Site Crashing After Mailchimp Blast

I have a client whose site is crashing every time they send an email blast to their database of 20,000 via Mailchimp. What's actually happening is, it will spin for about 5 minutes before first contenful paint.
They used to send to the whole database is one go, but now they schedule batches of 5 every 10 minutes, so effectively it's only sending 4,000 emails each time, and the problem is the same.

It's a CS-Cart 4.4.2 site with 3 storefronts.
Site is hosted with VentraIP on a VPS with 4 vCPU cores and 4GB memory.

When I check the server's resources in WHM, it doesn't even come close to using what's allocated, even during these email campaigns, but the site buckles almost immediately once the email is sent.

What's frustrating is that I can't see the actual concurrent users in Google Analytics, presumably because they're not loading enough of the site to trigger the script, and the Mailchimp report usually only shows a few clicks at the time it's happening (they told me the clicks showing on the report is basically in real time). But it's hard to believe that more than 30 users are INSTANTLY on the site at the same time.

The site is caching on AWS and we've already upgraded the VPS recently, and the client is starting to get frustrated that there's no clear answer.

What could be the possible source/s of this issue, and how can I confirm it?



If your issue (some of your post is unclear to me) is that the response from your mailing is overwhelming your site (good problem to have), then you need to get a systems person to setup a load balanced environment. A VPS won't cut it for large numbers of concurrent users.

Your problem is solvable but suggest you find someone who can setup a good load-balanced environment for you. It won't be cheap to setup nor to operate, but it sounds like there are sales there to justify it. Optionally you can spread your offer over several days to smaller numbers of users.

I had a similar issue and I found out that the problem was that another VPS at the same server with mine was stealing much resources and it was crashing my VPS. You can easily find this by connecting to your VPS via SSH and type the command “sar”. There you can see the category %Steal% and see if someone is using high percent of the resources. The problem is usually visible with the email marketing because it causes too many concurrent connections. The solution is change to another less busy VPS node