---name: rust-backenddescription: Rust coding guidelines for the Windmill backend. MUST use when writing or modifying Rust code in the backend directory.---

Windmill Rust Patterns

Apply these Windmill-specific patterns when writing Rust code in backend/.

Error Handling

Use Error from windmill_common::error. Return Result<T, Error> or JsonResult<T>:

use windmill_common::error::{Error, Result};

pub async fn get_job(db: &DB, id: Uuid) -> Result<Job> {
    sqlx::query_as!(Job, "SELECT id, workspace_id FROM v2_job WHERE id = $1", id)
        .fetch_optional(db)
        .await?
        .ok_or_else(|| Error::NotFound("job not found".to_string()))?;
}

Never panic in library code. Reserve .unwrap() for compile-time guarantees.

SQLx Patterns

Never use SELECT * — always list columns explicitly. Critical for backwards compatibility when workers lag behind API version:

// Correct
sqlx::query_as!(Job, "SELECT id, workspace_id, path FROM v2_job WHERE id = $1", id)

// Wrong — breaks when columns are added
sqlx::query_as!(Job, "SELECT * FROM v2_job WHERE id = $1", id)

Use batch operations to avoid N+1:

// Preferred — single query with IN clause
sqlx::query!("SELECT ... WHERE id = ANY($1)", &ids[..]).fetch_all(db).await?

Use transactions for multi-step operations. Parameterize all queries.

JSON Handling

Prefer Box<serde_json::value::RawValue> over serde_json::Value when storing/passing JSON without inspection:

pub struct Job {
    pub args: Option<Box<serde_json::value::RawValue>>,
}

Only use serde_json::Value when you need to inspect or modify the JSON.

Serde Optimizations

#[derive(Serialize, Deserialize)]
pub struct Job {
    #[serde(skip_serializing_if = "Option::is_none")]
    pub parent_job: Option<Uuid>,
    #[serde(skip_serializing_if = "Vec::is_empty")]
    pub tags: Vec<String>,
    #[serde(default)]
    pub priority: i32,
}

Async & Concurrency

Never block the async runtime. Use spawn_blocking for CPU-intensive work:

let result = tokio::task::spawn_blocking(move || expensive_computation(&data)).await?;

Mutex selection: Prefer std::sync::Mutex (or parking_lot::Mutex) for data protection. Only use tokio::sync::Mutex when holding locks across .await points.

Use tokio::sync::mpsc (bounded) for channels. Avoid std::thread::sleep in async contexts.

Module Structure & Visibility

• Use pub(crate) instead of pub when possible
• Place new code in the appropriate crate based on functionality
• API endpoints go in windmill-api/src/ organized by domain
• Shared functionality goes in windmill-common/src/

Code Navigation

Always use rust-analyzer LSP for go-to-definition, find-references, and type info. Do not guess at module paths.

Feature Telemetry

FEATURE_USAGE_KINDS in windmill-api-workspaces/src/workspaces.rs is an allowlist: a (feature, kind) pair missing from it is dropped by valid_feature_usage_event with a bare continue — no error, and the route still returns 204. Adding a counter on the frontend without registering it here records nothing. See docs/feature-telemetry.md.

Axum Handlers

Destructure extractors directly in function signatures:

async fn process_job(
    Extension(db): Extension<DB>,
    Path((workspace, job_id)): Path<(String, Uuid)>,
    Query(pagination): Query<Pagination>,
) -> Result<Json<Job>> { ... }