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What Happens Behind the Scenes of a Payment Gateway?

Picture of By Ram Nethaji

By Ram Nethaji

Founder

FinTech app development cost

User Interface Design

Custom software development
FinTech app development services
how payment gateway works​

When a card is declined at checkout, research consistently shows that more than a third of customers abandon the cart entirely rather than try another payment method. They do not retry, they leave. According to Stripe, 33% of customers do not retry after a payment failure at checkout, and those who encounter an error once are likely to abandon the transaction entirely, even after systems recover. That means a failed transaction is not just a missed sale. It is a lost customer. Most decision-makers know a payment gateway sits between the customer and the bank. Few know what happens in those two to three seconds, or which part of the chain is responsible when a transaction fails.

how payment gateway works​

What Is a Payment Gateway?

A payment gateway is a technology layer that securely captures a customer’s payment details, encrypts them, and routes them through the financial system for authorization. It acts as the digital equivalent of a card terminal at a physical store, handling the entire transaction remotely across multiple financial institutions in real time.

Traditionally, the gateway did not move money. That was the payment processor’s job. The gateway collected, secured, and relayed payment data from checkout to the banks and card networks involved, then returned the result to the merchant’s platform. Today, that distinction is less visible in practice. Modern providers like Stripe, Razorpay, and Adyen bundle the gateway, processor, acquiring bank, and orchestration layer into a single platform. Merchants rarely interact with these as separate components. But the layers still exist under the hood, and understanding them matters when something goes wrong, and you need to know where the failure actually happened.

Who Are the Key Players in a Payment Transaction?

Effective transaction security is not a single control. It is a layered architecture where each layer reinforces the others. A failure at one layer should be caught by the next.
Party Role
Customer Initiates the payment by entering card or wallet details
Merchant Receives the payment request and outcome via the gateway
Payment Gateway Encrypts and routes payment data between parties
Payment Processor Executes the transaction between the acquiring and issuing banks
Acquiring Bank The merchant’s bank, which requests authorization on the merchant’s behalf
Issuing Bank The customer’s bank, which approves or declines the transaction
Card Network The network (Visa, Mastercard, RuPay) routing data between both banks
Each party in this chain controls a different lever: fees, settlement speed, failure rates, and compliance obligations all trace back to specific roles.

What Actually Happens When a Customer Clicks "Pay"?

The entire flow completes in under three seconds. Here is what happens at each step:

  • Data capture: The customer enters payment details on the checkout page. The gateway collects this data using hosted fields or a JavaScript SDK, keeping it off the merchant’s servers and limiting PCI DSS compliance scope.
  • Tokenization: The gateway replaces raw card data with a unique reference token. This token is meaningless to anyone who intercepts it, which is why merchants are not required to store actual card numbers.
  • Authorization request: The gateway forwards the encrypted transaction to the acquiring bank, which routes it through the card network to the customer’s issuing bank.
  • Authentication (if triggered): For high-risk transactions, the issuing bank may trigger a two-factor check such as an OTP or 3D Secure (3DS) challenge. In India, this step is mandatory for most card transactions under RBI guidelines.
  • Approval or decline: The issuing bank evaluates the account balance, fraud flags, and card validity, then returns an approval or decline code back through the same chain.
  • Settlement: Authorization holds the funds but does not transfer them. Actual money movement from the issuing bank to the merchant’s account typically takes one to three business days. Delays happen because settlement runs in batches rather than per transaction. Cutoff times, banking hours, and the number of intermediaries in the chain all affect how quickly funds land. For high-volume merchants, those delays compound directly into cash flow gaps.

A timeout at step three, a failed OTP at step four, or an incorrect CVV at step one will result in a declined transaction the merchant may never fully diagnose without proper gateway reporting. Payment systems that were not designed with volume in mind tend to expose these gaps under load, which is a core reason why fintech apps struggle with scalability.

Why Do Payments Fail and Who Is Responsible?

Payment failures frustrate customers, but most businesses do not track them at the failure-point level. Knowing where the breakdown happens determines whether the fix lies with your gateway configuration, your bank relationships, or your checkout design.

  • Issuing bank decline: The customer’s bank rejects the transaction due to insufficient funds, card restrictions, or fraud flags. This is outside merchant control but can be reduced with retry logic and multiple payment method support.
  • Gateway timeout: The gateway does not receive a response from the processor within the required window. A gateway with low uptime SLAs will cause this more frequently.
  • Failed authentication: The customer does not complete the OTP or 3DS step because the prompt did not load or the session expired. This is a checkout UX issue as much as a technical one.
  • Tokenization mismatch: For recurring payments, the stored token fails if the customer’s card was reissued or expired without the token being updated. This is a common silent failure in subscription businesses.
  • PCI scope exposure: A merchant integration that handles raw card data, even briefly, does not trigger an immediate gateway rejection. The risk is longer-term: audit findings, fines from card networks, and in serious cases, loss of acquiring privileges. According to the PCI Security Standards Council, compliance obligations apply to every entity that stores, processes, or transmits cardholder data. How a gateway is architected determines how much of that scope falls on the merchant, and is worth understanding before any integration decision, particularly if you are evaluating whether to build a custom payment gateway.

What Should Businesses in India Look for in a Payment Gateway?

India’s payment ecosystem has specific requirements that generic guides do not address. According to the RBI Payment System Report, UPI accounted for 83% of India’s total digital payment volume in 2024, against an overall digital payments base of approximately 208.5 billion transactions. Any gateway deployed for an Indian audience without UPI support is already losing a majority of potential transactions. The compliance and infrastructure requirements in India are also distinct enough that creating a payment gateway in India warrants its own evaluation, separate from what a generic gateway comparison would reveal.

Key criteria for Indian businesses evaluating a payment gateway:

  • UPI support: Mandatory for consumer-facing products. Look for P2P and P2M flows, including UPI Lite for low-value transactions.
  • PCI DSS Level 1 certification: The highest compliance tier. Non-certified gateways shift significant liability onto the merchant.
  • 3D Secure v2 (3DS2) support: Required for card transactions in India under RBI two-factor authentication guidelines. 3DS2 reduces false declines compared to the older protocol.
  • Settlement speed: Standard settlement in India runs T+1 to T+2. Instant settlement options improve cash flow for high-volume merchants and marketplaces.
  • Multi-currency support: Essential for SaaS products and exporters. Confirm RBI-compliant handling of foreign inward remittances.
  • Webhook reliability: Server-side webhooks confirm final transaction status independently of the customer’s browser. Unreliable webhooks create reconciliation gaps.

Each of these criteria carries implementation cost implications that are easy to underestimate, particularly multi-currency support and compliance tooling. If you are scoping a fintech product, it is worth reading up on hidden fintech app development costs before locking in a gateway architecture.

Conclusion

The question worth asking is not whether your gateway works. It is whether you can tell when it stops working and why. Authorization rate drops, settlement delays, and silent subscription failures are all diagnosable if the gateway is instrumented correctly and the integration is built to the right compliance standard. Teams that can trace failures to their source fix them faster and lose less revenue in the process.

For businesses operating in India, getting this right starts with understanding what the integration actually involves. The full cost of building a payment system is often broader than the gateway license fee alone. Zethic works with product teams across fintech software development, ecommerce, and SaaS, helping ensure the gateway layer is configured, secured, and built to hold up at scale.

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Frequently Asked Questions

A payment gateway captures and encrypts payment data and routes it for authorization. A payment processor executes the actual fund movement between banks. Most modern platforms like Stripe or Razorpay bundle both, but they are distinct layers with separate responsibilities.
Authorization completes in two to three seconds. Settlement, where funds physically move to the merchant’s account, takes one to three business days depending on the gateway and acquiring bank. Some Indian gateways offer T+1 or instant settlement for an additional fee.
Yes, partially. PCI DSS applies to every entity that stores, processes, or transmits cardholder data. Merchants using hosted or redirect gateways carry a lower compliance burden (SAQ-A level) because they never handle raw card data directly. API-based integrations that touch card data carry significantly higher obligations and require formal audits.
Tokenization replaces a customer’s raw card details with a unique, non-sensitive reference string generated by the gateway. This token can be stored and reused for recurring charges without the merchant ever holding actual card data, which dramatically reduces the scope of a potential data breach and simplifies PCI DSS compliance.
Most enterprise-grade gateways support multi-currency processing, but the level of support varies significantly. Some handle only currency conversion at checkout, while others manage local payment methods, regional compliance, and settlement in the merchant’s home currency. For Indian businesses accepting international payments, the gateway must also be configured to handle RBI-compliant foreign inward remittances.
A hosted gateway redirects the customer to an external payment page to complete the transaction, keeping all card data off the merchant’s infrastructure. An API-based gateway processes the payment directly on the merchant’s platform, giving full control over the checkout experience but placing a higher compliance burden on the merchant. The right choice depends on the business’s technical capacity and its tolerance for PCI DSS scope.

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