To me, Nginx. I remember seeing this file structure and saying: "ohh, that's how...

To me, Nginx. I remember seeing this file structure and saying: "ohh, that's how it should be done"

    src/os/win32/ngx_alloc.c
    src/os/unix/ngx_alloc.c

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A few years later I stumbled upon this refactoring video by Uncle Bob and that was my second aha! moment.

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Many people here recommend Redis as an inspiring example.