How to use Micrometer Gauge the correct way?

Question

Say I have an application where a REST API updates the price of a product.
I want to use a Micrometer Gauge to expose the new price as a metric. I'm having trouble understanding from Micrometer documentation how this should be accomplished.

The only toDoubleFunction() that worked for me was to create a new method in my ProductService which returns its price. This seems like an overhead for every piece of data I want to expose as a metric.

What am I missing here? Why product.getPrice() isn't enough to update the Gauge?

Answer 1

Micrometer's Gauge would hold a reference to whatever it has to pull the value from. And that reference is a WeakReference by default.

For example:

• StatsdGauge
• DefaultGauge

This means that should your provided value get garbage collected, micrometer would have nothing to poll the value from.

I assume that when you call product.getPrice(); you never hold on to that value just passing it to something like meterRegistry.gauge("product.price",tags,value); Since after the completion of this block of code nothing holds a strong reference to that specific value it gets Garbage Collected (GC-ed).

You have couple solutions here: either build a Gauge using a builder and specifying the strongReference(true) or (better) make sure you hold your references and manage their values yourself.

Both are rather weird as you'll end up holding a lot of "Gauge sources" in memory.