The basic building blocks for the Internet of Things (IoT) are microcontrollers, which are usually programmed in low level programming languages such as C for efficiency. Unfortunately, writing correct software at this abstraction level is difficult and time-consuming. To make matters even worse, remote debugging is often not available. To alleviate this, we have created WARDuino, a virtual machine capable of running WebAssembly (Wasm) with live code update and debugging support. WARDuino only supports standard Wasm and cannot communicate over the internet, a crucial requirement of any IoT application.

In this work we extend the WARDuino VM to support fully fledged IoT programs written in high-level languages. We add support for the two most widely used IoT protocols: HTTP and MQTT. In order to support the asynchronous nature of MQTT we extend Wasm and the VM with a new callback system which allows us to execute callback functions defined in Wasm. We also construct a complete formal description for this extension. Finally we adapt the VM to also support programs written in high-level languages rather than only plain Wasm code. This involves creating a symbiosis between the foreign constructs of the high-level language and the basic numeric types of Wasm. We illustrate our symbiosis mechanism by adding support for Rust and AssemblyScript to WARDuino. To evaluate our work we created a fully fledged IoT demo in Rust and let it run continuously for several days. This showed our implementation is stable and has the expected performance.