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I have a binary executable named "alpha" that requires a linked library (libz.so.1.2.7) which is placed at /home/username/myproduct/lib/libz.so.1.2.7

I export the same to my terminal instance before spawning my binary executable by executing the following command.

export LD_LIBRARY_PATH=/home/username/myproduct/lib/:$LD_LIBRARY_PATH

Now, when I spawn another application "bravo" that requires the same library but of different version, i.e (libz.so.1.2.8) which is available in
/lib/x86_64-linux-gnu/libz.so.1.2.8, system throws the following error.

version `ZLIB_1.2.3.3' not found (required by /usr/lib/x86_64-linux-gnu/libxml2.so.2)

If I unset the LD_LIBRARY_PATH, "bravo" starts up fine. I understand that the above behaviour is because LD_LIBRARY_PATH takes precedence over the directory paths defined in /etc/ld.so.conf while looking for linked libraries and consequently the above error occurred. I am just curious about why have not the developers of UNIX/LINUX designed the OS to search for linked libraries in other directories according to the hierarchy if the first instance of library is of different version.

Simply put, UNIX/LINUX systems traverse through a set of directories until it finds the required library. But why does it not do the same until it finds the expected version rather than accepting the first instance of library irrespective of its version?

But why does it not do the same until it finds the expected version rather than accepting the first instance of library irrespective of its version?

It does, as far as it’s aware. zlib.so.1.2.7 and zlib.so.1.2.8 both have an soname of zlib.so.1, so your alpha and bravo binaries say they need zlib.so.1. The dynamic loader loads the first matching library it finds; it doesn’t know that version 1.2.8 provides additional symbols which bravo needs. (This is why distributions take pains to specify additional dependency information, such as zlib1g (>= 1.2.8) for bravo.)

You might think this should be easy to fix, but it isn’t, not least because binaries and libraries list the symbols they need separately from the libraries they need, so the loader can’t check that a given library provides all the symbols that are needed from it. Symbols can be provided in a variety of ways, and introducing a link between symbols and the libraries providing them could break existing binaries. There’s also the added fun of symbol interposition to complicate things (and make security-sensitive developers tear their hair out).

Some libraries provide version information which ends up being stored in .gnu.version_r, with a link to the providing library, which would help here, but libz isn’t one of them.

(Given the sonames, I’d expect your alpha binary to work fine with zlib.so.1.2.8.)