62 lines
3.9 KiB
Markdown
62 lines
3.9 KiB
Markdown
# Compatibility with multiple versions of the application running at the same time
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When adding or changing features, we must be aware that there may be multiple versions of the application running
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at the same time and connected to the same PostgreSQL and Redis databases. This could happen during a rolling deploy
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when the servers are updated one by one.
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During a rolling deploy, post-deployment DB migrations are run after all the servers have been updated. This means the
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servers could be in these intermediate states:
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1. Old application code running with new DB migrations already executed
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1. New application code running with new DB migrations but without new post-deployment DB migrations
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We must make sure that the application works properly in these states.
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For GitLab.com, we also run a set of canary servers which run a more recent version of the application. Users with
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the canary cookie set would be handled by these servers. Some URL patterns may also be forced to the canary servers,
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even without the cookie being set. This also means that some pages may match the pattern and get handled by canary servers,
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but AJAX requests to URLs (like the GraphQL endpoint) won't match the pattern.
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With this canary setup, we'd be in this mixed-versions state for an extended period of time until canary is promoted to
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production and post-deployment migrations run.
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## Examples of previous incidents
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### Some links to issues and MRs were broken
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When we moved MR routes, users on the new servers were redirected to the new URLs. When these users shared these new URLs in
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Markdown (or anywhere else), they were broken links for users on the old servers.
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For more information, see [the relevant issue](https://gitlab.com/gitlab-org/gitlab/-/issues/118840).
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### Stale cache in issue or merge request descriptions and comments
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We bumped the Markdown cache version and found a bug when a user edited a description or comment which was generated from a different Markdown
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cache version. The cached HTML wasn't generated properly after saving. In most cases, this wouldn't have happened because users would have
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viewed the Markdown before clicking **Edit** and that would mean the Markdown cache is refreshed. But because we run mixed versions, this is
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more likely to happen. Another user on a different version could view the same page and refresh the cache to the other version behind the scenes.
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For more information, see [the relevant issue](https://gitlab.com/gitlab-org/gitlab/-/issues/208255).
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### Project service templates incorrectly copied
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We changed the column which indicates whether a service is a template. When we create services, we copy attributes from the template
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and set this column to `false`. The old servers were still updating the old column, but that was fine because we had a DB trigger
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that updated the new column from the old one. For the new servers though, they were only updating the new column and that same trigger
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was now working against us and setting it back to the wrong value.
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For more information, see this [confidential issue](../user/project/issues/confidential_issues.md) `https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/9176`.
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### Sidebar wasn't loading for some users
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We changed the data type of one GraphQL field. When a user opened an issue page from the new servers and the GraphQL AJAX request went
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to the old servers, a type mismatch happened, which resulted in a JavaScript error that prevented the sidebar from loading.
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For more information, see [the relevant issue](https://gitlab.com/gitlab-com/gl-infra/production/-/issues/1772).
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### CI artifact uploads were failing
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We added a `NOT NULL` constraint to a column and marked it as a `NOT VALID` constraint so that it is not enforced on existing rows.
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But even with that, this was still a problem because the old servers were still inserting new rows with null values.
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For more information, see [the relevant issue](https://gitlab.com/gitlab-com/gl-infra/production/-/issues/1944).
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