Operations, scale and edge cases
GOV.UK Notify's late-2019 SMS failover that broke both providers, and how to run the week's platform: a delivery funnel measured from created_at, with oldest message age per lane; SLOs that stop at the provider hand-off (99.9% of critical in 10 seconds is about 14 minutes of stall a month); pages on symptoms and a runbook; campaigns as paced, pausable jobs that respect FCM's quota and ramp rules; priority lanes that reach the provider through reserved quota, separate SMS senders and short validity periods; breakers per provider and route; SMS always split and shifted 10 points at a time; APNs outages and OTPs that keep one code across failover; segments, GSM-7 and UCS-2, and halving a ~$396,000-a-day SMS bill; SMS pumping; losing a region (fence, replay, re-admit); time zones, stale tokens and duplicates; and the whole week as one 45-minute interview.
Day 5: operations, scale and edge cases.
Previously, on Day 4, you built what runs through the pipes: presence that expires by itself, fan-out only to people looking, one owner sequencing each conversation, and duplicate checks on both ends.