document crater

service-catalog/crater/README.md

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+# Crater
+
+`Crater` is a tool to run experiments across parts of the Rust ecosystem.
+
+The `crater` service is managed in the [terraform/crater] module, while the app is in the [rust-lang/crater] repository.
+
+You can find the detailed `crater` docs
+[here](https://github.com/rust-lang/crater/tree/master/docs).
+
+## Architecture
+
+The crater project contains the following components:
+- `crater`: server that assigns experiments (tasks) to agents
+- `agent`: worker that runs experiments and reports back to the server
+
+```mermaid
+graph LR
+    crater["Crater"]
+    agent1["Agent 1"]
+    agent2["Agent 2"]
+
+    agent1 --> crater
+    agent2 --> crater
+```
+
+The agents and crater communicate over HTTP.
+
+The agents communicate to crater their capabilities
+(e.g. "windows" or "hard-drive-bigger-than-1TB") and crater assigns experiments
+based on these capabilities.
+
+## Bot
+
+Crater can be controlled in the `rust-lang/rust` repo thanks to the GitHub bot
+[@craterbot](https://github.com/craterbot). The bot replies to every command in
+the comments of issues and pull requests, if the command is in its own line and
+is prefixed with the bot's username.
+
+For example, to check if the bot is alive you can write this comment:
+
+```
+@craterbot ping
+```
+
+And the bot will reply to you.
+
+## Versions
+
+These are the versions we need to keep up-to-date:
+
+- Ubuntu version specified in the [Dockerfile]
+- Cargo dependencies: specified in the [Cargo.lock]
+- GitHub Actions: specified in the [workflows] directory
+
+[Dockerfile]: https://github.com/rust-lang/crater/blob/master/Dockerfile
+[Cargo.lock]: https://github.com/rust-lang/crater/blob/master/Cargo.lock
+[workflows]: https://github.com/rust-lang/crater/tree/master/.github/workflows
+[rust-lang/crater]: https://github.com/rust-lang/crater
+[terraform/crater]: https://github.com/rust-lang/simpleinfra/tree/master/terraform/crater