Extreme MQTT on PicoRuby

This is the presentation material from my talk on Day 2 of RubyKaigi 2026 in Hakodate, Hokkaido.

https://rubykaigi.org/2026/presentations/ryosk7.html
I spoke about the challenges of implementing MQTT and MQTTS (MQTT + TLS) based on picoruby-socket on an RP2040 board.
The library developed for this presentation can be found at:
https://github.com/ryosk7/picoruby-net-mqtt-femto/tree/rubykaigi-2026
Please feel free to check it out.

Transcript

1. Extreme MQTT
 on PicoRuby Ryosuke Uchida @ryosk7 2026/04/23
2. Source: UchidaSakusen Co,.Ltd. / ಺ాᮓઘגࣜձࣾ
3. Switch of the original shape Source: https://www.amazon.co.jp/dp/B0FH1XVPY9?ref=ppx_yo2ov_dt_b_fed_asin_title
4. Day 3 !
5. How to achieve basic MQTT on RP2040?
6. How to achieve basic MQTT on RP2040? extreme
7. ˒ Ryosuke Uchida (@ryosk7) ˒ Freelancer ˒ Organizer of Roppongi.rb, a regional.rb for Rubyists. Hello, Hakodate!
10. Thank you, 
 hasumi san,
 yancya san!!!!!
12. What is MQTT?
13. ˒ Message Queuing Telemetry Transport ˒ Lightweight pub/sub messaging protocol over TCP ˒ Designed for IoT: low bandwidth, unreliable network ˒ Central Broker decouples Publishers and Subscribers What is MQTT?
15. picoruby-net-mqtt picoruby-net-mqtt-femto Implementation Pure Ruby C + lwIP bindings Target Any PicoRuby board RP2040 / Pico W only QoS 0 ✅ ✅ QoS 1 ✅ ✅ MQTTS ✅ ⚠ Experimental Portability ✅ RP2040 only
16. PicoRuby and FemtoRuby PicoRuby FemtoRuby Ruby VM mruby mruby/c memory size (RAM) < 400 KB < 40 KB
17. ˒ A Ruby implemention for microcontrollers ˒ Part of the PicoRuby ecosystem, maintained by @hasumikin ˒ R2P2: Ruby shell system for RP2040 What is FemtoRuby?
18. The hardware Source: ssci.to/8171
19. 264 KB That is all static RAM we have.
20. The challenge Fitting a network stack + Ruby VM into 264 KB
 is normally considered impossible This session is about how we made it work — and pushed it further
21. Layer RAM mruby/c VM baseline ~ 49 KB PicoRuby standard gems ~ 30 KB lwIP stack ~ 40 KB MQTT state machine ~ 10 KB Total ~ 129 KB / 264 KB
22. What is lwIP? lwIP (~40 KB) Application (Ruby / C) apps/mqtt ← we use this altcp_tls + mbedTLS TCP / UDP IP / ICMP / DNS / DHCP Netif: CYW43439 WiFi (SPI)
23. lwIP (~40 KB) Application (Ruby / C) apps/mqtt ← we use this altcp_tls + mbedTLS TCP / UDP IP / ICMP / DNS / DHCP Netif: CYW43439 WiFi (SPI) ˒ “lightweight IP” — an embedded 
 TCP/IP stack ˒ Provides TCP / UDP / DNS /
 DHCP + app clients
 (HTTP, MQTT…) ˒ Built into the Raspberry Pi Pico SDK
 — driven via cyw43_arch ˒ TLS through altcp_tls (mbedTLS backend)
24. Implementation
25. lwipopts.h build con fi guration lwIP MQTT driver ports/rp2040/mqtt.c mruby/c bindings src/mrubyc/mqtt.c Overview - 4 layers Ruby API mrblib/mqtt.rb
26. ˒ Net::MQTT::Client - public API ˒ connect / disconnect / publish / subscribe ˒ Reconnect helpers, retained session state ˒ Keeps the Ruby side idiomatic Design - Ruby Layer mruby/c bindings src/mrubyc/mqtt.c Ruby API mrblib/mqtt.rb
27. ˒ Thin bridge between mruby/c VM and C ˒ Type conversation: mrbc_value ⁶ C primitives ˒ NUL termination at every string boundary ˒ Error propagation into Ruby exceptions Design - C Bindings lwIP MQTT driver ports/rp2040/mqtt.c mruby/c bindings src/mrubyc/mqtt.c Ruby API mrblib/mqtt.rb
28. ˒ lwIP’s built-in MQTT client ˒ TCP connection lifecycle ˒ MQTT fi nite state machine —
 CONNECT / CONNACK / PUBLISH / PUBACK ˒ Callback dispatch back to C bindings Design - lwIP Layer lwipopts.h build con fi lwIP MQTT driver ports/rp2040/mqtt.c mruby/c bindings src/mrubyc/mqtt.c
29. ˒ CYW43439 WiFi chip
 — separate from RP2040 ˒ Talks to RP2040 over SPI ˒ cyw43_arch_poll() pumps WiFi events into lwIP ˒ Polling timing is critical (see the bug later) Design — Hardware lwipopts.h build con fi guration lwIP MQTT driver ports/rp2040/mqtt.c
30. MQTT Broker CYW43439 lwIP MQTT mruby/c Ruby client.connect cyw43_arch_poll() (outside lock) MQTT_connect_impl() TCP SYN (SPI) TCP handshake SYN-ACK WiFi event MQTT CONNECT CONNECT packet CONNACK WiFi event connection_cb() connected
31. MQTT Broker CYW43439 lwIP MQTT mruby/c Ruby Entry — Ruby → mruby/c → lwIP client.connect cyw43_arch_poll() (outside lock) MQTT_connect_impl() TCP SYN (SPI) TCP handshake SYN-ACK WiFi event MQTT CONNECT CONNECT packet CONNACK WiFi event connection_cb() connected
32. MQTT Broker CYW43439 lwIP MQTT mruby/c Ruby TCP handshake over SPI client.connect cyw43_arch_poll() (outside lock) MQTT_connect_impl() TCP SYN (SPI) TCP handshake SYN-ACK WiFi event MQTT CONNECT CONNECT packet CONNACK WiFi event connection_cb() connected
33. MQTT Broker CYW43439 lwIP MQTT mruby/c Ruby MQTT CONNECT / CONNACK client.connect cyw43_arch_poll() (outside lock) MQTT_connect_impl() TCP SYN (SPI) TCP handshake SYN-ACK WiFi event MQTT CONNECT CONNECT packet CONNACK WiFi event connection_cb() connected
34. MQTT Broker CYW43439 lwIP MQTT mruby/c Ruby Callback bubbles back up client.connect cyw43_arch_poll() (outside lock) MQTT_connect_impl() TCP SYN (SPI) TCP handshake SYN-ACK WiFi event MQTT CONNECT CONNECT packet CONNACK WiFi event connection_cb() connected
35. Step 1: lwipopts.h — build con fi guration Miss any one of these and client.connect hangs silently.
36. Step 2: lwIP MQTT driver
37. DNS resolve outside any lock — no lwIP operation yet
38. Alloc + callbacks every lwIP call wrapped
 in lwip_begin() / lwip_end()
39. State → CONNECTING set before calling connect
40. Send CONNECT packet inside lock;
 CONNACK arrives asynchronously via poll
41. ˒ cyw43_arch_poll() reads WiFi events from CYW43 and dispatches them into lwIP ˒ Called outside the lwIP lock — calling inside causes deadlock ˒ CONNACK and PUBACK only arrive through this poll ˒ sys_check_timeout() (inside lock) advances lwIP’s internal timers poll loop
42. Step 3: mruby/c bindings mruby/c strings can be GC’d soon as the binding returns. But lwIP keeps a pointer to client_id for the lifetime of the connections. Every Ruby-supplied string that lwIP holds onto must be copied into a
 context-owned buffer with a manual NUL terminator.
43. Step 3: mruby/c bindings
44. The Ruby API
45. Ruby API: connect and publish
46. Ruby API: subscribe and receive
47. poll_sleep_ms — not just sleep Without polling, PINGREQ / PINGRESP stops working.
 The broker closes the connection after keep_alive seconds of slience. poll_sleep_ms keeps the connection alive during idle periods.
48. Auto-recconect with backoff
49. with_reconnect block On ConnectionError, reconnects and re-enters the block automatically. Previously subscribed topics are resubscribed on reconnect.
50. More reconnect helpers
51. Wait helpers Useful for sequencing operations without busy-waiting.
52. Extreme
53. RAM at the edge mruby/c heap pool 150 KB lwIP / USB / CYW43 ( .bss ) 90.8 KB Initialized data ( .data ) 12.9 KB Core stacks (core0 + core1) 6 KB C heap ( PICO_HEAP_SIZE ) 1 KB Total 260.7 KB / 264 KB = 98.8%
54. ˒ QoS 1 ˒ Auto Resubscription ˒ MQTTS Goal
55. QoS 0 vs QoS 1
56. QoS 1 with native-state retry
57. with_reconnect + auto-resubscribe
58. with_reconnect + auto-resubscribe
59. Ring buffer that drops the oldest
60. Ring buffer that drops the oldest
61. MQTTS: the wall mbedTLS needs ≈ 40 KB for handshake and TLS record buffers. Current free RAM: 3.3 KB. Fix: shirink heap_pool 150 → 101 KB, raise PICO_HEAP_SIZE 1 → 50 KB
62. ˒ MQTT 3.1.1 on RP2040 / Pico W ˒ IwIP-native C bindings — minimal overhead ˒ QoS 0 + QoS 1 ˒ Auto-reconnect with exponential backoff ˒ Same Net::MQTT API as picoruby-net-mqtt Summary
63. Thank you