Flag Configuration Merging

Flagd can be configured to read from multiple sources at once, when this is the case flagd will merge all flag configurations into a single merged state.

For example:

In this example, source-A and source-B provide a single flag configuration, the foo flag and the bar flag respectively. The merge logic for this configuration is simple, both flag configurations are added to the store.

In most scenarios, these flag sources will be supplying n number of configurations, using a unique flag key for each configuration.

However, as multiple sources are being used, there is the opportunity for keys to be duplicated, intentionally or not, between flag sources. In these situations flagd uses a merge priority order to ensure that its behavior is consistent.

Merge order is dictated by the order that sync-providers and uris are defined, with the latest defined source taking precedence over those defined before it, as an example:

./bin/flagd start --uri file:source-A.json --uri file:source-B.json --uri file:source-C.json

When flagd is started with the command defined above, source-B takes priority over source-A, whilst source-C takes priority over both source-B and source-A.

Using the above example, if a flag key is duplicated across all 3 sources, then the configuration from source-C would be the only one stored in the merged state.

State Resync Events

Given the above example, the source-A and source-B 'versions' of flag configuration the foo have been discarded, so if a delete event in source-C results in the removal of the fooflag, there will no longer be any reference of foo in flagd's store.

As a result of this flagd will return FLAG_NOT_FOUND errors, and the OpenFeature SDK will always return 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 configuration being removed from the merged state, the full set of configurations 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.

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 configurations.

Resync events may lead to further resync events if the returned flag configurations result in further delete events, however the state will eventually be resolved correctly.