Are calls to `add_distribution_value` synchronous or asynchronous? Can they raise errors?

[thbar]

Are calls to `add_distribution_value` synchronous or asynchronous? Can they raise errors?

[unflxw]

Hi @thbar! TL;DR: It is safe to call add_distribution_value (or interface with AppSignal in other ways) from within a Telemetry handler. All operations involving the NIF are non-blocking.

1. Are these add_distribution_value calls synchronous?

Yes, they're "synchronous", in that they synchronously communicate with the AppSignal extension via the NIF. But they're not "synchronous" in that they'd block until the value is sent to AppSignal, or anything like that. Communicating through the NIF takes a very short time. This is to say, this should not be a problem -- the value is merely queued in the extension, which will send it to AppSignal asynchronously.

(Our own instrumentations interface with the NIF from within Telemetry handlers -- see Ecto for example)

2. Could they raise errors?

In practice, no. They could raise an error if they're not being invoked correctly -- say, if you were to pass a string as the distribution value instead of a double. As long as the right number of arguments are passed with the right types, no error will be raised.

While the integration's NIF interfaces with the AppSignal agent, which is not open source, the NIF bridge itself is publicly available, and you can take a look at the implementation for add_distribution_value.

That said, if an error was somehow raised, the consequence would be that Telemetry would detach the event handler that raised an error. This error would be logged, and your application would no longer emit this metric to AppSignal, but your application would not crash because of it.

[thbar]

Hi @unflxw! Thanks for the timely & thoughtful answer!

Yes, they're "synchronous", in that they synchronously communicate with the AppSignal extension via the NIF. But they're not "synchronous" in that they'd block until the value is sent to AppSignal, or anything like that.

This is what I hoped for, given the type of software 😄 thanks for the confirmation!

They could raise an error if they're not being invoked correctly -- say, if you were to pass a string as the distribution value instead of a double

Ok! I looked at the code before and indeed was wondering what would happen inside the NIF, would it block or not etc.

Many thanks, this is perfect and I can move forward with our implementation. Thank you!