When I check their dockfiles, alpine is like this (for Alpine v3.12 - 3.12.7)
ADD alpine-minirootfs-3.12.7-x86_64.tar.gz /
ADD busybox.tar.xz /
But as https://alpinelinux.org/about/ says
Alpine Linux is built around musl libc and busybox.
So what is exactly the difference ?
I am also curious that many docker images, (nodejs/nginx/php just name a few) provide images based on alpine but not on busybox. Why is that ? What is use case for busybox image then ? I need to emphasize that I am not looking for an answer about why A is better than B or vise versa or software recommendation.
I have been experiencing intermittent DNS lookup failure for my alpine docker, as here musl-libc - Alpine's Greatest Weakness and here Does Alpine have known DNS issue within Kubernetes? said. That is one of reasons I asked my question.
It previously used uClibc as its C standard library instead of the traditional GNU C Library (glibc) most commonly used. Although it is more lightweight, it does have the significant drawback of being binary incompatible with glibc. Thus, all software must be compiled for use with uClibc to work properly. As of 9 April 2014, Alpine Linux switched to musl, which is partially binary compatible with glibc.
The key difference between these is that older versions of the
busybox image statically linked busybox against glibc (current versions dynamically link busybox against glibc due to use of libnss even in static configuration), whereas the
alpine image dynamically links against musl libc.
Going into the weighting factors used to choose between these in detail would be off-topic here (software recommendation requests), but some key points:
Comparing glibc against musl libc, a few salient points (though there are certainly many other factors as well):
Comparing the advantages of a static build against a dynamic build:
Honestly, these two images don't between themselves cover the whole matrix space of possibilities; there are situations where neither of them is optimal. There would be value to having an image with only busybox that statically links against musl libc (if everything you're going to add is in a non-C language), or an image with busybox that dynamically links against glibc (if you're going to add more binaries that need libc and aren't compatible with musl).