---
description: Harmony is an abstract extension system you can use to make any software extendable and composable. It is the engine that drives Bit extensibility and composability.
labels: ['aspect', 'composition', 'extendability']
---

## Basic features

Harmony allows you to implement Aspects as the main building blocks that compose your software. Each Aspect has:

* Dependency declaration - Where you declare other Aspects as dependencies.
* Dependency injection - An Aspect gets providers for all its dependencies.
* Slots - An easy way for an Aspect to provide API for another aspect to hook into any of its processes.
* Multiple runtimes - Aspect can annotate in which runtimes it can run (for example providing functionality to run on the server and on the browser).

## Harmony config file

Use a config file to describe your desired composition of Aspects and their configuration. Harmony uses a basic JSON syntax, where each key is an aspect.

```json
{
  "teambit.dependencies/dependency-resolver": {}, // load an Aspect without providing configuration
  "teambit.workspace/workspace": {                // Load an Aspect with configuration
    "name": "bit",
    "icon": "https://static.bit.dev/bit-logo.svg",
}
```

## Define an aspect

Harmony Aspect is  an object that implements the `ExtensionManifest` interface

```js
export type ExtensionManifest = {
  /**
   * Aspect's name.
   */
  name: string;

  /**
   * Aspect unique ID.
   */
  id?: string;

  /**
   * Array of dependencies.
   * These other Aspects to be installed and resolved prior to this Aspect activation.
   */
  dependencies?: ExtensionManifest[];

  /**
   * Reference to the Aspect factory function.
   */
  provider?: ProviderFn;

  /**
   * Default Aspect configuration. 
   */
  defaultConfig?: object;

  /**
   * Alias to the provider.
   */
  provide?: ProviderFn;

  /**
   * Array of slots the Aspect exposes.
   */
  slots?: SlotProvider<unknown>[],

  /**
   * Additional keys which might be expected by other Aspects.
   */
  [key: string]: any;
}
```

## Programmatic API

### Static API

```js
// Create instance of harmony with the provided aspects, on specific runtime, with provided config
static async load(aspects: Aspect[], runtime: string, globalConfig: GlobalConfig) {
```

### Instance API

```js
// load an Aspect into the dependency graph
async load(extensions: ExtensionManifest[])

// set extensions during Harmony runtime.
async set(extensions: ExtensionManifest[])

export type RequireFn = (aspect: Extension, runtime: RuntimeDefinition) => Promise<void>;

// Start harmony. 
// The require function is a function that get's the aspect and its runtime, and knows to require/load it
async run(requireFn?: RequireFn) 
```

## Assumptions

* Harmony does not allow circular dependencies between Aspects.

<!--

-------------------------for reference only - to remove ------------------

## composition model
Harmony proposes a graph composition model.
composition model should allow:

- full control over composition including dependency composition.
- easy overrides mechanism for configs.
- full encapsulation of an extension. 

bit.config.js
```js
import FlowSchemaPlugin from '@bit/plugins.flow-schema';

export default () => {
  return [ // returns an array of PluginInstance
    [FlowSchemaPlugin, {

    }],
    [ReactSchemaPlugin, {
      eslintrc: './.eslintrc'
    }]
  ];
}
```

configured extensions in bit.json
```json
{
  "extensions": {
    "bit.envs/eslint@0.0.4": {
      "eslintrc": "./.eslintrc"
    },
    "bit.envs/babel@0.0.1": {
      "babelrc": "./.babelrc",
      "strict": true,
      "__alias": "compile"
    },
    "bit.envs/webpack@0.0.1": {
      "config": "./webpack.config.js",
      "mode": "prod",
      "__alias": "bundle"
    },
    "bit.envs/mocha@0.0.1": {
      "reporter": "json",
      "mochaOptions": "./mocha.opts",
      "__alias": "test-mocha"
    }
  },
  "pipes": {
    "tag": [""]
  },
  "dist": {
    "target": "",
    "entry": ""
  }
}
```

### Open questions
- How do I install a Bit extensions? Where the component is configured?
- Is the extension installed with Bit/NPM?
- What's the impl. behind `bit use <plugin/extension name>`
- maybe an composition model can co-exist as json and js? what are the tradeoffs?

## extension registration
```js
import { register } from 'harmony';
import { DocGen } from '@bit/bit.exts.doc-gen';

const extensions = register([DocGen]);
```

## lifecycle event invocation

```js
import { invoke } from 'harmony';

invoke('tag', ...data);
```

## lifecycle event registration
```js
import { Lifecycle } from 'harmony';

@Lifecycle(Tag);
function tag() {
  
}
```

## state / schema management
```ts
@Lifecycle(Tag)
function tag(component: Component) {
  const docs = component.get('docs'); // returns `Maybe` type?
  const docs: Docs = docs.get();
}
```

## function extension 

```js
export default function foo() {
  context.a
}
```

## configuration
an extension can declaratively ask for configuration type. this can be reflected
- extensions can be configured during instantiation.
- configuration types can be built from multiple sources

## context
- standard context can be shared between all extensions in the same instance.

## extension composition
- hook invocation from an extension
- extension dependencies? how can an extension declare a dependency as part of its execution? 
- contextual/namespaced hooks
- config api

## extension resolution
```js
import { resolve } from 'harmony';

const extension = resolve('doc-gen');
```

## Questions (?)
- can extensions be added/configured during runtime? DI
- is there a difference between extension composition to registration?
- how to make autocomplete work for hooks?
- how to avoid unintentional hook invocation (decorators?) 
- how to declare a new hook..
- schema validator
- dev experience of an extension.
- how do manage different runtime environments? should I? Why is that relevant?
- runtime capsules?
- how can one extension run from both server and client? how capsule is related?
- in which process extensions will run from? also, what will happen from the backend?
- 
-->