[Nix-dev] understanding Nix: dependencies

[Shea Levy]

On 01/10/2013 02:02 PM, Christopher Howard wrote:
> A few questions, to help me understand how Nix works: Nix being a fully
> deterministic package management system, does that mean that the exact
> dependencies required by each package are fixed, i.e. do not change over
> time for a specific package (version)? Does that include the
> dependencies of those dependencies?

Yes. If any dependency changes in any way (in version, in command used 
to build it, etc.), the hash of the package will be different and thus 
it will be rebuilt and installed into a different path in the store.

> Assuming the answer to the proceeding two questions is yes, what is done
> in the event that a deep dependency must be updated, while all the
> higher level packages that use it do not? For example, each time openssl
> is updated for security reasons, does this require you to release new
> packages for hundreds of programs in your repository?

The short answer is yes, but with binary patching provided by the build 
farm it's not as bad as it sounds (you just download a diff between the 
two outputs, which should be nearly identical).

The long answer is: Currently, the way we build most of our packages the 
build time dependencies (e.g. where do I get my library headers) and the 
run time dependencies (e.g. which library do I link to) are the same, so 
updating the run time dependencies will require a rebuild. There are 
some proposed solutions to take one package built against one version of 
a library and relink it to another without recompiling it (so you still 
need to 'rebuild' the package, but the rebuild is trivial), but a) 
libraries are never as good at compatibility as they claim and b) we've 
found that the cost of recompiling everything isn't actually all that 
high, all things considered. In general, the question of whether and how 
to separate what something was compiled against and what it uses is 
still open, but what we have for now works very well.

>
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