# Backend (Rust)

- **Coding patterns**: MUST use the `rust-backend` skill when writing Rust code
- **Validation**: `docs/validation.md` — which `cargo check` flags to use
- **Enterprise**: `docs/enterprise.md` — EE file conventions and PR workflow
- **DB schema**: `backend/summarized_schema.txt`
- **API routes entry point**: `windmill-api/src/lib.rs`
- **OpenAPI spec**: `windmill-api/openapi.yaml`
- **DuckDB local jobs**: build the dynamic FFI library before running DuckDB scripts locally:
  ```bash
  cd backend/windmill-duckdb-ffi-internal && ./build_dev.sh
  ```
  Re-run after clean builds or when `target/debug/libwindmill_duckdb_ffi_internal.*` is missing.
  The bundled DuckDB compile (~2min) is cached in a per-user dir shared across
  worktrees (keyed by the crate's `Cargo.lock`), so a fresh worktree reuses it and
  the build is near-instant — you don't pay the full compile per worktree. Editing
  the FFI crate's own source falls back to an isolated per-worktree `./target`.
  The engine is a **patched fork** of duckdb-rs, not the crates.io crate — read
  `docs/duckdb-isolation.md` before bumping it or touching the isolation transform.
- **Running data pipelines (DuckLake) from source**: see the section below — a plain build
  advertises the `duckdb` tag but cannot execute DuckDB scripts and has no working S3 proxy.

## Cargo features & running the dev backend

The dev backend runs under `cargo watch` and is launched by default with **only
`--features quickjs`** (see the tmux backend pane). That baseline compiles fast but
**deliberately omits most functionality** — notably S3/object storage, the S3 proxy, all
EE code, MCP, and every non-JS language runtime. A running server never gains a feature you
didn't compile in: feature-gated routes 404 or return a `"requires <feature>"` stub. So if
you touch code behind a feature gate, or need to *exercise* such a feature at runtime, you
MUST **restart the backend with the appropriate features** for what you're working on.

### Restarting the dev backend with the right features

Restart in the **same pane**, so the relaunch inherits that pane's `DATABASE_URL`, `BACKEND_PORT`
and the rest of `runtime.env`. Scope every kill to this worktree — **never**
`pkill -f target/debug/windmill`, which kills every sibling worktree's backend.

1. Find the backend pane by what it is running, not by index. The index depends on the webmux
   profile: pane 1 is the backend under `full`, but the *frontend* under `frontendOnly`.

   ```bash
   WIN=$(tmux display-message -p -t "$TMUX_PANE" '#{window_id}')
   tmux list-panes -t "$WIN" -F '#{pane_index} #{pane_current_command} #{pane_pid}'
   ```

2. Read the feature set it is **actually** running. `CARGO_FEATURES` in `runtime.env` is only what
   the pane started with, and goes stale the first time anyone restarts by hand:

   ```bash
   ps --ppid <pane_pid> -o args=
   # /home/hugo/.cargo/bin/cargo-watch watch -x run --features quickjs
   ```

3. Stop it and relaunch with the extended set. `PORT` in the pane shell can be stale, so pass it
   explicitly:

   ```bash
   tmux send-keys -t "$WIN.<idx>" C-c
   tmux send-keys -t "$WIN.<idx>" 'PORT=$BACKEND_PORT cargo watch -x "run --features quickjs,private,parquet"' Enter
   ```

   Carry over every feature the old command had unless you mean to drop one — rebuilding the list
   from memory is how a backend silently loses `quickjs`.

4. Persist the new set so a recreated pane starts with it: set `CARGO_FEATURES` in the
   worktree's `.env.local`, which `scripts/post-create.sh` writes and webmux reads. Do **not**
   edit `runtime.env` for this — webmux regenerates it from metadata and `.env.local` every time
   the worktree is opened, so an edit there is lost on the next reopen. Either way the change
   only affects a future pane; step 3 is what takes effect now.

5. Re-capture the pane until `health check completed` appears before hitting the API. A cold
   rebuild takes ~60s, and the previous run's success line is still in the scrollback, so a
   capture taken too early reads as ready when it isn't.

cargo-watch only re-runs on a file change, so after an idle/failed run `touch README.md` (from
`backend/`, where the watch runs) is a cheap retrigger (touching a `.rs` forces a full rebuild).

### An orphaned backend is holding the port

If the pane's `cargo watch` looks alive but the API never answers, or the build ends in an
address-already-in-use error, a backend from an earlier run is probably still bound to the port.
It gets reparented to `systemd --user` when its shell dies, so it survives everything that looks
like a cleanup.

Confirm all three before killing anything — a dozen sibling worktrees run their own backend, and
`pkill -f windmill` (or `-f target/debug/windmill`) kills every one of them:

```bash
ss -ltnp | grep ":$BACKEND_PORT"          # 1. which pid holds the port
readlink /proc/<pid>/cwd                   # 2. must be THIS worktree's backend/
ps -o ppid= -p <pid>                       # 3. parent is systemd/pid 1, not your pane's cargo-watch
```

Only when the port owner is this worktree's backend **and** it is orphaned, kill that single pid
(`kill <pid>`, then `kill -9` if it does not exit). Ask first when there is a human in the loop;
unattended, the three checks are what make it safe. Then `touch README.md` to retrigger the
watch.

### What each feature gate does (the ones you'll actually toggle)

`backend/Cargo.toml` `[features]` is the source of truth; this is the practical dev map. Combine
only what you need — build time scales with the set.

| Feature | Enables | Need it for |
|---|---|---|
| `quickjs` | Embedded JS engine for inline JS eval (the default dev baseline). | Keep in every dev set. |
| `private` | Compiles the `*_ee.rs` files (symlinked from `windmill-ee-private`). Gates **all** EE code, including the real S3 helpers, the S3 proxy, and advanced S3 permission checks. | Any EE code path, S3/object storage. |
| `enterprise` | EE business logic (autoscaling, SAML hooks, advanced S3 rule **enforcement**, WAP, forks, …). Pulls in `license`. | Running EE features. Advanced S3 permission rules only take effect with this. |
| `license` | License-key/plan plumbing (`LICENSE_KEY`). Pulled in by `enterprise`. Having the feature compiled does **not** require a license *key* at runtime — CE defaults to a free plan and most EE paths still run keyless. | License-gated behavior. |
| `parquet` | S3/object-storage support: the `job_helpers/*` and `apps_u/*` S3 endpoints, parquet/CSV preview, workspace large-file storage. Without it those routes return `"requires parquet"`. | Anything touching S3/object storage or datasets. |
| `duckdb` | DuckDB script executor (also needs the FFI dylib — see above). | DuckDB scripts, DuckLake. |
| `python` `rust` `php` `java` `ruby` `csharp` `nu` `deno_core` `mysql` `mssql` `bigquery` `snowflake` `oracledb` `rlang` | Each enables that language/DB runtime for job execution. | Running jobs in that language. |
| `mcp` | MCP gateway routes (baseline `quickjs` does NOT include it → MCP routes 404). | MCP work. |
| `websocket` `http_trigger` `kafka` `nats` `mqtt_trigger` `sqs_trigger` `gcp_trigger` `azure_trigger` `postgres_trigger` `native_trigger` | Each native trigger kind; none on by default (creating one 404s without its feature). | Working on / exercising that trigger. |
| `no_auth` | Treats every request as an admin superadmin (`CLOUD_HOSTED`-guarded). | Local auth-free experiments only. |

Convenience bundles (`ce`, `ee`, `oss`, …) exist in `[features]` but are heavy — prefer the
minimal explicit set for dev.

**Common combinations** (run from `backend/`):

| Goal | `--features` |
|---|---|
| Plain dev baseline (JS eval only) | `quickjs` |
| S3 / object storage / datasets (CE) | `quickjs,private,parquet` |
| S3 + EE (advanced S3 rules, on-behalf app reads, WAP, forks) | `quickjs,enterprise,private,parquet` |
| DuckLake / DuckDB (CE) | `quickjs,duckdb,parquet,private` (+ build the FFI) |
| + Python jobs | append `,python` |

## Workspace object storage in dev — use the local filesystem

For a dev workspace you don't need MinIO/S3: use the built-in **`FilesystemStorage`** large-file
storage (a root path on local disk). It is intentionally hidden from the settings-UI storage
dropdown (dev-only), so set it via the API. Requires the backend built with `parquet` (+ `private`
for the real S3 helpers, + `enterprise` if you want advanced permission rules enforced):

```bash
curl -X POST "$BASE/api/w/<ws>/workspaces/edit_large_file_storage_config" \
  -H "Authorization: Bearer <admin-token>" -H "Content-Type: application/json" \
  -d '{"large_file_storage":{"type":"FilesystemStorage","root_path":"/abs/writable/dir",
       "public_resource":false,"advanced_permissions":null,"secondary_storage":{}}}'
```

Optional `advanced_permissions` (EE) is a list of `{"pattern":"<glob>","allow":"read[,write,delete,list]"}`
rules: admins bypass them, non-admins are confined to matching grants. Uploads/reads then flow
through the normal `job_helpers/*` (viewer-scoped) and `apps_u/*` (app-author on-behalf) S3
endpoints. Caveat: direct DuckDB access rejects filesystem stores (`"Filesystem is not supported
in DuckDB"`) — DuckLake/datatable go through the S3 proxy instead, which works.

## Running data pipelines (DuckLake) from source

DuckLake pipelines need **both** the right cargo features **and** the prebuilt DuckDB FFI. A
plain `cargo run` (or `cargo run --features quickjs`) does **not** suffice, and the failure modes
are silent-ish, so agents lose time. Verify feature names against `backend/Cargo.toml` `[features]`.

**Feature sets** (run from `backend/`):

| Goal | Command |
|---|---|
| CE DuckLake (DuckDB scripts + S3 proxy) | `cargo run --features quickjs,duckdb,parquet,private` |
| + Python scripts | add `,python` |
| EE features (WAP, partitioning, forks, …) | add `,enterprise,license` |

`enterprise` already pulls in `license`, but list both when you want the license-gated paths.
`quickjs` is for JS eval, not DuckLake per se — keep it if your baseline build had it.

**Before running any DuckDB script**, build the FFI (see the bullet above):
`cd backend/windmill-duckdb-ffi-internal && ./build_dev.sh`.

**Two gotchas that a wrong feature set produces:**

1. **`duckdb` tag advertised, feature missing.** The `duckdb` worker tag is in the *unconditional*
   default tag list (`windmill-common/src/worker.rs`, `DEFAULT_TAGS`), so a worker advertises it even
   without the `duckdb` feature. Jobs then dispatch but fail at execution with
   `"Duck DB requires the duckdb feature to be enabled"` (`windmill-worker/src/worker.rs`). Fix:
   compile with `--features duckdb`.
2. **DuckLake writes 404 (no S3 proxy).** The workspace S3 proxy (`/w/{ws}/s3_proxy/*`) that
   DuckLake uses for reads/writes only mounts the real service under
   `#[cfg(all(feature = "private", feature = "parquet"))]` (`windmill-api/src/s3_proxy_oss.rs`);
   otherwise it's an empty router and every proxied request 404s. Fix: compile with **both**
   `private` and `parquet`.

## Cloud vs self-hosted gating

The `cloud` cargo feature is compiled into **all** EE builds, so `#[cfg(feature = "cloud")]` is **not** a "cloud-only" runtime gate — it only means the code is present. The real gate for behavior specific to the managed cloud (app.windmill.dev) is the runtime flag `*CLOUD_HOSTED` (`windmill_common::worker::CLOUD_HOSTED`, from the `CLOUD_HOSTED` env var; note it's loaded from `.env` via `dotenv`, so it won't show in `/proc/<pid>/environ` — check the running behavior, not the exec env).

Cloud-only logic must be behind `if *CLOUD_HOSTED { ... }`: feature-gate the helper so it compiles, then **runtime-gate the call**. `#[cfg(feature = "cloud")]` on its own is only sufficient for:
- pure helper/struct definitions (they only run when a gated caller invokes them),
- code already inside an `if *CLOUD_HOSTED { ... }` block,
- handlers that early-return on `!*CLOUD_HOSTED`,
- idempotent no-ops that are harmless off-cloud (e.g. cache invalidation).

## Verifying Backend Changes

`cargo check` and the unit tests do not exercise a worker code path. **If you changed how
a job runs — an executor, `handle_child`, anything spawning or reading from a
subprocess — run an actual job of that kind** and confirm it completed, then say so.
Whole classes of defect compile and unit-test clean:

- **Stack overflow from a large buffer in an async block.** An array declared across an
  `.await` is baked into the future's state; once that future is boxed a few layers deep
  by the job poller, two 16 KB arrays abort the worker *process* (`thread
  'tokio-runtime-worker' has overflowed its stack`). Heap-allocate read buffers
  (`vec![0u8; N]`, not `[0u8; N]`).
- Deadlocks from draining only one of a child's pipes, missed cancellation or timeout
  propagation, and anything depending on the real engine's output format.

A crash like this takes down every job on that worker, not just yours, so check the
backend log after the run rather than only the job's own status. If you cannot run one,
say which path went unexercised instead of implying it was verified.

### How many jobs a worker runs at once

`NUM_WORKERS > 1` falls back to 1 outside native mode (`backend/src/main.rs`, unless
`I_ACK_NUM_WORKERS_IS_UNSAFE`), so a worker serving script tags — `go`, `python3`,
`dependency`, `flow`, … — runs one job at a time: a per-job resource budget (memory, CPU,
temp space) shares the worker with the worker process alone.

Native mode is the exception, and budgets for its tags must divide by its concurrency:
it forces 8 workers, and `NATIVE_TAGS` includes executors that already claim a per-job
share of the worker's memory (`postgresql` and `mysql` through `MAX_SQL_RESULT_SIZE`), so
up to 8 of those run against the same limit at once.