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docs: replace outdated State Resync Events section #2026

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24 changes: 12 additions & 12 deletions docs/concepts/syncs.md
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Expand Up @@ -150,20 +150,20 @@ Using the above example, if a flag key is duplicated across all 3 sources, then

![flag merge 2](../images/flag-merge-2.svg)

### State Resync Events
### Duplicate flags across sources

Given the above example, the `source-A` and `source-B` 'versions' of flag definition the `foo` have been discarded, so if a delete event in `source-C` results in the removal of the `foo`flag, there will no longer be any reference of `foo` in flagd's store.
Given the above example, the `source-A` and `source-B` 'versions' of the `foo` flag definition have been discarded.
flagd stores a single definition per flag set ID and flag key, so lower priority duplicates are not retained alongside the winning one.

As a result of this flagd will return `FLAG_NOT_FOUND` errors, and the OpenFeature SDK will always return the default value.
This matters when a flag is deleted.
If `source-C` removes its definition of `foo`, that definition is removed from the store and no definition of `foo` remains.
flagd does not fall back to the definition from `source-B`, and does not rebuild the merged state from the remaining sources.
Evaluations of `foo` then result in `FLAG_NOT_FOUND` errors, and the OpenFeature SDK returns the default value.

To prevent flagd falling out of sync with its flag sources during delete events, resync events are used.
When a delete event results in a flag definition being removed from the merged state, the full set of definition is requested from all flag sources, and the merged state is rebuilt.
As a result, the value of the `foo` flag from `source-B` will be stored in the merged state, preventing flagd from returning `FLAG_NOT_FOUND` errors.
!!! note

![flag merge 3](../images/flag-merge-3.svg)
Earlier versions of flagd emitted a state resync event in this situation, re-requesting the full flag definition from every source so that the `source-B` definition could take over.
That behavior was removed when flag storage moved to keying flags by flag set ID and key, and flagd no longer reconciles the same flag across multiple sources.

In the example above, a delete event results in a resync event being fired, as `source-C` has deleted its 'version' of the `foo`, this results in a new merge state being formed from the remaining definition.

![flag merge 4](../images/flag-merge-4.svg)

Resync events may lead to further resync events if the returned flag definition result in further delete events, however the state will eventually be resolved correctly.
For this reason, the same flag, identified by its flag set ID and flag key, must not be supplied by more than one source.
Flags that do not set a `flagSetId` all belong to the same implicit flag set, so duplicating a flag key across sources without flag sets collides in exactly the same way.
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