Fallback Routing (SMS -> Email) Module

Problem

Implement an extensible message-delivery module that tries channels in order (SMS, then email, then push) until one succeeds, with retry/backoff on failure.

Example

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

Constraints

• Channels pluggable; transient vs permanent failures differ

Approach

Strategy/chain pattern over channels; exponential backoff; idempotency to avoid double-send.