Caching

flagd has a caching strategy implementable by providers that support server-to-client streaming.

Cacheable flags

flagd sets the reason of a flag evaluation as STATIC when no targeting rules are configured for the flag. A client can safely store the result of a static evaluation in its cache indefinitely (until the configuration of the flag changes, see cache invalidation).

Put simply in pseudocode:

if reason == "STATIC" {
    isFlagCacheable = true
}

Cache invalidation

flagd emits events to the server-to-client stream, among these is the configuration_change event. The structure of this event is as such:

{
    "type": "delete", // ENUM:["delete","write","update"]
    "source": "/flag-configuration.json", // the source of the flag configuration
    "flagKey": "foo"
}

A client should invalidate the cache of any flag found in a configuration_change event to prevent stale data. If the connection drops all cache values must be cleared (any number of events may have been missed).