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Events

Events enable the ability to react to state changes in the provider or underlying flag management system. These include changes in provider readiness, error status, or perhaps most interestingly, flag configuration changes.

Event Handlers​

Event handlers can be attached to a client or to the global API object, for any provider event type. Handlers attached to the global API object run when any provider emits the associated events; this makes global handlers ideal for configuration troubleshooting, monitoring and other cross-cutting concerns. Handlers attached to a client will run only when the provider bound with that client emits the associated event; this makes client handlers ideal for reacting to flag-state changes in an application domain logic.

Handlers are passed an event details structure, which contains data about the event, including a list of keys that have changed (if applicable and available).

// attach a `PROVIDER_READY` handler
client.addHandler(ProviderEvents.Ready, (eventDetails: EventDetails) => {
  // do something when the provider is ready
});

// attach a `PROVIDER_CONFIGURATION_CHANGED` handler
client.addHandler(ProviderEvents.ConfigurationChanged, (eventDetails: EventDetails) => {
  // do something when flags settings have changed
});

// attach a `PROVIDER_ERROR` handler
client.addHandler(ProviderEvents.Error, (eventDetails: EventDetails) => {
  // do something when the provider has entered an error state
});

// attach a `PROVIDER_STALE` handler
client.addHandler(ProviderEvents.Stale, (eventDetails: EventDetails) => {
  // do something when the provider has gone stale
});

// handlers can also be added globally
OpenFeature.addHandler(ProviderEvents.Ready, (eventDetails: EventDetails) => {
  // do something when the provider is ready
});

Event Types​

See event types specification.

PROVIDER_READY​

The provider is ready to perform flag evaluations.

Application authors may wish to wait to evaluate flags until the provider has fully started. In that case, they can await the PROVIDER_READY event before evaluating any flags. This can be especially useful for hiding UI elements or otherwise deferring evaluation until associated flags can be resolved accurately.

PROVIDER_CONFIGURATION_CHANGED​

A change was made to the backend flag configuration.

If the flag management system supports it, providers may emit a PROVIDER_CONFIGURATION_CHANGED event signifying that flag configurations have changed. This might mean the value of a flag (or flags) have changed, or that a rule has been updated. Generally, the associated event details will indicate which flag (or flags) have been changed with the flags changed field.

PROVIDER_ERROR​

The provider signaled an error.

If a provider becomes unavailable, evaluations will typically default. Handlers associated with PROVIDER_ERROR events can be used to alert monitoring systems about the provider failure, or enable custom fallback mechanisms.

PROVIDER_STALE​

The provider's cached state is no longer valid and may not be up-to-date with the source of truth.

Some providers maintain a connection to a management system, but are also tolerate of disconnection. The PROVIDER_STALE indicates this situation.