> ## Documentation Index
> Fetch the complete documentation index at: https://iii.dev/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Functions

> Registering and invoking functions in your iii project.

## Register a function

Inside a worker, `worker.registerFunction(id, handler)` makes a function callable from anywhere in
the iii system. The `id` follows the `service::name` form; the handler receives the call's payload
and returns the result.

<Tabs>
  <Tab title="Node / TypeScript">
    ```typescript theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
    import { registerWorker } from "iii-sdk";

    const url = process.env.III_URL;
    if (!url) throw new Error("III_URL must be set");
    const worker = registerWorker(url);

    worker.registerFunction("math::add", async (payload: { a: number; b: number }) => {
      return { c: payload.a + payload.b };
    });
    ```
  </Tab>

  <Tab title="Python">
    ```python theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
    import os
    from iii import register_worker, InitOptions

    worker = register_worker(
        os.environ.get("III_URL"),
        InitOptions(worker_name="math-worker"),
    )

    def add_handler(payload: dict) -> dict:
        return {"c": payload["a"] + payload["b"]}

    worker.register_function("math::add", add_handler)
    ```
  </Tab>

  <Tab title="Rust">
    ```rust theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
    use iii_sdk::{InitOptions, RegisterFunction, register_worker};

    let url = std::env::var("III_URL").expect("III_URL must be set");
    let worker = register_worker(&url, InitOptions::default());

    worker.register_function("math::add", RegisterFunction::new(|input: AddInput| {
        Ok(serde_json::json!({ "c": input.a + input.b }))
    }));
    ```
  </Tab>
</Tabs>

## Triggering (Invoking) functions

A function runs when a trigger fires. The same function can be invoked from many trigger types at
once: direct CLI calls (`iii trigger`), in-process SDK calls (`worker.trigger`), or bindings to
event-source workers like iii-http, iii-cron, iii-queue, iii-state, and iii-stream. All paths leave
the handler unchanged.

The two most common ways to invoke a function directly are from worker code with `worker.trigger` or
from the terminal with the command `iii trigger`. The engine routes the call to whatever worker
registered the function; no trigger registration is involved. The `action` field controls delivery:
by default the call waits for the function to return its result, or for the configured timeout to
fire. Pass a different `TriggerAction` to change that.

<Tabs>
  <Tab title="CLI">
    ```bash theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
    iii trigger math::add a=2 b=3
    ```
  </Tab>

  <Tab title="Node / TypeScript">
    ```typescript theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
    import { TriggerAction } from "iii-sdk";

    const result = await worker.trigger({
      function_id: "math::add",
      payload: { a: 2, b: 3 },
      // action: TriggerAction.Void(),                       // fire-and-forget
      // action: TriggerAction.Enqueue({ queue: "math" }),   // route through iii-queue
    });
    ```
  </Tab>

  <Tab title="Python">
    ```python theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
    from iii import TriggerAction

    result = worker.trigger({
        "function_id": "math::add",
        "payload": {"a": 2, "b": 3},
        # "action": TriggerAction.Void(),                    # fire-and-forget
        # "action": TriggerAction.Enqueue(queue="math"),     # route through iii-queue
    })
    # result = await worker.trigger_async({...})             # awaitable form for asyncio callers
    ```
  </Tab>

  <Tab title="Rust">
    ```rust theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
    use iii_sdk::{TriggerAction, TriggerRequest};
    use serde_json::json;

    let result = worker
        .trigger(TriggerRequest {
            function_id: "math::add".into(),
            payload: json!({ "a": 2, "b": 3 }),
            action: None,
            // action: Some(TriggerAction::Void),                                  // fire-and-forget
            // action: Some(TriggerAction::Enqueue { queue: "math".to_string() }), // route through iii-queue
            timeout_ms: None,
        })
        .await?;
    ```
  </Tab>
</Tabs>

Some common actions are:

* **Default (synchronous)**. No `action` set. The call waits for the function to return its result
  or for the configured timeout to fire.
* **`TriggerAction.Void()`**. Fire-and-forget. The call returns immediately; the function still runs
  but the caller doesn't see the result.
* **`TriggerAction.Enqueue({ queue })`**. Provided by
  [iii-queue](https://workers.iii.dev/workers/iii-queue). Routes the invocation through a named
  queue with retries; the call returns once the message is enqueued.

<Note>
  Functions can also be registered (or bound) to Triggers such as an `http` request, a `cron`
  schedule, a `state` change, and so on. To bind a function to an event source, see [Triggers /
  Register a trigger](./triggers#register-a-trigger).
</Note>

<Note>
  Workers can provide their own `TriggerAction`s. Check each [worker's
  documentation](https://workers.iii.dev) for the action types it offers.
</Note>

<Note>
  In Python, every blocking method has an awaitable twin (`trigger_async`, `shutdown_async`,
  `create_channel_async`) for use inside `asyncio`. See the [Python SDK
  reference](../api-reference/sdk-python).
</Note>

## Trigger functions from the CLI

`iii trigger` is a useful development tool. Pass `--help` to any function to see its arguments and a
description of what it does:

```bash theme={"theme":{"light":"catppuccin-latte","dark":"dark-plus"}}
iii trigger function::id --help
```

Because `iii trigger` runs through the exact same code paths as the rest of the system, you can do
real work straight from the terminal: figure out what a function does, apply database modifications,
make state changes, or try any other manipulation or new code interactively. That makes it a useful
tool for development and debugging.

<Note>
  Request and response formats are what make `iii trigger <function> --help` work. Functions can carry JSON
  Schemas for their request payload and response shape. For how to create these schemas when
  registering a function, see [Creating Workers /
  Functions](../creating-workers/functions#attach-request-and-response-schemas).
</Note>

## Common functions

A handful of functions ship with the iii engine and the standard workers. You'll likely call them
from almost every iii project. They look like any function you'd register yourself and are invoked
the same way (via [`iii trigger`](./cli) or [`worker.trigger`](#triggering-invoking-functions)). The
only thing special about them is that you didn't have to register them.

### Engine functions (`engine::*`)

The engine itself registers a small set of introspection and lifecycle functions. Full request and
response schemas are in the
[engine protocol reference](../sdk-reference/engine-sdk#engine-discovery-functions).

| Function                    | What it does                                                                                                                            |
| --------------------------- | --------------------------------------------------------------------------------------------------------------------------------------- |
| `engine::functions::list`   | List every registered function. Pass `{ include_internal: true }` to include engine internals.                                          |
| `engine::workers::list`     | List every connected worker with its metrics. Pass `{ worker_id: "<uuid>" }` to look one up.                                            |
| `engine::triggers::list`    | List every registered trigger binding.                                                                                                  |
| `engine::channels::create`  | Allocate a streaming channel reader / writer pair. The SDK wraps this as `worker.createChannel()`; rarely called directly.              |
| `engine::workers::register` | Publish the calling worker's metadata (runtime, version, OS, PID, optional `description`). The SDK calls this automatically on connect. |

The engine also publishes two subscription triggers in the same family. Bind a function to one of
these to react to the registry changing:

| Trigger                       | Fires when                                |
| ----------------------------- | ----------------------------------------- |
| `engine::functions-available` | A function is registered or unregistered. |
| `engine::workers-available`   | A worker connects or disconnects.         |

### Common workers

Each of these is published by a separate worker. Function ids, payload shapes, and per-function
behaviour are in the worker's own docs at [workers.iii.dev](https://workers.iii.dev):

* **State**: KV-style state with scoped namespaces and reactive triggers on create/update/delete.
  See [iii-state](https://workers.iii.dev/workers/iii-state).
* **Stream**: Real-time push to connected clients over WebSocket. See
  [iii-stream](https://workers.iii.dev/workers/iii-stream).
* **Queue**: Durable, ordered job processing with retries, concurrency limits, and a dead-letter
  queue. See [iii-queue](https://workers.iii.dev/workers/iii-queue).
* **Pub/Sub**: Lightweight in-engine topic subscription for fan-out without durability guarantees.
  See [iii-pubsub](https://workers.iii.dev/workers/iii-pubsub).
* **Observability**: Traces, logs, metrics, alerts, sampling rules, and rollups. See
  [iii-observability](https://workers.iii.dev/workers/iii-observability).