That's actually not true. Binary caches have a priority field that determines the order (see the Priority field in https://cache.nixos.org/nix-cache-info).

@edolstra what happens if two caches have the same priority?

Then they're tried in the order in which they're defined in nix.conf.

@edolstra thanks!

I'm curious about why caches are allowed to define their own priority?

I have a few cachix substituters defined and would like to be able to tell nix to prefer binaries from cache.nixos.org over those from my cachix sustituters, but that doesn't seem to be possible if any cache can set itself to prio 1 without my consent?

@edolstra Would this be better?

Substituters with the same Priority value are queried from left to right.

Doesn't really explain what this Priority is, but at least it's not incorrect.

btw it isnt a good idea to depend on priority as a security feature, since it doesn't provide meaningful control over what you get from what cache.

The original use case was to ensure that cache.nixos.org (which is fast) is prioritized over hydra.nixos.org (which is slow). Nowadays hydra.nixos.org no longer provides a binary cache so it doesn't matter anymore.

BTW I think you can also set the priority in the substituter URL (e.g. http://bla?priority=123).

btw it isnt a good idea to depend on priority as a security feature, since it doesn't provide meaningful control over what you get from what cache.

@grahamc Could you elaborate a bit? I'm not sure I follow.

I marked this as stale due to inactivity. → More info

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/how-does-nix-choose-a-substituter-when-there-is-more-than-one-choice/3658/8

Is higher Priority better? The source says otherwise https://github.com/NixOS/nix/blob/master/src/libstore/store-api.hh#L195 but common usage and also experimental findings like in https://discourse.nixos.org/t/how-does-nix-choose-a-substituter-when-there-is-more-than-one-choice/3658 suggest otherwise.

@edolstra does specifying the priority in the substituter URL have precedence over the Priority field in the cache itself?

@Fuuzetsu where do you see disagreement on the sorting? There seems to be agreement on lower value = higher priority.

@asymmetric I set priority 50 in my s3 cache amd it gets picked over priority 40 official nixos cache.

In linked issue, nix-serve with priority 30 gets checked only after the official priority 40 cache.

@Fuuzetsu in the linked issue, priority 30 gets checked before 40:

My local nix.conf specifies both the internal CI server as well as cache.nixos.org as substituters.

I’ve noticed that the internal CI server is picked for paths that exist on BOTH caches.
I wondered how does it pick a CI server so I tried changing the order of the CI servers in nix.conf but it kept picking the internal CI server.

The CI server cache’s priority is set to 30.

in the linked issue, priority 30 gets checked before 40

Oh, you're right. I have a priority to change. Thank you for the doc update.