iii

iii - Cross-Language Backend Engine

SDK 0.3.0-alpha Breaking Change (verified 2026-02-18): The registerTrigger field is trigger_type (NOT type). HTTP triggers use trigger_type: "api" (NOT "http"). Using type: "http" causes silent 404s on all routes. This was confirmed in the tac-4-go rewrite from motia to direct iii-sdk.

SDK Note: The iii.dev docs reference @iii-dev/sdk with a Bridge class — this package does not exist on npm (404 as of 2026-02-17). Always install iii-sdk (no scope). The docs SDK uses different conventions (dot separators, trigger_type, function_path). See references/cross-language.md for translation guide.

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Core Concepts

iii has two fundamental primitives:

1. Register - Make a function callable by the engine
2. Call - Invoke any registered function regardless of language or process

Services connect to the iii engine over WebSocket. The engine handles routing, retries, observability, state, streams, cron, and HTTP API exposure.

---

Quick Start

1. Install the Engine

curl -fsSL https://install.iii.dev/iii/main/install.sh | sh
iii --version

Or run via Docker:

docker pull iiidev/iii:latest
docker run -p 3111:3111 -p 49134:49134 \
  -v ./iii-config.yaml:/app/config.yaml:ro \
  iiidev/iii:latest

2. Create Engine Config (iii-config.yaml)

modules:
  - class: modules::api::RestApiModule
    config:
      port: 3111
      host: 127.0.0.1
      default_timeout: 30000
      concurrency_request_limit: 1024
      cors:
        allowed_origins: ["*"]
        allowed_methods: [GET, POST, PUT, DELETE, OPTIONS]

  - class: modules::state::StateModule
    config:
      adapter:
        class: modules::state::adapters::KvStore
        config:
          store_method: file_based
          file_path: ./data/state_store.db

  - class: modules::shell::ExecModule
    config:
      exec:
        - bun run --enable-source-maps worker.js

See references/engine-modules.md for full module configuration including StreamModule, OtelModule, QueueModule, PubSubModule, CronModule, KvServer, LoggingModule, and BridgeClientModule.

3. Start the Engine

iii -c iii-config.yaml

4. Create a TypeScript Service

mkdir my-service && cd my-service
npm init -y
npm install iii-sdk

Set "type": "module" in package.json.

// src/worker.ts
import { init, getContext } from "iii-sdk";

const { registerFunction, registerTrigger, call, callVoid } = init(
  process.env.III_BRIDGE_URL ?? "ws://localhost:49134"
);

// Register a function callable by any connected service
const health = registerFunction({ id: "my-service::health" }, async () => {
  const { logger } = getContext();
  logger.info("Health check OK");
  return { status: 200, body: { healthy: true, timestamp: Date.now() } };
});

// Expose as HTTP endpoint via the engine's REST API module
registerTrigger({
  trigger_type: "api",
  function_id: health.id,
  config: { api_path: "health", http_method: "GET" },
});

console.log("Service started - listening for calls");

Run with: npx tsx src/worker.ts

Test: curl http://localhost:3111/health

---

Critical Rules

Always Do

• Use service-name::function-name convention for function IDs (e.g., "client::health", "data-service::transform")
• Set "type": "module" in package.json (SDK uses ESM)
• Use process.env.III_BRIDGE_URL ?? "ws://localhost:49134" for the engine address
• Start the iii engine before starting services
• Use getContext() for logging inside function handlers (provides structured OTEL logging)
• Use Promise.allSettled() when calling multiple remote functions in parallel (graceful partial failure)
• Use GET /health on port 3111 to check engine health before connecting services
• Use index (not scope) for KV Server function parameters — scope is for the State module only

Never Do

• Never hardcode ws://localhost:49134 without env var fallback
• Never use process.env in function IDs (they must be static strings)
• Never assume all services are available - handle missing services gracefully
• Never use 6-field cron expressions - iii supports 7 fields (seconds included): "*/30 * * * * * *"
• iii-sdk 0.3.0-alpha: Use trigger_type (NOT type) in registerTrigger. Use "api" (NOT "http") for HTTP triggers. Use "queue" for queue triggers, "cron" for cron triggers
• iii-sdk 0.2.0: Use type (NOT trigger_type). Use "http" for HTTP triggers
• Never import from iii-sdk/state or iii-sdk/stream for basic state operations - use sdk.call("state::set", ...) instead. Removed in 0.3.0
• Never npm install @iii-dev/sdk — this scoped package does not exist on npm (404). Use iii-sdk instead
• Never use dot separators in engine function paths — use :: (double colon). "kv_server::get" not "kv_server.get"

---

SDK Entry Points

Import	Purpose
iii-sdk	Core: init, getContext, withContext, Logger (+ types). init() returns ISdk with registerFunction, registerTrigger, call, callVoid, shutdown methods
iii-sdk/state	Advanced: Direct IState interface (get/set/delete/list/update)
iii-sdk/stream	Advanced: Direct IStream interface with groups
iii-sdk/telemetry	OpenTelemetry: initOtel, withSpan, getTracer, getMeter

For most use cases, use the core SDK and call("state::set", ...) / call("state::get", ...) for state.

---

Function Registration

const fn = registerFunction(
  {
    id: "service::action",           // Required: unique function ID
    description: "What it does",     // Optional: for discovery
    request_format: { /* schema */ },  // Optional: input schema
    response_format: { /* schema */ }, // Optional: output schema
    metadata: { version: "1.0" },    // Optional: custom metadata
  },
  async (payload) => {
    const { logger } = getContext();
    logger.info("Processing", { payload });
    return { result: "done" };       // Return value sent back to caller
  }
);

// fn.id = "service::action"
// fn.unregister() - removes the function

---

Calling Functions

// Awaitable call (returns result)
const result = await call<InputType, OutputType>("other-service::action", { data: "hello" });

// Fire-and-forget (no return value)
callVoid("log-service::audit", { event: "user_login" });

// Parallel calls with graceful failure
const [a, b] = await Promise.allSettled([
  call("service-a::process", payload),
  call("service-b::process", payload),
]);

Built-in Functions

// State management (uses 'scope')
await call("state::set", { scope: "shared", key: "VERSION", value: 1 });
const val = await call("state::get", { scope: "shared", key: "VERSION" });
const items = await call("state::list", { scope: "shared" });

// KV Server (uses 'index' — NOT 'scope')
await call("kv_server::set", { index: "default", key: "user:123", value: { name: "Alice" } });
const user = await call("kv_server::get", { index: "default", key: "user:123" });
await call("kv_server::delete", { index: "default", key: "user:123" });
const keys = await call("kv_server::list_keys_with_prefix", { prefix: "user:" });

// Queue — enqueue a message to a topic
await call("enqueue", { topic: "user.created", data: { id: "123", email: "[email protected]" } });

// Engine introspection
const functions = await call("engine::functions::list", {});
const workers = await call("engine::workers::list", {});

// Health check (HTTP, not via SDK)
// curl http://localhost:3111/health

// Graceful shutdown (new in 0.2.0)
await sdk.shutdown();  // Closes WebSocket, cleans up resources

Note: shutdown() was added in 0.2.0. Always call it during graceful process termination.

---

Triggers

HTTP Trigger (API)

Exposes a function as an HTTP endpoint on the engine's REST API (default port 3111).

// iii-sdk 0.3.0-alpha (current working — use this)
registerTrigger({
  trigger_type: "api",
  function_id: "service::handler",
  config: {
    api_path: "users/:id",           // Path params supported
    http_method: "POST",             // GET, POST, PUT, DELETE, OPTIONS
  },
});
// Accessible at: http://localhost:3111/users/123

Warning: Using type: "http" (0.2.0 syntax) with iii engine 0.4.0+ causes silent 404s on all routes. Always use trigger_type: "api" with 0.3.0-alpha SDK.

The handler receives an ApiRequest object:

registerFunction({ id: "service::handler" }, async (req: ApiRequest) => {
  const { path_params, query_params, body, headers, method } = req;
  return {
    status_code: 200,
    headers: { "Content-Type": "application/json" },
    body: { id: path_params.id, data: body },
  };
});

Cron Trigger

Supports 7-field cron expressions (seconds granularity):

registerTrigger({
  trigger_type: "cron",
  function_id: "service::cleanup",
  config: { expression: "*/30 * * * * * *" },  // Every 30 seconds
});
// Fields: seconds minutes hours day-of-month month day-of-week year

Queue Trigger

Invokes a function when a message is enqueued to a specific topic. Requires QueueModule in engine config.

const consumer = registerFunction(
  { id: "events::on-user-created" },
  async (data) => {
    console.log("User created", data);
    return { ok: true };
  }
);

registerTrigger({
  trigger_type: "queue",
  function_id: consumer.id,
  config: { topic: "user.created" },
});

// Enqueue from anywhere:
await call("enqueue", { topic: "user.created", data: { id: "123", email: "[email protected]" } });

Log Trigger

Invokes a function when log events match a level. Requires LoggingModule in engine config.

registerFunction({ id: "monitoring::on-error" }, async (logEntry) => {
  // logEntry: { trace_id, message, level, function_name, date }
  const { logger } = getContext();
  logger.info("Error captured", logEntry);
  // Send alert, store in database, etc.
});

registerTrigger({
  trigger_type: "log",
  function_id: "monitoring::on-error",
  config: { level: "error" },  // Optional: info, warn, error, debug (omit for all)
});

---

References

Need	File
Full SDK type definitions and state/stream APIs	references/api-reference.md
Engine module configuration schemas	references/engine-modules.md
Cross-language examples, OpenTelemetry, env vars	references/cross-language.md
Docker deployment and project structure	references/docker-deployment.md

---

Known Issues

• Alpha software: API may change between releases
• Port 49134 conflicts: The engine's WebSocket port is fixed at 49134; ensure nothing else uses it
• Docker networking: On Linux Docker <20.10, host.docker.internal requires explicit extra_hosts mapping
• Reconnection: SDK auto-reconnects with exponential backoff (1s initial, 30s max, infinite retries by default)
• CJS support: 0.2.0 adds CommonJS exports alongside ESM - both import and require now work

0.3.0-alpha Breaking Changes (confirmed working 2026-02-18)

These are current reality with [email protected] + iii engine 0.4.0:

• trigger_type replaces type in registerTrigger — use trigger_type: "api" (NOT type: "http")
• "api" replaces "http" as the HTTP trigger type name
• iii-sdk/state export removed — use call("state::set", ...) and call("state::get", ...)
• iii-sdk/stream renamed to iii-sdk/streams (plural)
• ISdk type no longer exported from the main entry point
• SDK transition: The docs-only @iii-dev/sdk (Bridge class, dot separators) may become the official SDK. Monitor npm for @iii-dev/sdk availability

Verified: tac-4-go project uses direct iii-sdk 0.3.0-alpha with trigger_type: "api" and all 4 HTTP endpoints + queue + cron work correctly.

---

Troubleshooting

Error	Cause	Fix
ECONNREFUSED 127.0.0.1:49134	Engine not running	Start with iii -c iii-config.yaml
Function call times out (30s)	Target service not connected	Start the service; check engine::workers::list
Cannot find module 'iii-sdk'	SDK not installed	npm install iii-sdk
HTTP endpoint returns 404	Wrong trigger field or type	With 0.3.0-alpha: use trigger_type: "api" (NOT type: "http"). Verify api_path matches URL
Cron not firing	Wrong expression format	Use 7-field format: sec min hour dom mon dow year
State returns null	Wrong scope or key	Check scope/key strings match exactly
trigger_type_not_found	SDK/engine version mismatch	0.2.0: use type. 0.3.0-alpha: use trigger_type
All routes 404 (silent)	Used type: "http" with 0.3.0-alpha SDK	Change to trigger_type: "api" — motia sends "http", iii engine 0.4.0 expects "api"
KV Server timeout	KV Server module not in config	Add modules::kv_server::KvServer to iii-config.yaml
module class not found	Wrong singular/plural path	Try stream vs streams in class path; check engine version
@iii-dev/sdk npm 404	Package not published	Use iii-sdk (no scope). @iii-dev/sdk is docs-only
Queue messages not processing	Missing queue trigger or wrong topic	Verify registerTrigger({ type: "queue", config: { topic } }) matches enqueue topic