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Installation

Add the Slung SDK to your Rust project:
Cargo.toml
[dependencies]
slung = "0.1.0"
Your workflow must compile to a WASM target: 
rustup target add wasm32-unknown-unknown

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Your First Workflow

Here’s a complete example that monitors temperature sensors and sends alerts when values exceed a threshold:
src/main.rs
use slung::prelude::*;

#[main]
fn main() -> Result<()> {
    // Subscribe to live stream updates.
    let handle = query_live("AVG:temp:[sensor=1]")?;
    poll_handle(handle, on_event, 100.0)?;

    Ok(())
}

fn on_event(event: Event, alert_threshold: f64) -> Result<()> {
    if event.value > alert_threshold {
        println!("event timestamp={} value={}", event.timestamp, event.value);
        for producer in event.producers {
            writeback_ws(producer, "ALERT: threshold exceeded")?;
        }
    }

    Ok(())
}

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Breaking Down the Example

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Import the Prelude

use slung::prelude::*;
The prelude includes all essential types and functions:
  • Event - Event payload structure
  • query_live() - Register live queries
  • query_history() - Execute historical queries
  • poll_handle() - Poll for new events
  • writeback_ws() - Send data to WebSocket clients
  • unix_micros() - Get current timestamp
  • write_event() - Ingest new events

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Main Entry Point

#[main]
fn main() -> Result<()> {
    // Workflow logic here
}
The #[main] macro marks your entry point. It must return std::io::Result<()>.

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Register a Live Query

let handle = query_live("AVG:temp:[sensor=1]")?;
Function signature: 
pub fn query_live(filter: &str) -> Result<QueryHandle>
This subscribes to the running average of the temp series filtered by sensor=1. The runtime returns a QueryHandle (type alias for u64).

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Poll for Events

poll_handle(handle, on_event, 100.0)?;
Function signature: 
pub fn poll_handle<F, A>(handle: QueryHandle, callback: F, args: A) -> Result<()>
where
    F: Fn(Event, A) -> Result<()>,
    A: Clone
This polls the handle forever, calling on_event for each event. The third parameter (100.0) is passed to the callback as alert_threshold.

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Process Events

fn on_event(event: Event, alert_threshold: f64) -> Result<()> {
    if event.value > alert_threshold {
        println!("event timestamp={} value={}", event.timestamp, event.value);
        for producer in event.producers {
            writeback_ws(producer, "ALERT: threshold exceeded")?;
        }
    }
    Ok(())
}
The callback receives:
  • event: Event - Contains timestamp, value, tags, and producers
  • alert_threshold: f64 - The argument passed to poll_handle()
When the value exceeds the threshold, it writes back an alert to all connected producers.

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Building Your Workflow

Compile to WebAssembly: 
cargo build --target wasm32-unknown-unknown --release
The compiled workflow will be at: 
target/wasm32-unknown-unknown/release/your_workflow.wasm

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Running Your Workflow

Load the workflow into Slung: 
slung workflow load your_workflow.wasm
The runtime will instantiate your WASM module and call your main() function.

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Error Handling

All SDK functions return std::io::Result<T>. Use ? to propagate errors: 
let handle = query_live("AVG:temp:[sensor=1]")?;
writeback_ws(producer, "message")?;
Note that:
  • query_live() returns Ok(0) on host registration failure (0 is the sentinel value)
  • query_history() returns 0.0 for both valid zero results and some failures
  • writeback_http() returns Ok(None) when there’s no response body or the request fails

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Next Steps

Live Queries

Learn about live query subscriptions and polling

Historical Queries

Query aggregated historical data