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Cargo: predictable dependency management

Cargo’s goal is to make modern application package management a core value of the Rust programming language.

In practice, this goal translates to being able to build a new browser engine like Servo out of 247 community-driven libraries—and counting. Servo’s build system is a thin wrapper around Cargo, and after a fresh checkout, you’re only one command away from seeing the whole dependency graph built:

   Compiling num-complex v0.1.32    Compiling bitflags v0.6.0    Compiling angle v0.1.0 (https://github.com/emilio/angle?branch=servo#eefe3506)    Compiling backtrace v0.2.1    Compiling smallvec v0.1.5    Compiling browserhtml v0.1.4 (https://github.com/browserhtml/browserhtml?branch=gh-pages#0ca50842)    Compiling unicase v1.4.0    Compiling fnv v1.0.2    Compiling heapsize_plugin v0.1.4    ... 

Why do these granular dependencies matter?

Concretely, they mean that Servo’s encoding library (and many components like it) is not a deeply nested part of Servo’s main tree, but rather an external library that anyone in the ecosystem can use. This makes it possible for other Rust libraries, like web frameworks, to easily use a browser-grade encoding library, sharing the costs and benefits of maintenance. And it flows both ways: recently, a new Rust-based text editor was announced, and happened to provide a fast line-breaking library. Within days, that library replaced Servo’s old custom linebreaker , decreasing Servo’s maintenance burden and increasing sharing in the Rust ecosystem.

The core concerns of dependency management

To make this all work at the scale of an app like Servo, you need a dependency management approach with good answers to a number of thorny questions:

  1. How easy is it to add an external library, like a new linebreaker, to Servo?

  2. If I build Servo on a different machine, for a different architecture, in CI or for release, am I building from the same source code?

  3. If I build Servo for testing, will its indirect dependencies be compiled with debug symbols? If I build Servo for release, will its indirect dependencies be compiled with maximum optimizations? How can I be sure?

  4. If someone published a new version of one of Servo’s dependencies after I commit to Servo, will my CI environment use the same source code as my machine? My production environment?

  5. If I add a new dependency (or upgrade one), can that break the build? Can it affect unrelated dependencies? Under what conditions?

All of these questions (and many more like them) have one thing in common: predictability. One solution to this problem, common in the systems space, is vendoring dependencies—forking them directly into an application’s repository—and then managing them manually. But this comes at a substantial per-project cost, since there’s more to manage and configure. It also comes at an ecosystem-wide cost, since the work involved cannot easily be shared between libraries; it has to be redone instead for each application that brings a set of libraries together. And making sure you can answer all of the questions above, all of the time, is hard work indeed.

Package managers for higher-level languages have shown that by turning dependency management over to a shared tool, you can have predictability, easy workflows that operate over the entire dependency graph, and increased sharing and robustness across the ecosystem. When we started planning Rust 1.0, we knew we wanted to bring these ideas to a systems setting, and making Cargo a central part of the way people use Rust was a big part of that.

Pillars of Cargo

Cargo is built on three major pillars:

  1. Building, testing, and running projects should be predictable across environments and over time.

  2. To the extent possible, indirect dependencies should be invisible to application authors.

  3. Cargo should provide a shared workflow for the Rust ecosystem that aids the first two goals.

We’ll look at each of these pillars in turn.

Predictability

Cargo’s predictability goals start with a simple guarantee: once a project successfully compiles on one machine, subsequent compiles across machines and environments will use exactly the same source code .

This guarantee is accomplished without incorporating the source code for dependencies directly into a project repository. Instead, Cargo uses several strategies:

  1. The first time a build succeeds, Cargo emits a Cargo.lock file, which contains a manifest of precisely which source code was used in the build. (more on “precise” in a bit)

  2. Cargo manages the entire workflow, from running tests and benchmarks, to building release artifacts, to running executables for debugging. This allows Cargo to ensure that all dependencies (direct and indirect) are downloaded and properly configured for these use-cases without the user having to do anything extra.

  3. Cargo standardizes important environment configuration, like optimization level, static and dynamic linking, and architecture. Combined with the Cargo.lock , this makes the results of building, testing and executing Cargo projects highly predictable.

Predictability By Example

To illustrate these strategies, let’s build an example crate using Cargo. To keep things simple, we’ll create a small datetime crate that exposes date and time functionality.

First, we’ll use cargo new to start us out:

$ cargo new datetime $ cd datetime $ ls Cargo.toml src $ cat Cargo.toml [package] name = "datetime" version = "0.1.0" authors = ["Yehuda Katz <wycats@gmail.com>"]  [dependencies] 

We don’t want to build the date or time functionality from scratch, so let’s edit the Cargo.toml and add the time crate from crates.io :

  [package]   name = "datetime"   version = "0.1.0"   authors = ["Yehuda Katz <wycats@gmail.com>"]    [dependencies] + time = "0.1.35" 

Now that we’ve added the time crate, let’s see what happens if we ask Cargo to build our package:

$ cargo build    Compiling winapi v0.2.6    Compiling libc v0.2.10    Compiling winapi-build v0.1.1    Compiling kernel32-sys v0.2.2    Compiling time v0.1.35    Compiling datetime v0.1.0 (file:///Users/ykatz/Code/datetime) 

Whoa! That’s a lot of crates. The biggest part of Cargo’s job is to provide enough predictability to allow functionality like the time crate to be broken up into smaller crates that do one thing and do it well .

Now that we successfully built our crate, what happens if we try to build it again?

$ cargo build  

Nothing happened at all. Why’s that? We can always ask Cargo to give us more information through the --verbose flag, so let’s do that:

$ cargo build --verbose        Fresh libc v0.2.10        Fresh winapi v0.2.6        Fresh winapi-build v0.1.1        Fresh kernel32-sys v0.2.2        Fresh time v0.1.35        Fresh datetime v0.1.0 (file:///Users/ykatz/Code/datetime) 

Cargo isn’t bothering to recompile packages that it knows are “fresh”, like make , but without having to write the Makefile .

But how does Cargo know that everything is fresh? When Cargo builds a crate, it emits a file called Cargo.lock that contains the precise versions of all of its resolved dependencies:

[root] name = "datetime" version = "0.1.0" dependencies = [  "libc 0.2.10 (registry+https://github.com/rust-lang/crates.io-index)",  "time 0.1.35 (registry+https://github.com/rust-lang/crates.io-index)", ]  [[package]] name = "kernel32-sys" version = "0.2.2" source = "registry+https://github.com/rust-lang/crates.io-index" dependencies = [  "winapi 0.2.6 (registry+https://github.com/rust-lang/crates.io-index)",  "winapi-build 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)", ]  ... 

The Cargo.lock contains a serialized version of the entire resolved dependency graph, including precise versions of all of the source code included in the build. In the case of a package from crates.io, Cargo stores the name and version of the dependency. This is enough information to uniquely identify source code from crates.io , because the registry is append only (no changes to already-published packages are allowed).

In addition, the metadata for the registry is stored in a separate git repository , and includes checksum for the relevant package. Before Cargo ever unpacks a crate it downloads, it first validates the checksum.

Collaborating

Now for the real test. Let’s push our code up to GitHub and develop it on a different machine. Ideally, we would like to be able to pick up right where we left off, with the exact same source code for all of our dependencies.

To do this, we check in our Cargo.lock and clone the repository on our new machine. Then, we run cargo build again.

$ cargo build    Compiling libc v0.2.10    Compiling winapi v0.2.6    Compiling winapi-build v0.1.1    Compiling kernel32-sys v0.2.2    Compiling time v0.1.35    Compiling datetime v0.1.0 (file:///Users/ykatz/Code/datetime) 

As expected, because we checked in our Cargo.lock we get exactly the same versions of all dependencies as before . And if we wanted to start collaborating with other developers on GitHub (or with other members of our team at work), we would continue to get the same level of predictability.

Common conventions: examples, tests, and docs

Now that we’ve written our snazzy new datetime crate, we’d love to write an example to show other developers how it should be used. We create a new file called examples/date.rs that looks like this:

extern crate datetime;  fn main() {     println!("{}", datetime::DateTime::now()); } 

To run the example, we ask Cargo to build and run it:

$ cargo run --example date    Compiling datetime v0.1.0 (file:///Users/ykatz/Code/datetime)      Running `target/debug/examples/date` 26 Apr 2016 :: 05:03:38 

Because we put our code in the conventional location for examples, Cargo knew how to do the right thing, no sweat.

In addition, once you start writing a few tests, cargo test will automatically build your examples as well, which prevents them from getting out of sync with your code, and ensures they continue to compile as long as your tests are passing.

Similarly, the cargo doc command will automatically compile not just your code, but that of your dependencies as well. The upshot is that the API docs it automatically produces include the crates you depend on, so if your APIs mention types from those crates, your clients can follow those links.

These are just a few examples of a general point: Cargo defines a common set of conventions and workflows that operate precisely the same way across the entire Rust ecosystem .

Updating

All of this means that your application won’t change if you don’t make any changes to your dependencies, but what happens when you need to change them?

Cargo adds another layer of protection with conservative updates . This means that if you modify your Cargo.toml , Cargo attempts to minimize the changes made to the Cargo.lock . The intuition of conservative updates is: if the change you made was unrelated to another dependency, it shouldn’t change .

Let’s say that after developing the library for a little while, we decide that we want to add support for time zones. First, let’s add in the tz dependency to our package:

  [package]   name = "datetime"   version = "0.1.0"   authors = ["Yehuda Katz <wycats@gmail.com>"]    [dependencies]   time = "0.1.35" + tz = "0.2.1" 

After using the crate in our library, let’s run cargo build again:

$ cargo build     Updating registry `https://github.com/rust-lang/crates.io-index`  Downloading tz v0.2.1  Downloading byteorder v0.5.1    Compiling byteorder v0.5.1    Compiling tz v0.2.1    Compiling datetime v0.1.0 (file:///Users/ykatz/Code/datetime) 

Cargo downloaded tz (and its dependency byteorder ) and compiled them, but it didn’t touch the packages we were already using ( kernel32-sys , libc , time , winapi and winapi-build ). Even if one of those package authors published an update in the meantime, you can be sure that adding new crates won’t mess with unrelated ones.

Conservative updates attempt to significantly reduce unexpected changes to your source code. It stands in stark contrast to ‘rebuild the world’, which allows a small change to dependencies to rebuild the entire graph, wreaking havoc in its wake.

As a rule, Cargo attempts to minimize the effects of intentional changes to direct dependencies.

Indirect Dependencies “Just Work”

One of the most basic goals of an application package manager is separating direct dependencies, which are required by the application, and indirect dependencies, which those dependencies need in order to work.

As we’ve seen in the datetime crate we built, we only needed to specify dependencies on time and tz , and Cargo automatically created the entire graph of dependencies needed to make that work. It also serialized that graph for future predictability.

Since Cargo manages your dependencies for you, it can also make sure that it compiles all of your dependencies (whether you knew about them directly or not) appropriately for the task at hand.

Testing, Benchmarking, Releasing, Oh My

Historically, people have shied away from the kinds of granular dependencies we’ve seen here because of the configuration needed for each new dependency.

For example, when running tests or type-checking your code, you would like to compile the code as fast as possible to keep the feedback loop fast. On the other hand, when benchmarking or releasing your code, you are willing to spend plenty of time waiting for the compiler to optimize your code if it produces a fast binary.

It’s important to compile not only your own code or your direct dependencies, but all indirect dependencies with the same configuration.

Cargo manages that process for you automatically. Let’s add a benchmark to our code:

#[bench] fn bench_date(b: &mut Bencher) {     b.iter(|| DateTime::now()); } 

If we then run cargo bench :

$ cargo bench    Compiling winapi v0.2.6    Compiling libc v0.2.10    Compiling byteorder v0.5.1    Compiling winapi-build v0.1.1    Compiling kernel32-sys v0.2.2    Compiling tz v0.2.1    Compiling time v0.1.35    Compiling datetime v0.1.0 (file:///Users/ykatz/Code/datetime)      Running target/release/datetime-2602656fcee02e68  running 1 test test bench_date ... bench:         486 ns/iter (+/- 56) 

Notice that we’re re-compiling all of our dependencies. This is because cargo bench defaults to release mode, which uses maximum optimizations. cargo build --release similarly builds in optimized mode by default.

As an aside, the default behavior of each command is configurable through profiles in the Cargo.toml . This allows you to configure things like the optimization level, whether to include debug symbols and more. Rather than forcing you to use a custom workflow if something doesn’t precisely meet your needs, the profiles feature allows you to customize the existing workflows and stay within Cargo’s flows.

Platforms and Architectures

Similarly, applications are often built for different architectures, operating systems, or even operating system version. They can be compiled for maximum portability or to make maximum use of available platform features.

Libraries can be compiled as static libraries or dynamic libraries. And even static libraries might want to do some dynamic linking (for example, against the system version of openssl ).

By standardizing what it means to build and configure a package, Cargo can apply all of these configuration choices to your direct dependencies and indirect dependencies.

Shared Dependencies

So far, we’ve looked at packages and their dependencies. But what if two packages that your application depends on share a third dependency?

For example, let’s say that I decide to add the nix crate to my datetime library for Unix-specific functionality.

  [package]   name = "datetime"   version = "0.1.0"   authors = ["Yehuda Katz <wycats@gmail.com>"]    [dependencies]   time = "0.1.35"   tz = "0.2.1" + nix = "0.5.0" 

As before, when I run cargo build , Cargo conservatively adds nix and its dependencies:

$ cargo build     Updating registry `https://github.com/rust-lang/crates.io-index`  Downloading nix v0.5.0  Downloading bitflags v0.4.0    Compiling bitflags v0.4.0    Compiling nix v0.5.0    Compiling datetime v0.1.0 (file:///Users/ykatz/Code/datetime) 

But if we look a little closer, we’ll notice that nix has a dependency on bitflags and libc . It now shares the dependency on libc with the date package.

If my datetime crate gets libc types from time and hands them off to nix , they better be the same libc or my program won’t compile (and we wouldn’t want it to!)

Today, Cargo will automatically share dependencies between crates if they depend on the same major version (or minor version before 1.0), since Rust uses semantic versioning . This means that if nix and datetime both depend on some version of libc 0.2.x , they will get the same version. In this case, they do, and the program compiles.

While this policy works well (and in fact is the same policy that system package managers use), it doesn’t always do exactly what people expect, especially when it comes to coordinating a major version bump across the ecosystem. (In many cases, it would be preferable for Cargo to hard-error than assume that a dependency on 0.2.x is simply unrelated to another dependency on 0.3.x .)

This problem is especially pernicious when multiple major versions of the same package expose global symbols (using #[no_mangle] for example, or by including other statically linked C libraries).

We have some thoughts on ways to improve Cargo to handle these cases better, including the ability for a package to more explicitly express when a dependency is used purely internally and is not shared through its public interface. Those packages could be more readily duplicated if needed, while dependencies that are used in a package’s public interface must not be.

You should expect to see more on this topic in the months ahead.

Workflow

As we’ve seen, Cargo is not just a dependency manager, but Rust’s primary workflow tool .

This allows Rust to have a rich ecosystem of interconnected dependencies, eliminating the need for applications to manually manage large dependency graphs. Applications can benefit from a vibrant ecosystem of small packages that do one thing and do it well, and let Cargo handle the heavy lifting of keeping everything up to date and compiling correctly.

Even a small crate like the datetime example we built has a few dependencies on small, targeted crates, and each of those crates has some dependencies of its own.

By defining shared, well-known workflows, like “build”, “test”, “bench”, “run”, and “doc”, Cargo provides Rust programmers with a way to think about what they’re trying to accomplish at a high level, and not have to worry about what each of those workflows mean for indirect dependencies.

This allows us to get closer to the holy grail of making those indirect dependency graphs “invisible”, empowering individuals to do more on their hobby projects, small teams to do more on their products, and large teams to have a high degree of confidence in the output of their work.

With a workflow tool that provides predictability, even in the face of many indirect dependencies, we can all build higher together.

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