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Introduction to the Wasm VM for IOTA Smart Contracts

IOTA Smart Contracts provide a very flexible way of programming smart contracts by providing an API to a sandboxed environment that allows you to interact with the IOTA Smart Contracts deterministically without any security risks. The API provides a generic way to store, access, and modify the state of the smart contract. The API can be used by any kind of Virtual Machine (VM) to implement a system to program, load, and run smart contracts on top of IOTA Smart Contracts. The actual VMs can be implemented by whoever wants to create them.

Wasp node IOTA Smart Contracts Host

Of course, we provide an example implementation of such a VM to allow anyone to get a taste of what it is like to program a smart contract for IOTA Smart Contracts. Our VM implementation uses WebAssembly (Wasm) code as an intermediate compilation target. The implementation of the Wasm VM currently uses the open-source Wasmtime runtime environment. The Wasm VM enables dynamic loading and running of smart contracts that have been compiled to Wasm code.

We chose Wasm to be able to program smart contracts from any programming language. Since more and more languages are becoming capable of generating the intermediate Wasm code this will eventually allow developers to choose a language they are familiar with.

Because each Wasm code unit runs in its own memory space and cannot access anything outside that memory by design, Wasm code is ideally suited for secure smart contracts. The Wasm runtime system will only provide access to external functionality that is needed for the smart contracts to be able to do their thing, but nothing more. In our case, we only provide access to the IOTA Smart Contracts host's sandbox API environment. The way we do that is by providing a simple library that can be linked to the Wasm code. This library, for obvious reasons, has been named WasmLib for now.

Wasm VM

As you can see we can have any number of smart contracts running in our Wasm VM. Each smart contract is a separately compiled Wasm code unit that contains its own copy of WasmLib embedded into it. Each WasmLib provides the IOTA Smart Contracts sandbox functionality to its corresponding smart contract and knows how to access the underlying smart contract state storage through the VM runtime system. This makes the IOTA Smart Contracts sandbox API access seamless to the smart contract by hiding the details of bridging the gap between the smart contract's memory space, and the IOTA Smart Contracts host's memory space. It also prevents the smart contract from accessing and/or modifying the IOTA Smart Contracts host's memory space directly.

The IOTA Smart Contracts sandbox environment enables the following functionality:

  • Access to smart contract metadata
  • Access to parameter data for smart contract function calls
  • Access to the smart contract state data
  • A way to return result data to the caller of a smart contract function
  • Access to tokens owned by the smart contract, and the ability to move them
  • Ability to initiate or call other smart contract functions
  • Access to logging functionality
  • Access to several utility functions provided by the host

The initial WasmLib implementation was created for the Rust programming language. Rust had the most advanced and stable support for generating Wasm code at the time when we started implementing our Wasm VM environment. In the meantime, we have also implemented fully functional Go and TypeScript implementations. The Go implementation uses the TinyGo compiler to generate Wasm code. The TypeScript implementation uses the AssemblyScript compiler to generate Wasm code.

All implementations use only a very small common subset of their host languages. This keeps the coding style very similar, barring some syntactic idiosyncrasies. The reason for this is that we wanted to make it as easy as possible for anyone to start working with our smart contract system. If you have any previous experience in any C-style language you should quickly feel comfortable writing smart contracts in any of the supported languages, without having to dive deeply into all aspects of the chosen language.

We will now dive deeper into the concepts that are central to WasmLib smart contract programming.