Designing Idempotent APIs for Mobile Clients: Retry Logic, Idempotency Keys, and the Patterns That Prevent Double Charges

--- title : " Designing Idempotent APIs for Mobile Clients" published : true description : " A hands-on guide to implementing database-backed idempotency keys in Ktor, client-side retry with exponential backoff, and preventing double charges in mobile APIs." tags : kotlin, android, api, architecture canonical_url : https://blog.mvpfactory.co/designing-idempotent-apis-for-mobile-clients --- ## What We Will Build In this workshop, we are building a Stripe-style idempotency layer for a mobile payment API. By the end, you will have a Ktor endpoint that deduplicates requests using PostgreSQL, a Kotlin retry utility with exponential backoff and jitter, and a pattern for handling partial failures across multi-step mutations. Let me show you a pattern I use in every project that handles money. ## Prerequisites - Kotlin 1.9+ and Ktor 2.x for the server - A PostgreSQL instance (local or Docker) - An Android or KMP client using Ktor Client or Retrofit - Familiarity with coroutines and `suspend` f