Hi! I’m trying to understand how the dependency graph generator works in Turborepo.

[Anxhul10]

Summary

Since Turborepo builds only the changed and affected packages, it needs a DAG (directed acyclic graph) that tracks package dependencies. While reading the implementation, I noticed that recursion stops when a package.json is found in a directory, so that folder becomes a node in the graph and traversal doesn’t go deeper.

I also noticed that the DAG generation part is implemented in Rust for performance reasons.

Is this understanding correct?
I’m curious whether the DAG builder walks through each directory, detects package.json, and reads its contents to build a node. If so, how is that traversal performed and how are the dependencies extracted?

Additional information

No response

Example

No response

[Anxhul10]

update: I have made a npm lib where it logs the dependents : dependency !

any feedback would be nice
thanks anyways !

[elpddev]

Interesting. I came here also curious about the mechanics.

It seems the model: it's glob-driven, not a recursive package.json walk. Workspace discovery is delegated entirely to the package manager. From crates/turborepo-repository/src/package_manager/mod.rs:

pub fn get_package_jsons(
    &self,
    repo_root: &AbsoluteSystemPath,
) -> Result<impl Iterator<Item = AbsoluteSystemPathBuf> + use<>, Error> {
    let globs = self.get_workspace_globs(repo_root)?;
    Ok(globs.get_package_jsons(repo_root)?)
}

And for pnpm specifically (package_manager/pnpm.rs):

pub fn get_configured_workspace_globs(repo_root: &AbsoluteSystemPath) -> Option<Vec<String>> {
    let pnpm_workspace = PnpmWorkspace::from_file(repo_root).ok()?;
    // ...
    Some(pnpm_workspace.packages)
}

So the source of truth is pnpm-workspace.yaml (or the workspaces field in root package.json for npm/yarn/bun). Turbo never recursively walks arbitrary directories looking for package.json. Maybe "stops at the first package.json" behavior you're seeing is really "this directory matched the configured workspace glob, so it's a node. Anything below the package boundary is its own concern."

For the edges, once each workspace's package.json is loaded, package_graph/builder.rs matches dependencies / devDependencies / peerDependencies names against the set of known workspace names: internal matches become graph edges:

for dependency in internal {
    let dependency_idx = self
        .node_lookup
        .get(&PackageNode::Workspace(dependency))
        .expect("unable to find workspace node index");
    self.workspace_graph.add_edge(*node_idx, *dependency_idx, ());
}

This is just the package graph. The actual run-order DAG is probably composed on top using turbo.json's dependsOn rules after the first graph is collected I think. Two phases.

What is dependents : dependency ! ? What lib?

[Anxhul10]

Thanks for detail explanation !!
I just made a simple lib to get me the list of dependant: dependency https://github.com/Anxhul10/dag-rs . But right now it fails in circular dependency.