Create documentation for CDNs

The documentation describes the current state with the most important
CloudFront distributions. The motivation to document this now is that we
are planning to introduce a second CDN, which we can collaborate on by
updating this document.

Author: jdno

src/SUMMARY.md

@@ -53,6 +53,7 @@
         - [AWS access management](./infra/docs/aws-access-management.md)
         - [Bastion server](./infra/docs/bastion.md)
         - [Bors](./infra/docs/bors.md)
+        - [CDN](./infra/docs/cdn.md)
         - [Crater agents](./infra/docs/crater-agents.md)
         - [Custom GitHub Actions runners](./infra/docs/gha-self-hosted.md)
         - [Dev Desktops](./infra/docs/dev-desktop.md)

src/infra/docs/cdn.md

@@ -0,0 +1,74 @@
+# Content Delivery Networks
+
+Users of the Rust programming language interact with the infrastructure of the
+project in various different ways. They access the project's website and
+documentation, query the crates index, and download Rust releases and crates.
+These resources are hosted by the Rust project and served through a [Content
+Delivery Network] (CDN).
+
+This document outlines why we use CDNs, for what, and how we have set them up.
+
+## Objectives
+
+- Reduce costs of outbound traffic by caching resources in the CDN
+- Reduce load on origin servers to save compute resources
+- Provide a way to rewrite legacy URLs for some resources
+
+## Infrastructure
+
+Most of the project's resources are hosted on [AWS]. Static content is stored in
+[Amazon S3], while dynamic content is loaded from a server. Both types of
+content are served through [Amazon CloudFront], the [Content Delivery Network]
+of AWS.
+
+When a user access a resource, e.g. they are trying to download a crate, they
+will access the resource through the CDN. Different _distributions_ map domain
+names to a configuration and a backend (called the _origin_). For example,
+downloading a crate from `static.crates.io` goes through a _distribution_ that
+fetches the crate from an S3 bucket and then caches it for future requests.
+
+```text
+                             ┌──► S3 (static content)
+                             │
+User ───────► CloudFront ────┤
+                             │
+                             └──► Server (dynamic content)
+```
+
+## Distributions
+
+There are many distributions, all of which are configured in the
+[rust-lang/simpleinfra] repository. However, their usage is very unevenly
+distributed. The following distributions are the most important ones for the
+project, both in terms of traffic and criticality for the ecosystem.
+
+### Rust Releases
+
+Whenever a user installs or updates Rust, pre-compiled binaries are downloaded
+from `static.rust-lang.org`. The same is true when Rust is installed in a CI/CD
+pipeline, which is why this distribution has by far the highest traffic volume.
+
+Rust binaries are static and are stored in [Amazon S3], from where they are
+served by the CloudFront distribution.
+
+The distribution for `static.rust-lang.org` has a custom router that runs in a
+[AWS Lambda] function. The router provides a way to list files for a release and
+rewrites the legacy URL for `rustup.sh`.
+
+The cache for Rust releases is invalidated nightly.
+
+### Crates
+
+Similar to Rust releases, crates are served from as static content from
+`static.crates.io`. While still being the second-largest distribution in our
+infrastructure, it is much smaller than the releases.
+
+Crates are static and stored in [Amazon S3], and served through a CloudFront
+distribution.
+
+[amazon cloudfront]: https://aws.amazon.com/cloudfront/
+[amazon s3]: https://aws.amazon.com/s3/
+[aws]: https://aws.amazon.com/
+[aws lambda]: https://aws.amazon.com/lambda/
+[content delivery network]: https://en.wikipedia.org/wiki/Content_delivery_network
+[rust-lang/simpleinfra]: https://github.com/rust-lang/simpleinfra