Implement mergeCustomers(oldId, newId) to merge two customer identities while preserving each account’s balances, transaction history, and transfer records (including pending/held). Specify how you remap ownership, handle customerId collisions, update indexes and caches (including the top-N payers structure), and ensure atomicity and consistency across the system.

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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Merge customer identities safely | Meta Interview Question

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

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).
Step-by-step algorithm for mergeCustomers(oldId, newId), including:
1. Ownership remapping strategy
2. Collision resolution rules
3. Index and cache updates (incl. top-N payers)
4. Atomicity/consistency guarantees and locking
Pseudocode-level detail sufficient for implementation.

Requirements

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).

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.

Merge customer identities safely

Quick Overview

System Design Task: mergeCustomers(oldId, newId)

Context

Task

Requirements

Deliverables

Solution

Comments (0)

Merge customer identities safely

Quick Overview

System Design Task: mergeCustomers(oldId, newId)

Context

Task

Requirements

Deliverables

Solution

Comments (0)