Over the Black Friday/Cyber Monday weekend, we noticed relatively high memory usage on Magento servers utilizing Varnish along with a Klaviyo module for managing marketing emails. After further review, we identified that the cause of this issue was the parameter that is being used for tracking user activity related to emails sent by the Klaviyo module. This parameter, “_kx” is currently not included in Magento’s list of parameters that should be stripped before the request is processed by Varnish.
We have already notified Klaviyo about the issue and in the meantime, if you would like us to apply a temporary fix to your account, please open a ticket with our support team.
If you’d like to better understand the impact of this issue, here’s what we’ve been observing: When a visitor goes to a site via an email that was generated by the Klaviyo module, a part of the URL is not removed before being processed and cached by Varnish. This results in each request being handled as a new object by PHP without pulling a cached page from Varnish since the parameter’s value is associated with the _kx parameter. The byproducts of this are that these visitors experience a reduction in speed (due to being served uncached pages) and that the servers consume more memory than usual as these unique URLs are cached as new pages (which are unlikely to be used by any other visitors).
If you are using Klaviyo’s module and do not open a ticket requesting that we apply a fix, your customers may experience reduced performance while browsing your website. This is only true for visitors that reached your website via an email that was created with or sent by Klaviyo’s module.