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When using AWS Aurora, a nullable column can be added to existing large (100k rows+?) tables without causing downtime. However, the migration may still timeout in GoCD - so please coordinate the release with DevOps.
- Make a new field addition in model with null=True.
- Generate the model-change migrations locally.
- Create a pull request containing the model change and newly-generated migrations.
- Merge the migration pull request.
- The release process will run the migration and add the nullable column to table.
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On AWS Aurora, indexes can be build on large tables without causing downtime, but this requires devops coordination as the migration may timeout in GoCD.
- Add the new index on model fields.
- Generate the model-change migrations locally.
- Make pull request containing the model change and newly-generated migrations.
- Merge the migration pull request.
- Release will run the migration and add the index to table.
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- Will the migration cause data loss?
- Will this have a performance impact? NB: We do not take maintenance windows for migrations. We vastly prefer to re-engineer the migration than to schedule a maintenance window and virtually all migrations should be able to be engineered to avoid downtime by being additive-only
- Is the migration against a large table (see section below) ?
- Is the migration against a busy/highly contentious table (many writes/deletes/etc - see section below)?
- For the community, if we anticipate a potentially significant migration, make a note on the Open edX Release page for the next release
- How long do you expect the migration to take to run? Options include naive local testing, using 158766629 or some other synthetic method that might give you a reasonable framework to guess / extrapolate from
- Do you expect the migration to block queries, particularly frequent / user facing ones? This is
- For example, adding/modifying indexes on non-Aurora databases
- Is the migration backward compatible? Does it remove or edit a schema that the previous version of the code expects to be there?
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