Migrate a monolithic wallet to microservices

Q: Migrate a monolithic wallet to microservices

This question evaluates a candidate's ability to design and plan a safe, compliant migration of a high-scale digital wallet from a monolithic architecture to microservices, covering service decomposition, data ownership, transactional consistency, security and key management, compliance, observability, and deployment strategies.

Q: How do I approach System Design interview questions?

System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

Q: What difficulty level is this interview question?

This is a hard difficulty System Design question, commonly asked during Technical Screen rounds at Apple.

Q: What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Apple during technical interviews.

Question

System Design: Migrating a Monolithic Digital Wallet (Apple Cash–like) to Microservices

Context

You operate a high-scale consumer digital wallet and peer‑to‑peer (P2P) payments app. The current system is a monolith with a single database handling identity/KYC, wallet balances, ledger, payment processing, risk decisions, and notifications. The system must meet strict reliability, security, and regulatory requirements.

Assumptions (adjust as needed):

Millions of monthly active users; peak thousands of TPS for wallet reads/writes.
Regulated environment; PCI DSS and PII obligations apply; sponsor bank and payment rails integrations (e.g., card/ACH/real-time payments).
Double-entry accounting is required; no loss or creation of funds.
99.99%+ availability targets for critical money-movement paths.

Task

Design a migration from the monolith to a microservices architecture. Address:

Service boundaries and responsibilities (e.g., identity/KYC, wallet, ledger, payments orchestration, risk/fraud, notifications, funding/settlement, limits/compliance, reporting).
Data ownership and a safe migration plan from the monolith’s database to service-owned data stores.
API design, versioning, and idempotency across retries and network faults.
Cross-service transaction patterns (saga vs. two-phase commit) and when to use each.
Consistency models and reconciliation (internal ledger and with external partners).
Authentication/authorization (user-to-service and service-to-service), key management, and secrets.
PCI/PII compliance and data governance (tokenization, encryption, retention, right-to-erasure).
Observability (tracing, metrics, logs, audit) and operability (SLOs, alerts, DLQs, replay).
Deployment and rollout strategy (strangler fig, canary, blue/green, rollback), including backward/forward compatibility.
A phased migration plan with milestones, key risks, and success metrics.

Migrate a monolithic wallet to microservices

Quick Overview

Migrate a monolithic wallet to microservices

System Design: Migrating a Monolithic Digital Wallet (Apple Cash–like) to Microservices

Context

Task

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