Banking & Financial Services
Payments & Transfers
Complete guide to payment systems — real-time transfers (IMPS/NEFT/RTGS), UPI, card payments, cross-border remittances, and emerging payment technologies.
13B+
Monthly UPI Txns
₹200T
Annual Value
<10s
IMPS Speed
40+
Countries
Understanding Payments & Transfers— A Developer's Domain Guide
Payments & Transfers encompass all systems and mechanisms that facilitate movement of money between parties. This includes domestic transfers (NEFT, RTGS, IMPS, UPI), card-based payments (credit/debit cards, POS terminals), digital wallets, cross-border remittances, and emerging technologies like CBDC. Modern payment systems require real-time processing, high availability, and robust security.
Why Payments & Transfers Domain Knowledge Matters for Engineers
- 1India processed 13B+ UPI transactions monthly in 2024
- 2Digital payments growing at 30%+ CAGR
- 3RBI mandates like tokenization creating new opportunities
- 4Cross-border payments undergoing major transformation
- 5Central Bank Digital Currencies (CBDC) emerging globally
- 6Payment orchestration platforms gaining traction
How Payments & Transfers Organisations Actually Operate
Systems & Architecture — An Overview
Enterprise Payments & Transfers platforms are composed of a set of core systems, data platforms, and external integrations. For a detailed, interactive breakdown of the core systems and the step-by-step business flows, see the Core Systems and Business Flows sections below.
The remainder of this section presents a high-level architecture diagram to visualise how channels, API gateway, backend services, data layers and external partners fit together. Use the detailed sections below for concrete system names, API examples, and the full end-to-end walkthroughs.
Technology Architecture — How Payments & Transfers Platforms Are Built
Modern Payments & Transfersplatforms follow a layered microservices architecture. The diagram below shows how a typical enterprise system in this domain is structured — from the client layer through the API gateway, backend services, data stores, and external integrations. This is the kind of architecture you'll encounter on real projects, whether you're building greenfield systems or modernising legacy platforms.
End-to-End Workflows
Detailed, step-by-step business flow walkthroughs are available in the Business Flows section below. Use those interactive flow breakouts for exact API calls, system responsibilities, and failure handling patterns.
Industry Players & Real Applications
🇮🇳 Indian Companies
NPCI
Payment Network
ISO 8583, Java
Operates UPI, IMPS, NACH, RuPay
RBI RTGS/NEFT
Settlement System
ISO 20022
Gross and net settlement systems
Paytm
Payment App
Java, Microservices
300M+ users, payments + banking
PhonePe
UPI App
Java, Spring Boot
500M+ users, market leader
Google Pay
UPI App
Android, Cloud
Major UPI player
Razorpay
Payment Gateway
Ruby, Go, AWS
8M+ businesses
🌍 Global Companies
Visa
GlobalCard Network
Mainframe, Modern APIs
65,000 txns/second capacity
Mastercard
GlobalCard Network
Cloud-native evolution
2.5B cards globally
SWIFT
GlobalMessaging Network
ISO 20022 migration
11,000+ institutions
Stripe
USAPayment Platform
Ruby, Go, AWS
Developer-focused, 46 countries
Adyen
NetherlandsPayment Platform
Java, Custom infra
Enterprise payments
Wise
UKCross-border
Java, Microservices
Low-cost international transfers
Core Systems
These are the foundational systems that power Payments & Transfers operations. Understanding these systems — what they do, how they integrate, and their APIs — is essential for anyone working in this domain.
Business Flows
Key Business Flows Every Developer Should Know.Business flows are where domain knowledge directly impacts code quality. Each flow represents a real business process that your code must correctly implement — including all the edge cases, failure modes, and regulatory requirements that aren't obvious from the happy path.
The detailed step-by-step breakdown of each flow — including the exact API calls, data entities, system handoffs, and failure handling — is covered below. Study these carefully. The difference between a developer who “knows the code” and one who “knows the domain” is exactly this: the domain-knowledgeable developer reads a flow and immediately spots the missing error handling, the missing audit log, the missing regulatory check.
Technology Stack
Real Industry Technology Stack — What Payments & Transfers Teams Actually Use. Every technology choice in Payments & Transfersis driven by specific requirements — reliability, compliance, performance, or integration capabilities. Here's what you'll encounter on real projects and, more importantly, why these technologies were chosen.
The pattern across Payments & Transfers is consistent: battle-tested backend frameworks for business logic, relational databases for transactional correctness, message brokers for event-driven workflows, and cloud platforms for infrastructure. Modern Payments & Transfersplatforms increasingly adopt containerisation (Docker, Kubernetes), CI/CD pipelines, and observability tools — the same DevOps practices you'd find at any modern tech company, just with stricter compliance requirements.
⚙️ backend
Java
Most payment switches, ISO 8583 processing
C/C++
High-performance card switches, legacy systems
Go
Modern payment systems (Stripe, newer gateways)
Node.js
Payment gateway APIs, merchant integrations
💡 messaging
ISO 8583
Traditional card payment message format
ISO 20022
Modern XML/JSON format, SWIFT migration
Apache Kafka
Event streaming, transaction processing
RabbitMQ
Message queuing for async processing
🗄️ database
Oracle
Enterprise payment systems, card switches
PostgreSQL
Modern payment platforms
Redis
Rate limiting, caching, deduplication
Cassandra
Transaction history, high-volume writes
💡 security
HSM
Hardware Security Module for key management
PCI DSS
Card data security standard compliance
Tokenization
Replace card numbers with tokens
3D Secure
Cardholder authentication protocol
Interview Questions
Q1.Explain the difference between NEFT, RTGS, and IMPS.
NEFT: Net settlement, batches every 30 min, no minimum, 24x7 since Dec 2019. RTGS: Gross settlement (each txn settled individually), minimum ₹2L, real-time, for high-value. IMPS: Instant 24x7, up to ₹5L, P2P and P2A, charges apply. IMPS is fastest but has limits; RTGS for high-value urgent; NEFT for regular transfers.
Q2.How does card tokenization work and why is it important?
Tokenization replaces actual card number (PAN) with unique token. Network tokens (Visa/MC) are generated per merchant/device. Device tokens stored in secure element (Apple Pay). Benefits: Reduced fraud risk, PCI scope reduction, seamless recurring payments. RBI mandated token-only storage for merchants from Oct 2022.
Q3.What happens when a UPI transaction fails?
Failure modes: 1) Timeout - NPCI marks as 'DEEMED', auto-reversal in 48hrs, 2) Debit success, credit fail - reversal initiated, 3) Technical decline - instant failure, no money moved. Customer can raise dispute via UDIR (UPI Dispute and Issue Resolution). NPCI has 5-day TAT for resolution.
Q4.Explain the 4-party model in card payments.
1) Cardholder - customer with card, 2) Merchant - accepts card payment, 3) Issuing Bank - issued card to customer, bears credit risk, 4) Acquiring Bank - processes merchant's transactions, bears fraud risk. Card network (Visa/MC) connects issuers and acquirers. Interchange fee flows from acquirer to issuer.
Q5.How do you ensure idempotency in payment APIs?
Idempotency ensures same request produces same result without double-processing. Implement via: 1) Idempotency key header (client-generated UUID), 2) Store request hash with response, 3) Return cached response for duplicate key, 4) Use database unique constraints on transaction ref. Critical for handling retries and network failures.
Glossary & Key Terms
NPCI
National Payments Corporation of India - operates UPI, IMPS, RuPay, NACH, etc.
PSP
Payment Service Provider - bank/entity that provides UPI services to customers
VPA
Virtual Payment Address - UPI identifier like yourname@bankname
IFSC
Indian Financial System Code - 11-char code identifying bank branch
Acquiring Bank
Bank that processes card transactions for merchants
Issuing Bank
Bank that issued the card to the customer
Interchange
Fee paid by acquirer to issuer for each card transaction
MDR
Merchant Discount Rate - fee charged to merchant for accepting payments
ISO 8583
International standard for financial transaction messages
3D Secure
Additional authentication layer for online card payments (OTP/biometric)
Chargeback
Disputed transaction reversed back to cardholder
Settlement
Actual movement of funds between banks after clearing