Merge customer identities safely
Overview
This question evaluates a candidate's understanding of system design, data modeling, transactional consistency, concurrency control, idempotency, and index/cache maintenance when merging customer identities in a payments platform.
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Aug 9, 2025, 12:00 AM
Software Engineer
Take-home Project
System Design
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System Design Task: mergeCustomers(oldId, newId)
Context
Your payments platform allows a single real person to accumulate multiple customer identities (duplicates). Each customer may own multiple accounts. Accounts have balances, immutable transaction history, and transfers (including pending/held states).
Task
Design and implement mergeCustomers(oldId, newId) to merge the identity represented by oldId into the identity represented by newId.
Requirements
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Preserve all accounts as-is (no balance moves, no ledger rewrites), including:
- Account balances
- Full transaction history
- Transfer records (completed, pending, held/authorized)
- Remap ownership from oldId to newId across the system.
- Handle customerId collisions (e.g., uniqueness constraints, duplicated identifiers, preferences).
- Update indexes and caches, including a top-N payers leaderboard/structure.
- Ensure atomicity, consistency, and idempotency; handle concurrent merges safely.
- Describe locking and failure handling.
Deliverables
- Clear invariants and data model assumptions (minimal and reasonable).
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Step-by-step algorithm for mergeCustomers(oldId, newId), including:
- Ownership remapping strategy
- Collision resolution rules
- Index and cache updates (incl. top-N payers)
- Atomicity/consistency guarantees and locking
- Pseudocode-level detail sufficient for implementation.
Solution
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