What is the term Registry in rust?

I started learning rust by reading the official book. So when coming to the second chapter -
Programming a guessing game, we need to add rand crate to our dependency.

In that specifically it was written

Now that we have an external dependency, Cargo fetches the latest versions of everything from the registry , which is a copy of data from Crates.io. Crates.io is where people in the Rust ecosystem post their open source Rust projects for others to use.

After updating the registry, Cargo checks the [dependencies] section and downloads any crates you don’t have yet

So does it mean even though if i want only rand crate to be included in my project , it fetches the latest versions of everything from registry means whether it downloads all the crates available in crates.io?

Or what it is refferring to? What exactly is registry? what exactly is updating the registry?

The registry is a list of crates that are available on Crates.io. You get a local copy of it so that cargo knows what crates are available, and it is updated (by cargo) on occasion by syncing with the remote Crates.io copy. This is how your local cargo invocation knows when crates get updates or whether they exist at all.


Specifically, the crates.io registry is maintained as a Git repository that's updated automatically whenever a new version of a package is released. The registry only contains the names of crates and some metadata about them, mainly the versions and which other crates they depend on. Fetching the source code for a package like rand is a separate step.


But why my local local cargo invocation wants to knows when crates get updates or whether they exist at all? If cargo.lock takes care of maintaing the same version of my dependencies for building why my local cargo instance wants to know the updates of other crates?

Got it. But what's the use of it?

Well, Cargo needs to know what versions of crates are available so it can create Cargo.lock in a new project, or update Cargo.lock with more recent versions when you run cargo update.


I imagine (not 100% sure though) that if the versions listed in the Cargo.lock file match the constraints listed in the Cargo.toml file, then cargo won't need to update the registry (since it doesn't need to update the .lock file); unless the explicit override of running cargo update were to be used.

But, if there is a crate listed in the Cargo.toml which doesn't have a mathcing "choice" in the current Cargo.lock file (e.g., because the lock file is not there, or because the dep was added after the generation of a .lock file), then:

  • since cargo tries to pick from the highest possible version compatible with the Cargo.toml constraints (which are usually semver constraints), cargo will then look for an updated list of all the existing crates on crates.io, that is, an updated registry, to try and make such choice.

  • you can tell cargo to "calm down" by adding the --offline flag when using any cargo command that requires a correct .lock file; as the name indicates, this forbids cargo from fetching any data from the network, which thus allows for an out-of-date registry / list of such crates, mainly the one cached in your machine.

    • this means that if you first run cargo metadata --offline ,
      or cargo update -w --offline, you'll generate a Cargo.lock file with your local copy of the list of available crates only, and if it succeeds, you'll then find yourself with a Cargo.lock file which needs no further updates. You can then run the cargo build-like commands as usually, without the --offline, to still allow cargo to download the source code of your dependencies, if needed.
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Thank you for your detailed explanation. Sounds relevant

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