Introducing pg_karnak: Transactional schema migration across tenant databases

When we need to describe Nile in a single sentence, we say "PostgreSQL re-engineered for multi-tenant apps". By multi-tenant apps, we mean applications like Stripe, Figma, Twilio, Notion, Workday, and Gusto - here a large number of customers is served from a shared application stack. In these types of applications, a key architectural challenge is deciding how to store data for each customer.

What makes this data architecture especially challenging is the tension between two opposing needs. On one hand, requirements like latency, compliance, and scalability often push toward placing tenants on multiple databases. On the other hand, a single database offers a significantly better developer experience and cost model. Broadly speaking, there are two main approaches.

This architecture provides isolation and flexibility but requires more resources and effort to operate. The other approach is to place all tenants in the same shared schema and add a tenant identifier to every table:

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