorised Payment Systems (RBI/2019-20/67). As of 20 September 2019. If a UPI payment to a merchant debits the payer but the merchant gets no confirmation, it must be reversed within T+5 days; after that, ₹100 a day is payable.3Reserve Bank of India. Harmonisation of Turn Around Time (TAT) and customer compensation for failed transactions using authorised Payment Systems (RBI/2019-20/67). As of 20 September 2019.
These deadlines shape system design. A bank needs automated status checks, reversal jobs that run well inside the limit and records that prove when each step happened.
Trade-offs
| Choice | You gain | You give up |
|---|---|---|
| Gross settlement | No settlement risk between banks | Banks need more money on hand in their settlement accounts |
| Net settlement | Far less money needs to move | Exposure between banks until the cycle settles |
| Credit the customer before settlement | An instant experience | The bank carries the risk until settlement completes |
| Strict time-outs | Fast answers for customers | More payments land in the pending state and need resolving |
Failure modes
- Duplicate payments when a retry is treated as new because references are missing or not checked.
- Stuck payments that stay pending because no job asks for their status or reverses them in time.
- Reconciliation breaks when a bank’s records and the operator’s report disagree and no one owns the difference.
- Sponsor-bank bottlenecks when many smaller participants depend on one direct member’s systems.
When to use it, and when to avoid it
Use this model, a central switch with a state machine per payment, for any system where many independent parties must agree on money. Avoid building a private payment network where a public rail already does the job; joining the existing rail is usually cheaper and safer. Inside a bank, avoid letting each channel talk to each rail directly. A single payment hub that owns references, states and reconciliation is easier to control.
Questions for a design review
- Where is the unique reference created, and does every retry reuse it?
- Which component owns each payment’s state, and can it answer a status query at any time?
- How are pending payments found and resolved inside the regulator’s time limits?
- What does reconciliation compare, how often, and who owns each exception?
- What happens to in-flight payments if the switch connection drops?