snapmaker_moonraker — the J1S bridge¶

A Go application that pretends to be Moonraker on one side and talks SACP to a Snapmaker J1S on the other.


Repo	`goeland86/snapmaker_moonraker`
Language	Go 1.22+
Listen	`:7125` (Moonraker default) — HTTP + WebSocket JSON-RPC
Talks to printer	SACP TCP `:8888`, Snapmaker HTTP API `:8080`
License	MIT
Latest

What it does¶

[Mainsail/Fluidd] ←─ HTTP/WebSocket ─→ [Bridge :7125] ←─ SACP/HTTP ─→ [J1S]

From the UI's perspective the J1S looks like a normal Klipper printer. The bridge exposes the same Moonraker JSON-RPC namespace, the same websocket notifications, and the same printer-object tree.

From the printer's perspective the bridge is just another SACP client issuing temperature/move/upload commands over TCP.

Why a bridge (and not patches to either end)¶

Snapmaker firmware is closed. No way to put Klipper on the J1S without losing the touchscreen and the closed-source slicer integration.
Moonraker is tied to Klipper. Its serial protocol assumes Klipper's MCU protocol on the other end. Adapting it to SACP would be a larger surgery than just reimplementing the Moonraker surface.

So the bridge owns the Moonraker contract on one side and the SACP contract on the other.

What it implements¶

Surface	Notes
`printer.info`, `printer.objects.list`, `printer.objects.query`, `printer.objects.subscribe`	Standard Klipper objects synthesised from J1S state.
`printer.gcode.script`, `printer.gcode.help`	GCode script execution. Some commands are intercepted natively; the rest are translated to SACP.
`printer.print.start`, `.pause`, `.resume`, `.cancel`	Print control.
`printer.emergency_stop`	Mapped to J1S emergency-stop SACP packet.
`server.files.*` (`list`, `upload`, `delete`, `metadata`, `move`, `directory`)	File management. Upload uses streaming `MultipartReader` (no RAM blow-up — see v1.6.0).
`server.spoolman.*`	Proxied to a configured Spoolman instance.
`server.database.*`	Persistent key-value namespace (used by `[save_variables]` emulation).
`notify_*` websocket notifications	`notify_status_update`, `notify_gcode_response`, `notify_klippy_ready`, etc.
`[respond] action:prompt_*` emulation	Bridge re-broadcasts intercepted `RESPOND TYPE=command MSG="action:..."` calls as `notify_gcode_response`, which Mainsail renders as a prompt dialog.
`printer.save_variables` object	Backed by the bridge's persistent database namespace. Read by Mainsail / macros / `NFC_STATUS`.

See the Bridge intercepts reference for the exact list of intercepted gcode commands.

What it does not implement (yet)¶

printer.bed_mesh.* — J1S handles bed levelling internally.
update_manager — no autoupdate; the bridge is self-contained and versioned independently.
Anything Klipper-MCU-specific (gcode_macro saving, pid_calibrate output, etc.).

If you hit a missing endpoint that Mainsail expects, file an issue with the request/response from a real Moonraker.

Configuration¶

server:
  host: "0.0.0.0"
  port: 7125

printer:
  ip: "192.168.1.100"        # Your J1S IP
  token: ""                  # Confirmed at the printer HMI on first connect
  model: "Snapmaker J1S"
  poll_interval: 2           # Status polling cadence (s)

files:
  gcode_dir: "gcodes"        # Local gcode staging directory

spoolman:
  url: "http://localhost:7912"   # Optional — only if running NFC daemon

How it runs on the J1S Pi image¶

The CI pipeline builds a turnkey Raspberry Pi 3 SD image:

[Browser] → [nginx :80] → [Mainsail static files]
                        → proxy_pass → [snapmaker_moonraker :7125]
                                            ↓
                                  [J1S via SACP/HTTP]

Raspberry Pi OS Lite (Bookworm, 32-bit)
nginx serving the latest upstream Mainsail
snapmaker_moonraker as a systemd unit (snapmaker-moonraker.service)
Optionally klipper-nfc-daemon as nfc-spoolman.service
SSH enabled, default user pi / password temppwd

See How-to: build the J1S Pi image.

Memory & systemd hardening (v1.6.0)¶

The systemd unit includes:

[Service]
SyslogIdentifier=snapmaker-moonraker
Environment=GOMEMLIMIT=600MiB

SyslogIdentifier ensures journalctl -u snapmaker-moonraker actually finds the bridge's stdout/stderr (default identifier would be the truncated process name snapmaker_moonr).
GOMEMLIMIT=600MiB is a defence-in-depth Go-runtime soft cap on the Pi's 921 MB RAM. Combined with the streaming upload pipeline this prevents the OOM-kills that historically plagued the Pi 3 build on large prints.

Provenance¶

The SACP implementation in sacp/ is adapted from sm2uploader (by @macdylan, MIT) and its ancestor snapmaker-sm2uploader (by @kanocz). The code is vendored rather than imported because sm2uploader is a standalone program (package main).