Would nix guarantee that all upstreams are available forever? Is nix planning to replace all upstreams? (PyPI, Conda, npm etc?) OR does it plan to keep a cache forever?
Not yet. There have been some efforts around adding IPFS support to Nix, as well as making its storage fully content addressable, which would allow for peer-to-peer archival and distribution of source. https://blog.ipfs.tech/2020-09-08-nix-ipfs-milestone-1/
Nix doesn't keep a cache of the upstreams, though there are some projects planning to try to do that I think.
The build recipes pull from the original source and use a hash to ensure that the source artifacts don't change. Nix caching is usually done at the build output layer, e.g. the resulting binaries.
So it does plan to replace all up streams eventually including apt, npm, pypi, gem etc by providing the ability to build and configure all software? I am not understanding this, currently the original maintainers release on their own stores. the build and configure steps may change over time. Unless the original maintainers start building for nix, would not this be too hard to maintain?
> the build and configure steps may change over time. Unless the original maintainers start building for nix, would not this be too hard to maintain?
This is indeed very hard, but Nix's deep locking makes this a tractable problem: if a version of a package builds once, it will keep building for as long as the source code is available.
So in some ways, this Nix has an easier job compared to traditional distribution mechanisms. On the other hand, Nix can be sufficiently weird and unwieldy as to limit the number of potential contributors.
Yes, at which point you'll have to update the build script... but until you do that, the current build script will still pull in the previous version of the software and all of its dependencies, including system-level ones, so you won't have incidental breakage from the surrounding environment changing out from under you.
This works going backwards, too. If you need an older version of some software, Nix can happily install it for you in a way that won't conflict with other software on your system. The only requirement is that you can still get the original sources and you can get a copy of the nixpkgs repo at the time the older version of that software was initially committed.
My point was that for every update, nixpkgs will have to be fixed if the build steps were changed. Currently, it is done by the actual software maintainers. Unless they all start building nixpkgs, it will be too much work to maintain.
Guix, a similar project to Nix, has been tackling this head on by automatically falling back to the Software Heritage archive when the original upstream is unavailable: https://guix.gnu.org/en/blog/2019/connecting-reproducible-de...