Commit 24d29100 authored by Michael Kozono's avatar Michael Kozono Committed by Marcia Ramos

Clarify feature flags are only a tool for backwards compatibility

parent 2f3f8240
......@@ -115,12 +115,24 @@ For major or minor version updates of Rails or Puma:
### Feature flags
One way to handle this is to use a feature flag that is disabled by
default. The feature flag can be enabled when the deployment is in a
consistent state. However, this method of synchronization **does not
guarantee** that customers with on-premise instances can [update with
zero downtime](https://docs.gitlab.com/omnibus/update/#zero-downtime-updates)
because point releases bundle many changes together.
[Feature flags](feature_flags/index.md) are a tool, not a strategy, for handling backward compatibility problems.
For example, it is safe to add a new feature with frontend and API changes, if both
frontend and API changes are disabled by default. This can be done with multiple
merge requests, merged in any order. After all the changes are deployed to
GitLab.com, the feature can be enabled in ChatOps and validated on GitLab.com.
**However, it is not necessarily safe to enable the feature by default.** If the
feature flag is removed, or the default is flipped to enabled, in the same release
where the code was merged, then customers performing [zero-downtime updates](https://docs.gitlab.com/omnibus/update/#zero-downtime-updates)
will end up running the new frontend code against the previous release's API.
If you're not sure whether it's safe to enable all the changes at once, then one
option is to enable the API in the **current** release and enable the frontend
change in the **next** release. This is an example of the [Expand and contract pattern](#expand-and-contract-pattern).
Or you may be able to avoid delaying by a release by modifying the frontend to
[degrade gracefully](#graceful-degradation) against the previous release's API.
### Graceful degradation
......
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