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And while I’m reflecting here, the resize routine should also feature adjusting Apache/MySQL configuration to match different up-scaled CPU/RAM configurations. This question is really more about response strategy than identifying the threshold, but alas I can’t seem to edit the original post.
I don’t think DigitalOcean cares how frequently you resize, but I would not recommend it as it have one drawback.
Resize is OK, but keep in mind that every resize is a downtime. To resize droplet you need to completely power it off, do resize, power it on. Resize sometimes takes longer time to execute. This is note from DigitalOcean Control Panel for CPU&RAM resize:
Note: This Resize event may take longer than normal
So when you get high spike, you have a lot of consumers using your site, and you just take it off for half hour in middle of that spike. I would say don’t do it!
Load balancing is your best bet. DigitalOcean have many tutorials on this topic. Take a look on it, also I can recommend What is high availability as it have explained how you can make your seutp.
This will take some time to research and setup, but it will fix your problem for long time, you will get better performance and maybe better uptime. Resizing will only get you downtime, for time it need to execute, and sometimes it can take long