A local lab that reproduces the SSR memory leak from a finite gcTime on the server QueryClient

[YeonKyuJeon]

I ran into a Next.js SSR server whose memory kept growing under load. The cause was a finite gcTime on the request-scoped server QueryClient: each query schedules a removal timer, and the timer keeps the whole QueryClient alive for gcTime after the response is sent.

I made a small lab to see it happen: https://github.com/jeonyeonkyu/ssr-query-leak-lab

It runs two production builds of the same app side by side (leak config and fixed config), sends both the same requests, and plots heap usage and the number of QueryClient instances that were not collected. In my run with 30 concurrent requests the leak server reached a 768 MB heap limit in about 35 seconds while the fixed one went back to about 33 MB.

It can also be pointed at your own two servers with LAB_LEAK_URL and LAB_FIXED_URL. The lab page is in Korean for now. Feedback on the measurement method is welcome, especially the FinalizationRegistry counting.