file-server/README.md

# FileServer

A minimal Julia service that receives files over HTTP and hands them off to a
pool of worker threads for processing. The HTTP endpoint does no real work: it
spools each uploaded file to disk, pushes a lightweight reference onto a work
queue, and responds immediately — staying free to accept the next upload.

Right now the "processing" is just logging the received filename, to prove the
flow. That's the seam where real heavy-lifting goes later.

## Architecture

```
        POST /upload (multipart)
                 │
                 ▼
        ┌─────────────────┐     spool bytes to disk
        │  HTTP handler    │────────────────────────► data/spool/<uuid>-<name>
        │  (Oxygen.jl)     │
        └────────┬─────────┘     enqueue reference (non-blocking)
                 │                        │
                 ▼                        ▼
          202 + job IDs           ┌───────────────┐
          (503 if full)           │  work queue   │  bounded, thread-safe
                                  │  (Channel-ish)│
                                  └───────┬───────┘
                                          │ dequeue
                          ┌───────────────┼───────────────┐
                          ▼               ▼               ▼
                      worker 1        worker 2   …    worker N   (Threads.@spawn)
                          │
              success ────┴──► data/done/<uuid>-<name>
              failure  ───────► data/failed/<uuid>-<name>
```

Key properties:

- **Fast intake:** the queue only ever carries small references; file bytes live
  on disk, so memory stays flat regardless of file size.
- **Backpressure:** the queue is bounded (default 1000). When full, uploads get
  `503 Service Unavailable` instead of silently piling up.
- **Crash-resilient:** files survive on disk. On startup, anything left in
  `data/spool/` is re-enqueued (`recovered = N` in the log).
- **Graceful shutdown:** SIGINT (Ctrl-C) and SIGTERM (systemd/Docker/k8s `stop`)
  both stop accepting uploads, drain the queue, wait for in-flight files to
  finish, then exit. (See "Shutdown" below for one cosmetic caveat on SIGTERM.)
- **Safe filenames:** client-supplied names are sanitized and prefixed with a
  server-minted UUID before touching the filesystem (no path traversal).

## The queue seam (→ RabbitMQ later)

The HTTP handler and workers only ever call `enqueue!`, `dequeue!`, and
`close!` on a `JobQueue` (see `src/queue.jl`). Today that's an in-process
`ChannelQueue`. To move to RabbitMQ (or any broker), implement a new `JobQueue`
subtype with those three methods and swap the construction in `run` — no handler
or worker code changes.

## Running

```bash
# install deps (first time)
julia --project=. -e 'using Pkg; Pkg.instantiate()'

# start the server; -t sets the number of OS threads available to workers
julia --project=. -t auto bin/server.jl
```

## Shutdown

Both SIGINT and SIGTERM trigger the same idempotent graceful drain
(stop serving → close queue → wait for workers → exit):

- **SIGINT** is caught as an `InterruptException` (we call
  `Base.exit_on_sigint(false)`), so shutdown is clean and quiet.
- **SIGTERM** can't be intercepted directly — Julia blocks it on worker threads
  and handles it in its own runtime, so a user `signal()` handler never fires.
  Instead we hook the drain into an `atexit` handler, which Julia's SIGTERM path
  does run. Caveat: Julia prints its own `signal 15: Terminated` backtrace
  *before* `atexit` runs. It's harmless noise — the drain still completes right
  after it — but if you want a fully quiet stop under a process manager,
  configure it to send SIGINT instead (systemd: `KillSignal=SIGINT`; Docker:
  `STOPSIGNAL SIGINT`). Give the stop timeout enough headroom to drain
  in-flight work (systemd: `TimeoutStopSec`).

## Configuration (environment variables)

| Variable            | Default        | Meaning                                  |
|---------------------|----------------|------------------------------------------|
| `FS_HOST`           | `127.0.0.1`    | Bind address                             |
| `FS_PORT`           | `8080`         | Port                                     |
| `FS_WORKERS`        | `nthreads()`   | Number of worker tasks                   |
| `FS_QUEUE_CAPACITY` | `1000`         | Max pending jobs before `503`            |
| `FS_SPOOL_DIR`      | `data/spool`   | Incoming files (pending)                 |
| `FS_DONE_DIR`       | `data/done`    | Files after successful processing        |
| `FS_FAILED_DIR`     | `data/failed`  | Files whose processing threw             |

> To get real parallelism, start Julia with enough threads (`-t N`) to match
> `FS_WORKERS`. If `FS_WORKERS` exceeds available threads you'll get a warning
> and workers will share threads.

## Usage

```bash
# health check
curl http://127.0.0.1:8080/health
# {"status":"ok"}

# upload one or more files (multipart/form-data)
curl -F "a=@report.pdf" -F "b=@data.csv" http://127.0.0.1:8080/upload
# 202 {"accepted":[{"id":"<uuid>","name":"report.pdf"}, ...]}
```

Each file in a request becomes its own job. Responses:

- `202 Accepted` — all files spooled and queued (with per-file job IDs)
- `400 Bad Request` — not multipart, or no files present
- `503 Service Unavailable` — queue full, retry later
- `500 Internal Server Error` — failed to write a file to disk

## Layout

```
src/
  FileServer.jl   module + run() (startup, recovery, workers, serve, shutdown)
  config.jl       Config struct + env parsing
  job.jl          Job (the queue reference)
  queue.jl        JobQueue seam + in-process ChannelQueue
  spool.jl        filename sanitizing, spool/move, startup recovery
  worker.jl       worker loop + per-job processing (placeholder)
  server.jl       HTTP routes/handlers
bin/
  server.jl       entry point
```