Metric collection is not always accurate

[edclement]

I started doing real world testing against a Verdaccio instance running in a Docker container that uses this plugin and discovered that the metrics being collected were not 100% accurate. The counts of package downloads were always slightly less then what I was expecting based on the tests I was running.

I ran and debugged a local instance and performed the same tests that were producing inaccurate results. What I discovered is that the Express response finish event does not always fire. The plugin currently listens for the finish event in order to collect final response data and label metrics appropriately. You can see the line of code for this here.

There isn't a lot of documentation available on when the different Node/Express response event types fire. After doing some additional research, I found two good references:

• Node.js: Difference between http finish event, response close event, and response end event
• Measuring requests duration in a Node.js + Express application properly

Specifically from the 2nd link above:

When does the request-response cycle end? The first and the obvious answer is when the response is sent. Node.js has a default event that is fired when the response has been sent. This hook is res.on('finish').

99% of software developers use only “finish” event, but I should notice that not every request comes to the “finish” state, in the real life client can close the connection before the response has been sent. In this case Node.js emits only the res.on('close') event.

This is exactly the type of behavior I was seeing in my testing and debugging. It seems that sometimes the npm client kills the connection prior to Express thinking the full response has been sent. The solution seems to be to rely on the close event instead of the finish event on the response object.