created_at2019-07-23 09:25:42.274254
updated_at2023-08-23 07:39:42.533418
description🎁 Alternative to cargo fetch



🎁 cargo-fetcher

Embark Embark Docs dependency status Build Status

Alternative to cargo fetch for use in CI or other "clean" environments that you want to quickly bootstrap with the necessary crates to compile/test etc your project(s).


  • You run many CI jobs in clean and/or containerized environments and you want to quickly fetch cargo registries and crates so that you can spend your compute resources on actually compiling and testing the code, rather than downloading dependencies.

Why not?

  • Other than the fs storage backend, the only supported backends are the 3 major cloud storage backends, as it is generally beneficial to store crate and registry information in the same cloud as you are running your CI jobs to take advantage of locality and I/O throughput.
  • cargo-fetcher should not be used in a typical user environment as it completely disregards various safety mechanisms that are built into cargo, such as file-based locking.
  • cargo-fetcher assumes it is running in an environment with high network throughput and low latency.

Supported Storage Backends


The gcs feature enables the use of Google Cloud Storage as a backend.

  • Must provide a url to the -u | --url parameter with the gsutil syntax gs://<bucket_name>(/<prefix>)?
  • Must provide GCP service account credentials either with --credentials or via the GOOGLE_APPLICATION_CREDENTIALS environment variable


The s3 feature enables the use of Amazon S3 as a backend.

  • Must provide a url to the -u | --url parameter, it must of the form http(s)?://<bucket>.s3(-<region>).<host>(/<prefix>)?
  • Must provide AWS IAM user via the environment AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY described here or run from an ec2 instance with an assumed role as described here.


The fs feature enables use of a folder on a local disk to store crates to and fetch crates from.

  • Must provide a url to the -u | --url parameter with the file: scheme


The blob feature enables the use of Azure Blob storage as a backend.

  • Must provide a url to the -u | --url parameter, it must of the form blob://<container_name>(/<prefix>)?
  • Must provide Azure Storage Account via the environment variables STORAGE_ACCOUNT and STORAGE_MASTER_KEY described here.


This is an example from our CI for an internal project.


  • 424 crates: cached - 38MB, unpacked - 214MB
  • 13 crates source from 10 git repositories: db - 27MB, checked out - 38MB


The following CI jobs are run in parallel, each in a Kubernetes Job running on GKE. The container base is roughly the same as the official rust:1.39.0-slim image.

  • Build modules for WASM
  • Build modules for native
  • Build host client for native

~ wait for all jobs to finish ~

  • Run the tests for both the WASM and native modules from the host client


All 3 build jobs take around 1m2s each to do cargo fetch --target x86_64-unknown-linux-gnu


All 3 build jobs take 3-4s each to do cargo fetcher --include-index sync.


cargo-fetcher has only 2 subcommands. Both of them share a set of options, the important inputs for each backend are described in Storage Backends.

In addition to the backend specifics, the only required optional is the path to the Cargo.lock lockfile that you are operating on. cargo-fetcher requires a lockfile, as otherwise the normal cargo work of generating a lockfile requires having a full registry index locally, which partially defeats the point of this tool.

-l, --lock-file <lock-file>
    Path to the lockfile used for determining what crates to operate on [default: Cargo.lock]


The mirror subcommand does the work of downloading crates and registry indexes from their original locations and re-uploading them to your storage backend.

It does have one additional option however, to determine how often it should take snapshots of the registry index(es).

-m, --max-stale <max-stale>
    The duration for which the index will not be replaced after its most recent update.

    Times may be specified with no suffix (default days), or one of:
    * (s)econds
    * (m)inutes
    * (h)ours
    * (d)ays

Custom registries

One wrinkle with mirroring is the presence of custom registries. To handle these, cargo fetcher uses the same logic that cargo uses to locate .cargo/config<.toml> config files to detect custom registries, however, cargo's config files only contain the metadata needed to fetch and publish to the registry, but the url template for where to download crates from is actually present in a config.json file in the root of the registry itself.

Rather than wait for a registry index to be downloaded each time before fetching any crates sourced that registry, cargo-fetcher instead allows you to specify the download location yourself via an environment variable, that way it can fully parallelize the fetching of registry indices and crates.


# .cargo/config.toml

embark = { index = "<secret url>" }

The environment variable is of the form CARGO_FETCHER_<name>_DL where name is the same name (upper-cased) of the registry in the configuration file.

CARGO_FETCHER_EMBARK_DL="https://secret/rust/cargo/{crate}-{version}.crate" cargo fetcher mirror

The format of the URL should be the same as the one in your registry's config.json file, if this environment variable is not specified for your registry, the default of /{crate}/{version}/download is just appended to the url of the registry.


The sync subcommand is the actual replacement for cargo fetch, except instead of downloading crates and registries from their normal location, it downloads them from your storage backend, and splats them to disk in the same way that cargo does, so that cargo won't have to do any actual work before it can start building code.


Contributor Covenant

We welcome community contributions to this project.

Please read our Contributor Guide for more information on how to get started.


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

Commit count: 112

cargo fmt