How do you design a reliable webhook system?

Webhooks deliver events to external URLs. Design: event queue (decouple from main flow), delivery workers, retry logic (exponential backoff), delivery logging. Reliability: at-least-once delivery (retry failures), idempotency keys (receiver dedupes), signatures (HMAC for authenticity), timeouts (protect against slow receivers). Management: registration API, secret rotation, pause/resume, delivery history. Challenges: receiver errors, high volume, security. Features: filtering (subscribe to specific events), batching, test endpoints. SLAs: define max delivery time, retry policy.