WASM Module Guide

Guest Check-In Module

This guide demonstrates how to build a simple but robust Guestbook module. It records the total number of physical visits to a location.

#Full Source Code

This module reads the alias (name) from the guest's Micro-UI, increments a global counter, and writes the guest's name to a specific Sled key.

src/lib.rs

#![no_std]

use qos_sdk::{state, input, output, log_msg, LogLevel};

#[no_mangle]
pub extern "C" fn run() -> i32 {
    // 1. Read alias from input
    let bytes = input::read().unwrap_or_default();
    let alias = core::str::from_utf8(&bytes).unwrap_or("Anonymous");
    
    // 2. Increment global counter atomically
    let mut current_count: u32 = 1;
    if let Some(val) = state::get("guestbook:total_visits") {
        if val.len() == 4 {
            let mut buf = [0u8; 4];
            buf.copy_from_slice(&val);
            current_count = u32::from_le_bytes(buf) + 1;
        }
    }
    state::set("guestbook:total_visits", &current_count.to_le_bytes()).unwrap();
    
    // 3. Save latest guest
    state::set("guestbook:latest_guest", alias.as_bytes()).unwrap();
    
    log_msg(LogLevel::Info, "New guest checked in!");
    
    // 4. Return receipt
    // In a real app, use a JSON serializer like microserde.
    let receipt = "{\"status\":\"success\"}";
    output::write(receipt.as_bytes());
    
    0
}

#Handling Concurrency

Because state::get and state::set run within the execution pipeline synchronously on the Edge Node, there is no risk of a race condition between reading the counter and writing the updated counter.

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