API Design Patterns for Fintech Platforms
API Design Patterns for Fintech Platforms
Financial APIs carry more weight than most. A poorly designed API in a social media app means a frustrating developer experience. A poorly designed API in a payment gateway means lost money, regulatory issues, or both. Here are the patterns I've settled on after years of building financial APIs.
Predictable Resource Modeling
Financial resources — accounts, transactions, settlements — should follow a consistent URL pattern. /v1/accounts/{id}/transactions is predictable. /getTransactions?accountId={id} is not. The first tells you the relationship immediately; the second requires reading documentation to understand the parameter name.
Versioning Strategy
URL-based versioning (/v1/, /v2/) is the only approach that works reliably for financial APIs. Header-based versioning breaks when intermediaries strip headers. Query parameter versioning creates caching issues. URL versioning is explicit, cacheable, and impossible for an API gateway to accidentally strip.
Idempotency Keys
Every mutating endpoint should accept an idempotency key. This isn't optional for financial APIs — it's a requirement. Without it, network retries can create duplicate charges. The key should be generated by the client and checked by the server before processing. If the server has seen the key before, return the original response without processing again.
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