A metric with the name xxx has already been registered

I believe the intention here https://github.com/siimon/prom-client/blob/c63689bbbb6a0395ce9d806b0cc17cf5d380f4a8/lib/registry.js#L83 was to use something like !_.isEqual so consecutive creation of the same metric won’t fail (behave like a getOrCreate).

[zbjornson]

Thanks for the clear explanation @trboltom! Based on that and the other comments in this thread, it sounds like we should keep the current behavior. I think that encourages clean code logic also (e.g. ensure your services are singletons).

Note that this is easy to handle upstream:

if (!registry.getSingleMetric("metric_name"))
  new Gauge(...)

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In mongodb when I run db.users.insert, and there’s no users collection, then it creates it, otherwise it uses the existing collection.

(Side note: this design decision frustrates me. I’ve made typos before in collection names and this behavior makes the error hard to find.)

[trboltom]

@zbjornson You are correct that registering two different metrics with the same name has to end with an error. We wanted to do this change in order to suppress this error in case you try to register the same metric twice.

This can happen for example when you are adding new collectors in the constructor of some service and then inject this service without making it a singleton. This will result in constructor of this service being called multiple times during one request, which will end up in try to register one metric multiple times.

This is an error/mistake on the developers side and so it’s probably not good enough reason to implement this. I thought that it could be a good idea to ignore second attempt and just dont do any action. However as I am writing this comment it has become clear to me that we probably really dont want to implement it in this way. Its probably better to keep this check there as is and just let developer know that something is not alright.

If you agree I will close that PR, but this issue is not mine and so someone else will have to close it if all agree