[grumple]

Automated scaling has been a persistent problem for me, especially if I try to scale on simple metrics, or even worse (in Slack's case) on metrics that could potentially compete. The situations in which multiple metrics could compete are sometimes difficult to conceive, but it will always happen if you aren't performing something more sophisticated than "up if metric > value, down if < value" for multiple metrics. I think you've got to combine these somehow into a custom metric and scale just on one metric. I'm totally unsurprised to see that autoscaling failed for both Slack and AWS in this case.

I think you really have to look at metric-based autoscaling and say: is it worth the X% savings per month? Or would I rather avoid the occasional severe headaches caused by autoscaling messing up my day? Obviously this depends on company scale and how much your load varies. I'd rather have an excess of capacity than any impact on users.