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Your First Signal Handler

This guide walks through building a simple application that emits and processes NTL signals.

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What We’re Building

A basic key-value store that:
  • Accepts Command signals to store values
  • Accepts Query signals to retrieve values
  • Responds with Data signals containing results
This demonstrates the core pattern: signals in, signals out.

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Setup

cargo new ntl-kv-store
cd ntl-kv-store
cargo add ntl tokio serde serde_json

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Define Signal Handlers

use ntl::{Node, Signal, SignalType, SignalHandler};
use std::collections::HashMap;
use std::sync::Arc;
use tokio::sync::RwLock;

type Store = Arc<RwLock<HashMap<String, serde_json::Value>>>;

struct SetHandler {
    store: Store,
}

impl SignalHandler for SetHandler {
    fn signal_type(&self) -> SignalType {
        SignalType::Command
    }

    fn tags(&self) -> Vec<&str> {
        vec!["kv", "set"]
    }

    async fn handle(&self, signal: Signal) -> Result<Option<Signal>, ntl::Error> {
        let key = signal.payload.get("key")
            .and_then(|v| v.as_str())
            .ok_or(ntl::Error::InvalidPayload("missing key"))?;

        let value = signal.payload.get("value")
            .ok_or(ntl::Error::InvalidPayload("missing value"))?;

        // Store the value
        let mut store = self.store.write().await;
        store.insert(key.to_string(), value.clone());

        // Respond with acknowledgment
        Ok(Some(
            Signal::data("kv-set-ack")
                .with_correlation(signal.id)
                .with_payload(serde_json::json!({
                    "status": "ok",
                    "key": key,
                }))
                .with_weight(0.5)
        ))
    }
}

struct GetHandler {
    store: Store,
}

impl SignalHandler for GetHandler {
    fn signal_type(&self) -> SignalType {
        SignalType::Query
    }

    fn tags(&self) -> Vec<&str> {
        vec!["kv", "get"]
    }

    async fn handle(&self, signal: Signal) -> Result<Option<Signal>, ntl::Error> {
        let key = signal.payload.get("key")
            .and_then(|v| v.as_str())
            .ok_or(ntl::Error::InvalidPayload("missing key"))?;

        let store = self.store.read().await;
        let value = store.get(key).cloned();

        Ok(Some(
            Signal::data("kv-get-result")
                .with_correlation(signal.id)
                .with_payload(serde_json::json!({
                    "key": key,
                    "value": value,
                    "found": value.is_some(),
                }))
                .with_weight(0.5)
        ))
    }
}

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Wire It Together

#[tokio::main]
async fn main() -> Result<(), ntl::Error> {
    // Shared store
    let store: Store = Arc::new(RwLock::new(HashMap::new()));

    // Initialize node
    let node = Node::builder()
        .with_config_file("~/.ntl/config.toml")
        .build()
        .await?;

    // Register handlers
    node.register_handler(SetHandler { store: store.clone() }).await?;
    node.register_handler(GetHandler { store: store.clone() }).await?;

    // Announce capabilities
    Signal::discovery()
        .with_payload(serde_json::json!({
            "service": "kv-store",
            "operations": ["set", "get"],
            "tags": ["kv"],
        }))
        .with_scope(ntl::PropagationScope::Flood { max_hops: 3 })
        .emit(&node)
        .await?;

    println!("KV Store node running. Listening for signals...");

    // Keep running
    node.run_until_shutdown().await?;

    Ok(())
}

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Test It

In another terminal, use the NTL CLI: 
# Set a value
ntl emit \
  --type command \
  --tags kv,set \
  --payload '{"key": "greeting", "value": "hello from NTL"}' \
  --wait-correlation 5s

# Output:
# ✓ Signal emitted: 01HYX4A...
# ✓ Correlated response received:
#   {"status": "ok", "key": "greeting"}

# Get the value
ntl emit \
  --type query \
  --tags kv,get \
  --payload '{"key": "greeting"}' \
  --wait-correlation 5s

# Output:
# ✓ Signal emitted: 01HYX4B...
# ✓ Correlated response received:
#   {"key": "greeting", "value": "hello from NTL", "found": true}

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What Just Happened

  1. Your KV store node registered handlers for Command and Query signals tagged with kv
  2. It announced its capabilities via a Discovery signal
  3. The CLI emitted signals that propagated through the network
  4. Your handlers processed the signals and emitted correlated responses
  5. The CLI received the responses via correlation matching
No URLs. No endpoints. No API routes. Just signals flowing through the network and being processed by capable nodes.

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Key Patterns

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Handler Registration

Handlers declare what signal types and tags they process. The node’s propagation controller uses this to determine which incoming signals to route to which handlers.

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Discovery Announcement

By emitting a Discovery signal, your node tells the network what it can do. Other nodes learn about your capabilities and can route relevant signals your way.

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Correlation for Request-Response

When you need a response to a specific signal, use correlation. The emitter sets a signal ID, the handler includes that ID as correlation_id in its response. The --wait-correlation flag in the CLI demonstrates this pattern.

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Returning None

Handlers can return None to process a signal without emitting a response. Useful for event logging, metrics collection, or side effects.

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Next Steps

Building Adapters

Expose your signal handlers via HTTP

SiafuDB Integration

Persist state across signal handlers