Add ability to write csv stats files

[aldenpeterson-wf]

I started reviewing #445 but found that it had enough problems that it'd be just easier to rewrite the entire PR.

This:

• Ability to write csv files for both the requests and distribution csv as a command line flag
• Refactored the logic in the web handler to reuse this csv generation for better portability

I also fixed #598 because that issue has been annoying me for a while.

[mambusskruj]

Hi! As I understand one row will write in csv per 2 seconds? Why not per one second? It will be great for some TSDB like InfluxDB.

[aldenpeterson-wf]

@mambusskruj I copied the interval here: https://github.com/locustio/locust/blob/master/locust/stats.py#L519

I'd actually be more inclined to decrease the frequency given that it's blocking IO for locust and on slower systems that might affect your loadtest performance.

[mambusskruj]

@aldenpeterson-wf well, if locust used in master-slave mode, this is not a big issue I think.

[aldenpeterson-wf]

@mambusskruj I could make it a user defined option but I'm not sure that it's worth the added complexity.

I'd be curious what other people think. I could also define the time at the module level so anyone who really wanted to could set the timeframe for both the printing/csv save.

[cgoldberg]

why the 2 different imports from .stats and then locust.stats?

[aldenpeterson-wf]

.. no reason, will fix this

[cgoldberg]

I could make it a user defined option but I'm not sure that it's worth the added complexity.

I think 2 secs for a writing interval sounds good for the initial implementation. Once we gauge impact of this change on performance, we can revisit or make it configurable.

First pass at a review LGTM, but I haven't tested it. Also, can something be added to the docs or README that describes stats output?

[aldenpeterson-wf]

@cgoldberg will update the docs.

I also might just make the sleep time to be constants in that file, so someone can change them it if they want since.. python.

[aldenpeterson-wf]

I'm not 100% sure where to put this in the docs. There's not really a good section detailing all the flags but I added them, given activity on GH it seems an option like this would be very commonly used.

Also @mambusskruj I added the ability to easily customize how frequently the stats CSV is written.

[aldenpeterson-wf]

@cgoldberg this is updated and ready for review

[aldenpeterson-wf]

@cgoldberg bump :)

[mbeacom]

LGTM