Payment Webhooks & Immutable Ledger

01 / Context

Overview & purpose

A Blue ICT × Horse Capital technical assessment: a TypeScript service that authenticates payment notifications, records each payment once and reconciles missing transactions without rewriting financial history.

Idempotent payment processing with PostgreSQL deciding concurrency.

02 / Contribution

My work

Implemented the webhook service, transactional ledger, reconciliation CLI and documented database validation.

03 / Functionality

Inside the application

• Raw-body HMAC verification
• Atomic event and ledger insertion
• Database uniqueness and immutable triggers
• Provider-file reconciliation and recovery

04 / Engineering

Technical perspective

The database makes the final concurrency decision. Successful acknowledgement follows transaction commit, and failures allow provider retries. This is a technical assessment, not a commissioned client payment product.