# Grafn

Graph + Fn: Execute functions as a graph.

## What it's for?

Grafn lets you execute sequences of async functions by defining them as
vertices in a graph.

This allows you to easily define and manipulate complex dependencies in
asynchronous code.

## How can I use it?

In order to use it you must first create a graph.

```javascript
const Grafn = require('grafn');

const graph = new Grafn();
```

Next you need to add some vertexes.

```javascript
graph.vertex({
  name: 'root',
  action() {

    return 5;
  }
});
```

This is a vertex without dependencies, it means it can be executed without
waiting for anything else. The return value of this function will be stored
so it can be used by its dependents.

You can add dependencies by listing the names as an array in the vertex definition.

```javascript
graph.vertex({
  name: 'addition',
  dependencies: ['root'],
  action(state) {

    return state.root + state.root;
  }
});
```

Notice how the result of the `root` vertex is available in this action.

Vertexes will be executed as soon as all of their dependencies are met. For
example, consider the following graph

```javascript
const Grafn = require('grafn');

const graph = new Grafn();

graph.vertex({
  name: 'root',
  action() {

    return 5;
  }
});

graph.vertex({
  name: 'addition',
  dependencies: ['root'],
  action(state) {

    return state.root + state.root;
  }
});

graph.vertex({
  name: 'subtraction',
  dependencies: ['root'],
  action(state) {

    return state.root - state.root;
  }
});

graph.vertex({
  name: 'multiplication',
  dependencies: ['root'],
  action(state) {

    return state.root * state.root;
  }
});

graph.vertex({
  name: 'division',
  dependencies: ['root'],
  action(state) {

    return state.root / state.root;
  }
});

graph.vertex({
  name: 'tally',
  dependencies: ['addition', 'subtraction', 'multiplication', 'division'],
  action(state) {

    console.log('+', state.addition);
    console.log('-', state.subtraction);
    console.log('*', state.multiplication);
    console.log('/', state.division);
  }
});
```

The `addition`, `subtraction`, `multiplication`, and `division` vertices will
all run as soon as `roott` is executed. However, the final `tally` vertex won't
run until all other operations are complete.

You can visualize the graph by using the `graph.toString()` method. This
will output a graphviz digraph:

```graphviz
digraph {
    root
    addition
    root -> addition
    subtraction
    root -> subtraction
    multiplication
    root -> multiplication
    division
    root -> division
    tally
    addition -> tally
    subtraction -> tally
    multiplication -> tally
    division -> tally
}
```

![execution graph, before execution][graph-pre-exec]

You can easily change the shape of the graph by changing the dependencies array.

In order to start execution of the graph you should select which node to start
from.

```
graph.run('root');
```

After an execution, you can use `graph.toString()` to check the result of an
execution. It will highlight which nodes were executed successfully (green), and
which nodes threw an error (red).

![execution graph, after execution][graph-post-exec]

## Acknowledgements

This project implements the same ideas behind [fluorine][fluorine], a similar
graph based library. Grafn wouldn't exist without it.

[fluorine]: https://github.com/freshout-dev/fluorine
[graph-pre-exec]: doc/images/graph-pre-exec.svg
[graph-post-exec]: doc/images/graph-post-exec.svg