How We Migrated 55,000 Customers

The constraint

55,000 people had purchased on Shopify. They expected continued access. We were moving to a completely new system. Different auth. Different hosting. Different everything.

Nobody should re-purchase. Nobody should lose access. Nobody should create a new account. Log in with the same email. Everything works.

The approach

Two layers:

1. Cache every Shopify customer email in the database. If your email is in the list, you have access. No Shopify API call at runtime.
2. Email-only auth. No passwords to migrate. Verify with the same email you purchased with. The system recognizes you automatically.