System Design - Ledger System

The Ledger System problem is a cornerstone of the Coinbase interview process, appearing in both the CodeSignal online assessment and system design rounds. It typically asks candidates to design or implement a core banking service that manages customer account balances with strict financial correctness. YouTube +2 2 0 6 3

1. Problem Statement Overview

You are tasked with building a core ledger service for a fintech platform. The system must support customer account balances, ensuring full auditability and correctness even under high concurrency and potential failures. Reddit +1 6 0

2. Functional Requirements

• Transactions: Support basic operations like deposits, withdrawals, and internal transfers between accounts.
• Balance Queries: Distinguish between "posted" (finalized) and "available" balances.
• Audit Trail: Maintain a full transaction history with support for pagination and filtering.
• External Integration: Handle deposits and reconciliations from external sources like ACH, cards, and PayPal. YouTube +1

3. Non-Functional & Technical Requirements

• ACID Semantics: Ensure atomicity for multi-account transfers to prevent "double-spending" or funds disappearing during a crash.
• Double-Entry Bookkeeping: Every transaction must have a corresponding debit and credit to ensure the books always balance for audit purposes.
• Idempotency: Requests must have an "exactly-once" effect; retrying a failed network request should not result in a duplicate charge.
• Concurrency Control: The system must handle high throughput without race conditions, often using optimistic or pessimistic locking on account records. Reddit +1

4. Common Extensions & Edge Cases

• Reconciliation: How to resolve discrepancies when a transaction is initiated but not yet reflected in the external bank.
• State Machines: Modeling the lifecycle of a transfer (e.g., PENDING →right arrow→ SETTLED or FAILED).
• Fraud Checks: Integrating spending limits or risk rules into the transaction flow. YouTube +2

5. Interview Format Context

• CodeSignal (LLD): Usually 4 parts where each subsequent problem builds on the last (e.g., Part 1: Basic Get/Set; Part 2: Transactions; Part 3: TTL/History; Part 4: Advanced filtering).
• System Design (HLD): Focuses on scalability, sharding/partitioning strategies, and choosing between SQL vs. NoSQL based on consistency needs.

Are you preparing for a specific stage? I can provide:

• A Python/Java implementation of a simple thread-safe ledger.
• A system design checklist for the high-level round.
• CodeSignal-specific tips for the 4-part problem format.

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