Define the integration contract
List each operation, required fields, response shapes and permitted actors. Specify which system owns customer, order or stock data and how changes flow. Version contracts deliberately and distinguish validation failures from temporary service errors. Document rate limits and pagination. A successful request in a development console is only the beginning of integration testing.
Authentication and access
Keep provider secrets in server-side configuration, separate test and production accounts, and request the smallest useful permission set. Verify webhook signatures using the provider’s documented method. Rotation and revocation should be possible without editing a browser bundle. An API used by multiple organizations must check organization access on every record operation.
Retries, duplicates and ordering
Providers may deliver the same event repeatedly or events may arrive out of order. Use stable event identifiers and idempotent processing. Retry transient failures with a bounded policy and send unresolved work to an inspectable queue. Reconciliation should compare the local state with the provider, so a missed callback can be detected and repaired.
Integration acceptance
Test expired credentials, missing permissions, malformed payloads, timeouts and duplicate callbacks. Use provider sandboxes where available, then verify approved production flows without fabricating payment, OTP or delivery success. Agree monitoring ownership and maintenance responsibility when an external API changes. Bring provider documentation, account access requirements and sample sanitized payloads to the scope review.