Miron Marczuk's wroclove.rb 2023 talk (his first public tech talk). Tells the story of Apply4's journey from a single-region UK multi-tenant Rails SaaS to a fully separated multi-region multi-tenant setup after US clients required their data to be stored in-region. Compares three architectural options (separate data only, fully separate applications, or a hybrid) and explains why Apply4 picked full separation based on three framing questions (can tenants move across regions? how much data is shared? can users operate across regions?). Breaks the migration into two stages: (1) redirect users to the correct region using Route 53 geolocation DNS and load-balancer path-based rules (app.apply4.com → region-specific URLs, with 302 redirects after 301 was cached too aggressively by Chrome, plus SEO alternate links); (2) separate the data by categorizing tables as shared vs region-specific, adding an integer 'bucket' column (via a Rails enum with values not_bucketed/region_1/region_2/shared), top-down bucketing starting from the Authority model's country attribute, previewing via default_scope, and swapping tables by creating an empty copy, inserting only in-region records, and renaming (with a 10% ID buffer to avoid ID clashes). Also covers file-system split by adding region-aware parent folders and using AWS DataSync. Go-live advice: inform clients, schedule low-usage maintenance window, write a detailed runbook, prepare contingency/revert plan, rehearse repeatedly, and use DNS records as the final trigger. Q&A covered logging and business-report aggregation across regions, and why table swap beats deletion.