How much capacity will you need for your app?
Or asked another way if wearing the vendor hat, how much money ya got?
We’re generally lousy at estimating infrastructure capacity requirements and even when a more scientific approach is taken (and it’s frequently not), we’re still lousy at estimating user behaviour in real world circumstances and the impact it will have on system performance.
Now, put that situation in a cloud environment and it has the potential to go a couple of ways. One is that you have underestimated and by courtesy of the glorious ability to increase resource very quickly, your bill goes nuts. Another is that you’ve overestimated and you end up paying for resources you really don’t need. I’ve recently gone through scaling challenges with both the website and the Azure SQL database on Have I been pwned? (HIBP). For me, it’s never about having access to enough scale (that’s pretty much limited by your wallet), rather it’s about trying to both keep the cost down and the perf up and frankly, I don’t really want to compromise on either! Here’s what I’ve done with the website and I’ll write more about the database another time.