# Online schema migrations

> Changes a big table without holding its lock, so writes keep flowing through a migration.

## Prerequisites

Research these first: [Query analysis](https://wiki.playquorum.dev/research/query-analysis.md).

It leads to [Expand and contract](https://wiki.playquorum.dev/research/expand-contract.md).

## Incidents it prevents

- [Schema migration lock](https://wiki.playquorum.dev/incidents/migration-lock.md)

## Facts

Online schema migrations
- **Track:** Data
- **Era:** [Series A](https://wiki.playquorum.dev/eras/series-a.md)
- **Cost:** 6.00k
- **Requires:** [Query analysis](https://wiki.playquorum.dev/research/query-analysis.md)

## See also

- [Research tree](https://wiki.playquorum.dev/research.md)

Source: https://wiki.playquorum.dev/research/online-migrations