An EU employee uploads a file from the US. Where is it stored?

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An employee of an EU company is travelling in the US. They upload a file. It lands on a US server. Where should it be stored?

The tempting answer: store it where the request landed. That would put an EU tenant's data on US infrastructure.

The request lands on a US app server. The storage router asks the tenant region service: which region is this tenant? The answer is EU. So the file goes to the EU bucket, under the tenant's ID.

The tenant's region was assigned at onboarding, from its country. Some contracts lock it. Changing it later means a data migration. Every lookup is cached for an hour.

But here's the catch. A tenant is migrated from the US to the EU. The code updates the database, but forgets the cache key. For up to an hour, uploads may still go to the US. So assigning a region always clears it.

Not where the user is. Not where the request lands. Residency follows the tenant.

Full breakdown in the lecture.