Architecture

System Overview

iii uses a centralized engine + distributed worker architecture connected via WebSocket. The engine coordinates routing and discovery, while workers execute functions and manage trigger types.

The engine is a single Rust process. Workers are separate processes in any language (Node.js, Python, Rust, etc.) that connect via WebSocket.

The Engine

The engine is the central orchestration hub written in Rust. It manages:

WebSocket connections from workers
Function registry for discovering and routing calls
Trigger registry for event-to-function mappings
Worker registry for tracking connected workers
Modules for built-in functionality (HTTP API, Queue, Cron, etc.)
Invocation handling for request/response coordination

Engine Structure

pub struct Engine {
    pub worker_registry: Arc<WorkerRegistry>,
    pub functions: Arc<FunctionsRegistry>,
    pub trigger_registry: Arc<TriggerRegistry>,
    pub service_registry: Arc<ServicesRegistry>,
    pub invocations: Arc<InvocationHandler>,
    pub channel_manager: Arc<ChannelManager>,
}

All registries use thread-safe concurrent data structures (DashMap, Arc<RwLock<T>>) for lock-free access.

Default Ports

Port	Service
49134	WebSocket (worker connections)
3111	HTTP API
3112	Stream API (WebSocket)
9464	Prometheus metrics

Starting the Engine

# Default configuration
iii

# With custom config
iii --config config.yaml

# Custom WebSocket port
iii --port 50000

Workers

Workers are processes that connect to the engine and provide functions. Workers can be written in any language with a iii SDK.

Worker Capabilities

Register functions: Make code executable by the engine
Register trigger types: Declare support for event sources (http, cron, etc.)
Register triggers: Connect event sources to functions
Invoke functions: Call other functions across the system
Stream telemetry: Send OpenTelemetry traces, metrics, and logs

Worker Lifecycle

Worker Implementation

import { init } from 'iii-sdk';

const iii = init('ws://localhost:49134');

// Register capabilities
iii.registerFunction({ id: 'hello' }, async (input) => {
  return { message: `Hello, ${input.name}!` };
});

iii.registerTrigger({
  type: 'http',
  function_id: 'hello',
  config: { api_path: 'hello', http_method: 'POST' }
});

// SDK handles connection, reconnection, heartbeat
await iii.connect();

WebSocket Protocol

Workers and the engine communicate via JSON messages over WebSocket. The protocol is bidirectional and async.

Message Types

Message	Direction	Purpose
`WorkerRegistered`	Engine → Worker	Confirm connection, provide worker ID
`RegisterFunction`	Worker → Engine	Register a callable function
`UnregisterFunction`	Worker → Engine	Remove a function
`RegisterTriggerType`	Worker → Engine	Declare trigger type support
`RegisterTrigger`	Worker → Engine	Create a trigger instance
`UnregisterTrigger`	Worker → Engine	Remove a trigger
`TriggerRegistrationResult`	Engine → Worker	Confirm trigger registration
`InvokeFunction`	Bidirectional	Request function execution
`InvocationResult`	Bidirectional	Return function result
`RegisterService`	Worker → Engine	Register a logical service grouping
`Ping` / `Pong`	Bidirectional	Keep-alive heartbeat

Example Protocol Flow

Protocol Message Schemas

RegisterFunction

{
  "type": "registerfunction",
  "id": "users.create",
  "description": "Create a new user",
  "request_format": {
    "email": {"type": "string"},
    "name": {"type": "string"}
  },
  "response_format": {
    "userId": {"type": "string"}
  },
  "metadata": null,
  "invocation": null
}

InvokeFunction

{
  "type": "invokefunction",
  "invocation_id": "550e8400-e29b-41d4-a716-446655440000",
  "function_id": "users.create",
  "data": {
    "email": "[email protected]",
    "name": "John Doe"
  },
  "traceparent": "00-abc123...-def456...-01",
  "baggage": "key1=value1,key2=value2"
}

Note: invocation_id is optional. Omit for fire-and-forget calls.

InvocationResult

{
  "type": "invocationresult",
  "invocation_id": "550e8400-e29b-41d4-a716-446655440000",
  "function_id": "users.create",
  "result": {
    "userId": "user-123"
  },
  "error": null,
  "traceparent": "00-abc123...-def456...-01",
  "baggage": null
}

Either result or error will be set, not both.

RegisterTrigger

{
  "type": "registertrigger",
  "id": "trigger-uuid",
  "trigger_type": "http",
  "function_id": "users.create",
  "config": {
    "api_path": "users",
    "http_method": "POST"
  }
}

Binary Protocol Extensions

In addition to JSON messages, iii supports binary WebSocket frames for high-performance telemetry:

OTLP prefix (OTLP): OpenTelemetry trace spans
MTRC prefix (MTRC): OpenTelemetry metrics
LOGS prefix (LOGS): OpenTelemetry logs

These prefixes allow SDKs to stream telemetry without JSON serialization overhead.

Invocation Flow

When a function is invoked, the engine coordinates the request/response lifecycle:

Same-Worker Invocation

Cross-Worker Invocation

Fire-and-Forget

Omit invocation_id for async calls that don’t need a response:

// No await, no response
iii.call('notifications.send', { userId: '123' });

The engine routes the call but doesn’t track the invocation.

Modules

Modules are built-in engine plugins that provide functionality. They run inside the engine process and can register functions, trigger types, and services.

Default Modules

Module	Class	Purpose
HTTP	`RestApiModule`	HTTP API server, maps routes to functions
Queue	`QueueModule`	Redis-backed pub/sub queue
Cron	`CronModule`	Distributed cron scheduling
Stream	`StreamModule`	Real-time WebSocket streaming
Observability	`OtelModule`	Telemetry collection and export
Shell	`ExecModule`	File watcher for dev workflows

Module Configuration

Modules are configured in config.yaml:

modules:
  - class: modules::rest_api::RestApiModule
    config:
      host: 0.0.0.0
      port: 3111
      
  - class: modules::queue::QueueModule
    config:
      adapter:
        class: adapters::redis::RedisQueueAdapter
        config:
          url: redis://localhost:6379
          
  - class: modules::cron::CronModule
    config:
      enabled: true

Custom Modules

You can create custom modules in Rust:

use iii::modules::module::Module;

#[derive(Clone)]
pub struct MyModule {
    engine: Arc<Engine>,
}

#[async_trait]
impl Module for MyModule {
    fn name(&self) -> &'static str {
        "MyModule"
    }

    async fn create(engine: Arc<Engine>, config: Option<Value>) 
        -> anyhow::Result<Box<dyn Module>> 
    {
        Ok(Box::new(Self { engine }))
    }

    async fn initialize(&self) -> anyhow::Result<()> {
        // Register functions, trigger types, etc.
        Ok(())
    }
}

See examples/custom_queue_adapter.rs for a complete example.

Observability

iii has built-in observability using OpenTelemetry:

Distributed Tracing

All function invocations are traced with parent-child span relationships:

HTTP POST /users
  └─ users.create [Worker 1]
      ├─ users.validate [Worker 1]
      └─ users.store [Worker 2]
          └─ database.insert [Worker 2]

Traces include:

traceparent: W3C Trace Context propagation
baggage: Cross-cutting context metadata
Automatic span creation and linking

Metrics

Engine metrics exported on :9464/metrics (Prometheus format):

iii_workers_active: Current worker count
iii_workers_spawns_total: Total workers connected
iii_functions_registered_total: Total functions registered
iii_invocations_total: Function invocation count
iii_invocation_duration_seconds: Invocation latency histogram

Logs

Structured logging with tracing:

2026-03-03T12:00:00Z INFO [REGISTERED] Function users.create
2026-03-03T12:00:01Z INFO Worker 550e8400 connected
2026-03-03T12:00:05Z DEBUG Invoking function users.create

Scalability Considerations

Horizontal Workers

Add more workers to scale function execution capacity

Single Engine

Engine is single-process, vertically scaled (multi-threaded Rust)

Function Routing

O(1) hash lookup, no coordination overhead

WebSocket Limits

Tested with 10,000+ concurrent worker connections

The engine is currently single-instance. For HA deployments, use external load balancers with session affinity, or run multiple isolated engine instances with separate worker pools.

Security

Network Security

Engine binds to 127.0.0.1 by default (localhost only)
Configure host: 0.0.0.0 to accept external connections
Use TLS-terminating reverse proxy (Caddy, nginx) for production

Worker Authentication

Currently, worker connections are unauthenticated. For production:

Run engine behind a firewall
Use VPN or private networking
Implement custom auth in modules

Function Invocation Security

HTTP functions support auth configurations (Bearer, API key)
External functions can use auth field for credentials
Internal function calls are trusted (no auth)

Next Steps

Functions

Learn about registering and invoking functions

Triggers

Connect event sources to functions

Configuration

Configure modules and engine settings

Development

Set up a local development environment

Get Started

Core Concepts

Modules

SDKs

Deployment

Advanced

​System Overview

​The Engine

​Engine Structure

​Default Ports

​Starting the Engine

​Workers

​Worker Capabilities

​Worker Lifecycle

​Worker Implementation

​WebSocket Protocol

​Message Types

​Example Protocol Flow

​Protocol Message Schemas

​Binary Protocol Extensions

​Invocation Flow

​Same-Worker Invocation

​Cross-Worker Invocation

​Fire-and-Forget

​Modules

​Default Modules

​Module Configuration

​Custom Modules

​Observability

​Distributed Tracing

​Metrics

​Logs

​Scalability Considerations

Horizontal Workers

Single Engine

Function Routing

WebSocket Limits

​Security

​Network Security

​Worker Authentication

​Function Invocation Security

​Next Steps

Functions

Triggers

Configuration

Development

Build docs developers (and LLMs) love

System Overview

The Engine

Engine Structure

Default Ports

Starting the Engine

Workers

Worker Capabilities

Worker Lifecycle

Worker Implementation

WebSocket Protocol

Message Types

Example Protocol Flow

Protocol Message Schemas

Binary Protocol Extensions

Invocation Flow

Same-Worker Invocation

Cross-Worker Invocation

Fire-and-Forget

Modules

Default Modules

Module Configuration

Custom Modules

Observability

Distributed Tracing

Metrics

Logs

Scalability Considerations

Security

Network Security

Worker Authentication

Function Invocation Security

Next Steps