Developing Custom Modules - Iii

Custom modules in the iii Engine allow developers to extend the core functionality of the system. A module acts as a container for logic that can register functions, triggers, and integrate with external systems.

Introduction

Modules are dynamically loaded and configured, often utilizing an Adapter pattern to allow for swappable backend implementations (e.g., swapping an in-memory event bus for a Redis-backed one).

The engine provides a trait-based system where modules implement the CoreModule trait for lifecycle management and the ConfigurableModule trait for handling configuration and adapter injection.

mermaid

graph TD
    Engine[iii Engine] -->|Loads| Module[Custom Module]
    Module -->|Uses| Adapter[Adapter Interface]
    Adapter -.->|implements| Redis[Redis Adapter]
    Adapter -.->|implements| Memory[In-Memory Adapter]
    Adapter -.->|implements| Custom[Custom Adapter]

    Module -->|Registers| Functions[Functions]
    Module -->|Registers| Triggers[Triggers]

Module Architecture

The module system is built around two primary traits: CoreModule and ConfigurableModule.

Core Traits

Trait	Description	Key Methods
CoreModule	The base trait for all modules. Handles lifecycle, identification, and function registration.	`name()`, `create()`, `initialize()`, `register_functions()`, `start_background_tasks()`, `destroy()`
ConfigurableModule	Extends `CoreModule` to support typed configuration and pluggable adapters.	`build()`, `registry()`, `adapter_class_from_config()`

Lifecycle Flow

The following diagram illustrates the lifecycle of a module from creation to initialization.

mermaid

sequenceDiagram
    participant Engine
    participant Builder
    participant Module
    participant Adapter

    Note over Builder, Module: Module Registration Phase
    Builder->>Module: create(engine, config)

    alt is ConfigurableModule
        Module->>Module: create_with_adapters(engine, config)
        Module->>Adapter: factory(engine, config)
        Adapter-->>Module: Arc<AdapterInstance>
        Module->>Module: build(engine, config, adapter)
    end

    Builder->>Module: initialize()
    activate Module
    Module->>Engine: register_trigger_type() (Optional)
    Module-->>Builder: Result
    deactivate Module

    Builder->>Module: register_functions(engine)
    activate Module
    Module->>Engine: register_function()
    deactivate Module

Implementing a Configurable Module

Developing a custom module typically involves defining an adapter interface, implementing specific adapters, and then wrapping them in a module structure.

Step 1: Define the Adapter Trait

Define an async_trait that specifies the behavior your module's backend must implement. This allows users to switch implementations via configuration.

rust

use async_trait::async_trait;
use serde_json::Value;

#[async_trait]
pub trait CustomEventAdapter: Send + Sync + 'static {
    async fn emit(&self, topic: &str, event_data: Value);
    async fn subscribe(&self, topic: &str, id: &str, function_id: &str);
    async fn unsubscribe(&self, topic: &str, id: &str);
}

Why async_trait? Rust's async traits require this macro to handle the complexity of async function pointers.

Step 2: Implement Adapter Registration

To make adapters discoverable by the configuration system, you must define a registration struct and use the inventory crate.

rust

use std::sync::Arc;
use std::future::Future;
use std::pin::Pin;
use iii::Engine;

pub type CustomEventAdapterFuture = Pin<
    Box<dyn Future<Output = anyhow::Result<Arc<dyn CustomEventAdapter>>> + Send>
>;

pub struct CustomEventAdapterRegistration {
    pub class: &'static str,
    pub factory: fn(Arc<Engine>, Option<Value>) -> CustomEventAdapterFuture,
}

// Implement AdapterRegistrationEntry trait
impl AdapterRegistrationEntry<dyn CustomEventAdapter> for CustomEventAdapterRegistration {
    fn class(&self) -> &'static str {
        self.class
    }

    fn factory(&self) -> fn(Arc<Engine>, Option<Value>) -> CustomEventAdapterFuture {
        self.factory
    }
}

// Register the type with inventory
inventory::collect!(CustomEventAdapterRegistration);

Purpose: This registration system allows the engine to discover and instantiate adapters dynamically based on configuration.

Step 3: Create Adapter Factories

Define factory functions that instantiate your specific adapter implementations (e.g., InMemory or Logging).

rust

use iii::register_adapter;

fn make_inmemory_adapter(
    engine: Arc<Engine>,
    config: Option<Value>
) -> CustomEventAdapterFuture {
    Box::pin(async move {
        Ok(Arc::new(InMemoryEventAdapter::new(config, engine).await?)
            as Arc<dyn CustomEventAdapter>)
    })
}

// Register the specific adapter implementation
register_adapter!(
    <CustomEventAdapterRegistration>
    "my::InMemoryEventAdapter",
    make_inmemory_adapter
);

Step 4: Implement Adapter Logic

Create the actual adapter implementations.

<AccordionGroup> <Accordion title="In-Memory Adapter" icon="memory-stick"> Simple in-memory implementation for development and testing.

```rust
use std::collections::HashMap;
use tokio::sync::RwLock;

pub struct InMemoryEventAdapter {
    subscriptions: Arc<RwLock<HashMap<String, HashMap<String, String>>>>,
    engine: Arc<Engine>,
}

impl InMemoryEventAdapter {
    pub async fn new(
        _config: Option<Value>,
        engine: Arc<Engine>
    ) -> anyhow::Result<Self> {
        Ok(Self {
            subscriptions: Arc::new(RwLock::new(HashMap::new())),
            engine,
        })
    }
}

#[async_trait]
impl CustomEventAdapter for InMemoryEventAdapter {
    async fn emit(&self, topic: &str, event_data: Value) {
        let subs = self.subscriptions.read().await;
        if let Some(by_id) = subs.get(topic) {
            for function_id in by_id.values() {
                let _ = self.engine.call(function_id, event_data.clone()).await;
            }
        }
    }

    async fn subscribe(&self, topic: &str, id: &str, function_id: &str) {
        let mut subs = self.subscriptions.write().await;
        subs.entry(topic.to_string())
            .or_insert_with(HashMap::new)
            .insert(id.to_string(), function_id.to_string());
    }

    async fn unsubscribe(&self, topic: &str, id: &str) {
        let mut subs = self.subscriptions.write().await;
        if let Some(by_id) = subs.get_mut(topic) {
            by_id.remove(id);
        }
    }
}
```

</Accordion> <Accordion title="Logging Wrapper Adapter" icon="file-text"> Wrapper adapter that logs all events while delegating to another adapter.

```rust
pub struct LoggingEventAdapter {
    inner: Arc<dyn CustomEventAdapter>,
}

#[async_trait]
impl CustomEventAdapter for LoggingEventAdapter {
    async fn emit(&self, topic: &str, event_data: Value) {
        tracing::info!(
            topic = %topic,
            event_data = %event_data,
            "Emitting event"
        );
        self.inner.emit(topic, event_data).await;
    }

    async fn subscribe(&self, topic: &str, id: &str, function_id: &str) {
        tracing::info!(topic = %topic, "Subscribing to topic");
        self.inner.subscribe(topic, id, function_id).await;
    }

    async fn unsubscribe(&self, topic: &str, id: &str) {
        tracing::info!(topic = %topic, "Unsubscribing from topic");
        self.inner.unsubscribe(topic, id).await;
    }
}
```

</Accordion> </AccordionGroup>

Step 5: Implement the Module Logic

The module struct holds the Engine reference and the injected Adapter.

rust

use serde::Deserialize;
use once_cell::sync::Lazy;
use tokio::sync::RwLock;

#[derive(Debug, Clone, Deserialize, Default)]
#[serde(deny_unknown_fields)]
pub struct CustomEventModuleConfig {
    #[serde(default)]
    pub adapter: Option<AdapterEntry>,
}

#[derive(Clone)]
pub struct CustomEventModule {
    adapter: Arc<dyn CustomEventAdapter>,
    engine: Arc<Engine>,
    _config: CustomEventModuleConfig,
}

#[async_trait]
impl ConfigurableModule for CustomEventModule {
    type Config = CustomEventModuleConfig;
    type Adapter = dyn CustomEventAdapter;
    type AdapterRegistration = CustomEventAdapterRegistration;
    const DEFAULT_ADAPTER_CLASS: &'static str = "my::InMemoryEventAdapter";

    // Define how to access the registry
    async fn registry() -> &'static RwLock<HashMap<String, AdapterFactory<Self::Adapter>>> {
        static REGISTRY: Lazy<RwLock<HashMap<String, AdapterFactory<dyn CustomEventAdapter>>>> =
            Lazy::new(|| RwLock::new(CustomEventModule::build_registry()));
        &REGISTRY
    }

    // Builder method
    fn build(
        engine: Arc<Engine>,
        config: Self::Config,
        adapter: Arc<Self::Adapter>
    ) -> Self {
        Self {
            engine,
            _config: config,
            adapter
        }
    }
}

Registering Functions

Modules expose functionality to the engine (and thus to workers) by registering functions. This is typically done in the initialize method or register_functions.

Registration Request Structure

When registering a function, you must provide a RegisterFunctionRequest.

Field	Type	Description
`function_id`	`String`	Unique function ID (e.g., "custom::emit")
`description`	`Option<String>`	Human-readable description of the function
`request_format`	`Option<Value>`	JSON Schema defining the expected input
`response_format`	`Option<Value>`	JSON Schema defining the expected output

Example Registration

rust

use iii::RegisterFunctionRequest;

#[async_trait]
impl CoreModule for CustomEventModule {
    fn name(&self) -> &str {
        "custom_event"
    }

    async fn initialize(&self) -> anyhow::Result<()> {
        self.engine.register_function(
            RegisterFunctionRequest {
                function_id: "custom::emit".to_string(),
                description: Some("Emit a custom event".to_string()),
                request_format: Some(serde_json::json!({
                    "type": "object",
                    "properties": {
                        "topic": { "type": "string" },
                        "data": { "type": "object" }
                    },
                    "required": ["topic", "data"]
                })),
                response_format: None,
                metadata: None,
            },
            Box::new(self.clone()), // The handler
        );
        Ok(())
    }

    async fn register_functions(&self, _engine: Arc<Engine>) -> anyhow::Result<()> {
        // Additional function registrations can go here
        Ok(())
    }
}

Handling Function Invocations

To handle invocations, the module (or a specific handler struct) must implement the FunctionHandler trait.

rust

use iii::{FunctionHandler, FunctionResult};

#[async_trait]
impl FunctionHandler for CustomEventModule {
    async fn handle(&self, input: Value) -> FunctionResult {
        // 1. Parse Input
        let topic = input.get("topic")
            .and_then(|v| v.as_str())
            .ok_or_else(|| anyhow::anyhow!("Missing 'topic' field"))?;

        let data = input.get("data")
            .cloned()
            .unwrap_or(Value::Null);

        // 2. Execute Logic (using the adapter)
        self.adapter.emit(topic, data).await;

        // 3. Return Result
        FunctionResult::Success(None)
    }
}

Registering Triggers

Modules can also act as sources of events by registering TriggerTypes. This allows the engine to route external events (like Cron ticks or HTTP requests) to specific functions.

Trigger Architecture

mermaid

graph TD
    Module[Core Module] -->|Registers| TT[TriggerType]
    TT -->|Contains| Registrator[TriggerRegistrator]

    Worker -->|Sends registertrigger| Engine
    Engine -->|Delegates to| Registrator
    Registrator -->|Stores| TriggerDefinition

    ExternalEvent[External Event e.g., Timer/HTTP] --> Module
    Module -->|Look up| TriggerDefinition
    Module -->|Invoke| Engine
    Engine -->|Route to| Worker

Implementation

To support triggers, a module implements TriggerRegistrator.

rust

use iii::{TriggerRegistrator, Trigger, TriggerType};
use std::future::Future;
use std::pin::Pin;

impl TriggerRegistrator for CustomEventModule {
    fn register_trigger(
        &self,
        trigger: Trigger,
    ) -> Pin<Box<dyn Future<Output = Result<(), anyhow::Error>> + Send + '_>> {
        Box::pin(async move {
            // Extract configuration
            let config = trigger.config;
            let subscribes = config.get("subscribes")
                .and_then(|v| v.as_array())
                .ok_or_else(|| anyhow::anyhow!("Missing 'subscribes' array"))?;

            // Subscribe to each topic
            for topic in subscribes {
                let topic_str = topic.as_str()
                    .ok_or_else(|| anyhow::anyhow!("Invalid topic"))?;

                self.adapter.subscribe(
                    topic_str,
                    &trigger.id,
                    &trigger.function_id
                ).await;
            }

            Ok(())
        })
    }
}

Then, register the trigger type during initialization:

rust

async fn initialize(&self) -> anyhow::Result<()> {
    // Register functions
    // ... (function registration code)

    // Register trigger type
    let trigger_type = TriggerType {
        id: "event".to_string(),
        registrator: Box::new(self.clone()),
        description: Some("Event-based trigger".to_string()),
    };

    self.engine.register_trigger_type(trigger_type).await?;

    Ok(())
}

Configuration

Modules are configured via iii-config.yaml or JSON passed during initialization. The ConfigurableModule trait maps this configuration to a Rust struct.

Configuration Struct

rust

#[derive(Debug, Clone, Deserialize, Default)]
#[serde(deny_unknown_fields)]
pub struct CustomEventModuleConfig {
    #[serde(default)]
    pub adapter: Option<AdapterEntry>,
}

Usage in Config File

yaml

modules:
  - class: my::CustomEventModule
    config:
      adapter:
        class: my::LoggingEventAdapter
        config:
          inner_adapter: my::InMemoryEventAdapter

Nested Adapters: The logging adapter wraps the in-memory adapter, creating a decorator pattern for cross-cutting concerns.

Complete Example

Here's a complete custom module implementation:

examples/custom_event_module.rs

rust

use async_trait::async_trait;
use serde::Deserialize;
use serde_json::Value;
use std::collections::HashMap;
use std::sync::Arc;
use tokio::sync::RwLock;
use iii::{
    Engine, CoreModule, ConfigurableModule, FunctionHandler,
    FunctionResult, RegisterFunctionRequest, TriggerRegistrator,
    Trigger, TriggerType, AdapterEntry
};

// 1. Define Adapter Trait
#[async_trait]
pub trait CustomEventAdapter: Send + Sync + 'static {
    async fn emit(&self, topic: &str, event_data: Value);
    async fn subscribe(&self, topic: &str, id: &str, function_id: &str);
    async fn unsubscribe(&self, topic: &str, id: &str);
}

// 2. Implement In-Memory Adapter
pub struct InMemoryEventAdapter {
    subscriptions: Arc<RwLock<HashMap<String, HashMap<String, String>>>>,
    engine: Arc<Engine>,
}

impl InMemoryEventAdapter {
    pub async fn new(_config: Option<Value>, engine: Arc<Engine>) -> anyhow::Result<Self> {
        Ok(Self {
            subscriptions: Arc::new(RwLock::new(HashMap::new())),
            engine,
        })
    }
}

#[async_trait]
impl CustomEventAdapter for InMemoryEventAdapter {
    async fn emit(&self, topic: &str, event_data: Value) {
        let subs = self.subscriptions.read().await;
        if let Some(by_id) = subs.get(topic) {
            for function_id in by_id.values() {
                let _ = self.engine.call(function_id, event_data.clone()).await;
            }
        }
    }

    async fn subscribe(&self, topic: &str, id: &str, function_id: &str) {
        let mut subs = self.subscriptions.write().await;
        subs.entry(topic.to_string())
            .or_insert_with(HashMap::new)
            .insert(id.to_string(), function_id.to_string());
    }

    async fn unsubscribe(&self, topic: &str, id: &str) {
        let mut subs = self.subscriptions.write().await;
        if let Some(by_id) = subs.get_mut(topic) {
            by_id.remove(id);
        }
    }
}

// 3. Define Module Configuration
#[derive(Debug, Clone, Deserialize, Default)]
#[serde(deny_unknown_fields)]
pub struct CustomEventModuleConfig {
    #[serde(default)]
    pub adapter: Option<AdapterEntry>,
}

// 4. Implement Module
#[derive(Clone)]
pub struct CustomEventModule {
    adapter: Arc<dyn CustomEventAdapter>,
    engine: Arc<Engine>,
    _config: CustomEventModuleConfig,
}

#[async_trait]
impl CoreModule for CustomEventModule {
    fn name(&self) -> &str {
        "custom_event"
    }

    async fn initialize(&self) -> anyhow::Result<()> {
        // Register emit function
        self.engine.register_function(
            RegisterFunctionRequest {
                function_id: "custom::emit".to_string(),
                description: Some("Emit a custom event".to_string()),
                request_format: Some(serde_json::json!({
                    "type": "object",
                    "properties": {
                        "topic": { "type": "string" },
                        "data": { "type": "object" }
                    },
                    "required": ["topic", "data"]
                })),
                response_format: None,
                metadata: None,
            },
            Box::new(self.clone()),
        );

        // Register trigger type
        let trigger_type = TriggerType {
            id: "event".to_string(),
            registrator: Box::new(self.clone()),
            description: Some("Event-based trigger".to_string()),
        };
        self.engine.register_trigger_type(trigger_type).await?;

        Ok(())
    }
}

#[async_trait]
impl FunctionHandler for CustomEventModule {
    async fn handle(&self, input: Value) -> FunctionResult {
        let topic = input.get("topic").and_then(|v| v.as_str()).unwrap_or("");
        let data = input.get("data").cloned().unwrap_or(Value::Null);

        self.adapter.emit(topic, data).await;
        FunctionResult::Success(None)
    }
}

impl TriggerRegistrator for CustomEventModule {
    fn register_trigger(
        &self,
        trigger: Trigger,
    ) -> std::pin::Pin<Box<dyn std::future::Future<Output = Result<(), anyhow::Error>> + Send + '_>> {
        Box::pin(async move {
            let config = trigger.config;
            let subscribes = config.get("subscribes")
                .and_then(|v| v.as_array())
                .ok_or_else(|| anyhow::anyhow!("Missing 'subscribes' array"))?;

            for topic in subscribes {
                let topic_str = topic.as_str()
                    .ok_or_else(|| anyhow::anyhow!("Invalid topic"))?;
                self.adapter.subscribe(topic_str, &trigger.id, &trigger.function_id).await;
            }

            Ok(())
        })
    }
}

iii-config.yaml

yaml

modules:
  - class: my::CustomEventModule
    config:
      adapter:
        class: my::InMemoryEventAdapter

Best Practices

<AccordionGroup> <Accordion title="Use Adapter Pattern"> Always use adapters for external integrations to allow swapping implementations.

```rust
// Good: Adapter-based design
pub trait StorageAdapter {
    async fn save(&self, key: &str, value: Value);
}

// Avoid: Hard-coded implementation
pub struct Module {
    redis: RedisClient,  // Tightly coupled
}
```

</Accordion> <Accordion title="Implement Graceful Shutdown"> Handle cleanup in the module's drop implementation or provide shutdown hooks.

```rust
impl Drop for CustomEventModule {
    fn drop(&mut self) {
        // Clean up resources
        tracing::info!("Shutting down CustomEventModule");
    }
}
```

</Accordion> <Accordion title="Provide JSON Schemas"> Always define request and response formats for functions to enable validation and documentation.

```rust
request_format: Some(serde_json::json!({
    "type": "object",
    "properties": {
        "email": { "type": "string", "format": "email" },
        "age": { "type": "number", "minimum": 0 }
    },
    "required": ["email"]
}))
```

</Accordion> <Accordion title="Use Structured Logging"> Use the `tracing` crate for structured logging with context.

```rust
tracing::info!(
    topic = %topic,
    subscriber_count = by_id.len(),
    "Emitting event to subscribers"
);
```

</Accordion> </AccordionGroup>

Next Steps

<CardGroup cols={2}> <Card title="Core Modules" href="/modules" icon="cubes"> Explore built-in core modules </Card> <Card title="Adapters" href="/advanced/adapters" icon="gear"> Learn more about the adapter pattern </Card> </CardGroup>