Feature request: minimum request volume

[arkwright]

Hey, thanks for writing such an awesome library!

It would be really helpful if opossum could support a minimum request volume within the rolling window, below which the circuit would never open, even if 100% of requests result in errors. I am currently experiencing an issue wherein traffic occasionally drops to a trickle, and then the circuit opens because the first error after a long, silent period tips the statistics beyond the errorThresholdPercentage. I could increase the rollingCountTimeout, though this would effectively decrease the circuit breaker's sensitivity when traffic volume is sufficient.

[lance]

Hi @arkwright thanks for the suggestion and nice words. :) This is very similar to the options.warmUp property that was recently added. But I can see that it is different in that the warm up phase only happens when the application starts up. Your suggestion seems to encompass both a warm up phase, and an idle phase, so I need to give it some thought. I believe it should be possible to have a single property that handles both cases. I'd rather not have two separate code paths doing essentially the same thing.

Brainstorming a hack/workaround until I figure this out... Could your fallback function check stats.fires to see if it meets your threshold, and if it does not then call circuit.close()?

[arkwright]

Hey @lance, thanks for giving this some thought!

I recently considered writing a fallback function which looks at stats.fires, and that would provide the functionality I'm looking for. I just felt that this would be such a common use case that it would make sense to expose it as a configuration option. circuit-breaker-js provides a volumeThreshold option, for example.

I'm not sure if warmUp could handle both cases, but I think the inverse API might work. If warmUp was deprecated and replaced with a volumeThreshold:

1. At application start up, the circuit would remain closed until at least volumeThreshold requests have been sent, providing behavior similar to warmUp.
2. During idle periods, the circuit would remain closed until volumeThreshold requests occur within the rolling window, providing the desired low volume protection.

Of course, if backward compatibility is a concern, these two options could coexist safely.

I'm not sure if that helps, but that's how I might approach it!