Fallback Routing (SMS -> Email) Module

Problem

Implement an extensible message-delivery module that tries channels in priority order (SMS, then email, then push) until one succeeds. On a channel failure, retry that channel with backoff before moving on; distinguish transient failures (retry) from permanent ones (skip immediately). New channels must be pluggable without editing existing ones.

Requirements

• send(msg) -> DeliveryResult: attempt channels in order, return success with the channel used, or failure if all are exhausted.
• Per-channel retry with exponential backoff, capped attempts.
• Idempotency so a retry never causes a double-send.

Constraints

• Channels are pluggable; a transient failure should be retried, a permanent one should short-circuit to the next channel.

Example

send(msg): try SMS -> transient fail -> retry SMS -> permanent fail -> try email -> success