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RailsConf 2019 - rails db:migrate:safely by Matt Duszynski
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When you're dealing with fifty million records, simple migrations can become very dangerous. Come and learn from my mistakes instead of making your own. We'll talk about what's going on behind the scenes in your database, and how to safely write and run some common migrations. Remember, uptime begins at $HOME.

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In the talk titled "rails db:migrate:safely" at RailsConf 2019, Matt Duszynski presents valuable insights on safely handling database migrations when working with large datasets, specifically focusing on relational databases like PostgreSQL. The discussion emphasizes the risks associated with copying and modifying millions of records during migrations, urging developers to learn from real-world mistakes.

**Key points discussed include:**
- **Understanding Locks:** Migrations can obtain exclusive locks, causing system downtime when adding or modifying columns, leading to user frustrations. For example, adding a column with a default value can lock the entire table, preventing all access until the operation completes.
- **Transactions and Their Role:** Transactions can delay updates as they require locks to be released only after all operations complete. The problem intensifies with large data sets, where all rows are locked during bulk updates.
- **Best Practices for Migrations:**
  - Avoid adding columns with default values.
  - Perform data updates in smaller transactions, rather than a single large batch, to keep the system responsive.
  - Disable DDL transactions when necessary to avoid data and schema change collisions.
  - Utilize concurrent index creation to prevent table locks during index updates.
- **Anecdotal Case Study:** Duszynski shares a cautionary tale about an indexing operation at Weedmaps that took down production due to unexpected database concurrency issues caused by not realizing the read operations generated during auditing processes.
- **Testing and Monitoring:** Regular performance tests of database operations before, during, and after migrations are crucial to ensure stability and performance. Tools like `strong_migrations` can help catch unsafe migration practices early in the development cycle.

**Conclusions and Takeaways:**
- Developers must prioritize writing safe and efficient database migrations. By following best practices and being aware of database behavior, particularly during migration processes, one can significantly reduce risks of downtime or performance degradation during migrations. Uptime is paramount, and actions taken towards safe migrations reflect directly on user satisfaction and system reliability. Ultimately, learning from past mistakes and being proactive in testing and monitoring can lead to a pain-free migration experience.

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