The document discusses feature flagging as a method to mitigate risks during database migrations, detailing various approaches including maintenance windows, zero-downtime, and feature-flagged methods. The feature-flagged approach allows for gradual rollouts and easy rollbacks without downtime, though it requires keeping both old and new databases operational for a longer duration. Examples from the author's experience at LaunchDarkly illustrate the need for careful migration planning and monitoring.

1. Feature Flagging
to Reduce Risk in
Database Migrations
By Patrick Kaeding
Lead Engineer at LaunchDarkly
@pkaeding @launchdarkly

2. About Me & LaunchDarkly
• Current: Lead Engineer at LaunchDarkly
• Past: Engineer at Atlassian
• LaunchDarkly is a cloud-based feature flag management
platform
• Separate deploying code from releasing features
• Manage risk of releasing features
• Measure impact (both KPI and Ops performance)
@pkaeding @launchdarkly

3. Database Migrations
• Sooner or later, it is inevitable
• Schema changes
• Moving to larger database cluster
• Moving from one kind of DB to another
@pkaeding @launchdarkly

4. Maintenance window
Database Migrations (Bad Old Days)
Application Old Database
@pkaeding @launchdarkly

5. Database Migrations (Bad Old Days)
Old Database New Database
Migrate
Maintenance window (application offline)
@pkaeding @launchdarkly

6. Maintenance window
Database Migrations (Bad Old Days)
Application New Database
@pkaeding @launchdarkly

7. Maintenance Window Approach
• Pros
 Simplicity
 Can always be used (since both code paths don’t need to
work at the same time)
• Cons
 Downtime
 If there is a problem, redeploy old code and restore from
backup
 All or nothing
@pkaeding @launchdarkly

8. Zero-Downtime Approach
Database Migrations (The Less Bad Days)
Application Old Database
Write
Read
@pkaeding @launchdarkly

9. Zero-Downtime Approach
Database Migrations (The Less Bad Days)
Application Old Database
Write
Read
New Database
@pkaeding @launchdarkly

10. Zero-Downtime Approach
Database Migrations (The Less Bad Days)
Application New Database
Write
Read
@pkaeding @launchdarkly

11. Zero-Downtime Approach
• Pros
 No downtime
 If there is a problem, you can keep running from the old
database/schema
• Cons
 All or nothing
 Need to write a reverse migration if you want to roll back
@pkaeding @launchdarkly

12. The Feature-Flagged Approach
Database Migrations (Modern Times)
Application Old Database
Write
Read
@pkaeding @launchdarkly

13. Feature-Flagged Approach
Database Migrations (Modern Times)
Application Old Database
Write 100%
Read 100%
New Database
@pkaeding @launchdarkly

14. Feature-Flagged Approach
Database Migrations (Modern Times)
Application Old Database
Write 100%
Read 100%
New Database
@pkaeding @launchdarkly

15. Feature-Flagged Approach
Database Migrations (Modern Times)
Application Old Database
Write 100%
Read 100%
New Database
@pkaeding @launchdarkly

16. Feature-Flagged Approach
Database Migrations (Modern Times)
Application Old Database
Write 100%
Read 100%
New Database
@pkaeding @launchdarkly

17. The Feature-Flagged Approach
Database Migrations (Modern Times)
Application New Database
Write
Read
@pkaeding @launchdarkly

18. Feature-Flagged Approach
• Pros
 No downtime
 Gradual rollout allows you to monitor
 You can roll back just as easily (or partially roll back)
 Compare results with live data
• Cons
 Longer period for both databases to be operational
@pkaeding @launchdarkly

19. Demonstration
@pkaeding @launchdarkly

20. This demo is based on a true story…
• LD Events & user data were stored in Mongo
 Events drive A/B test results, feature status indicators, auto-
complete widgets, and more
• Unsharded replica set couldn’t handle the load
• Start sharding, or move on to Dynamo
• Integrity checks found issues with null/empty values in our
dynamo code
@pkaeding @launchdarkly

21. What about…
• Schema changes? New fields? Restructured data?
• Which users get the new DB?
• Open source alternatives?
@pkaeding @launchdarkly

22. Questions
• pkaeding@launchdarkly.com
• @pkaeding
• @launchdarkly
• https://launchdarkly.com
• Blog article: http://kaed.in/db-blog
• Demo code: http://kaed.in/dbcode
@pkaeding @launchdarkly