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

What difficulty level is this interview question?

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

What role is this question designed for?

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

Migrate a monolithic wallet to microservices

Quick Overview

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.

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.

Quick Overview

Context

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

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

Context

Task

Solution

Comments (0)

Migrate a monolithic wallet to microservices

Quick Overview

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

Context

Task

Solution

Comments (0)