Microprofile Config: Dynamic ConfigSource values for faulttolerance metric tags

Question

i'm currently working on a solution to setting metric tags for the microprofile fault-tolerance framework. We're using it together with metrics, but one cannot directly set tags via the fault-tolerance annotations.

So we came up with a workaround setting a ThreadLocal value via an Interceptor, which then is read by a custom ConfigSource. The ConfigSource checks for "mp.metrics.tags" and "MP_METRICS_TAGS" config keys in it's getValue(final String propertyName) method. This would basically work if the getValue would get called every time a fault-tolerance annotation is processed. But is seems like this is not the case and the invocations of the method happen randomly.

In my oppinion ConfigSources and their getValue(final String propertyName) should always get called as a developer might rely on config values which could change every second.

Any ideas why the config source is not called?

Answer 1

It looks like the custom ConfigSource does no longer get called when returning null multiple times or at least during the server startup phase. In the mentioned scenario this can be bypassed by returning an empty string. Then the ConfigSource also gets called for every getValue() method call at runtime.

Answer 2

The MicroProfile Config 1.4 specification indicates that no caching of a ConfigSource's value should occur, so if your MicroProfile Config Config implementation (you don't say which implementation it is) is caching the results of a call to ConfigSource#getValue() it is not compliant, as best as I can tell (the specification is flawed, not very rigorous, and its TCK is spotty but it certainly seems to be pretty clear on this issue).

(Do note that a given ConfigSource implementation may, of course, decide to return cached values from its getValue() method.)