# Gearbox

Gearbox is a functional state machine with an easy-to-use API, inspired by both
[Fsm](https://github.com/sasa1977/fsm) and [Machinery](https://github.com/joaomdmoura/machinery).

Gearbox does not run in a process, so there's no potential for a GenServer bottleneck.
This way there's also less overhead as you won't need to setup a supervision tree/manage your state machine processes.

> Note: Gearbox is heavily inspired by [Machinery](https://github.com/joaomdmoura/machinery),
> and also took inspiration from [Fsm](https://github.com/sasa1977/fsm).

Gearbox is **very** similar to [Machinery](https://github.com/joaomdmoura/machinery) in term of
the API usage, however it differs in the ways below:

- **Gearbox does not use a GenServer as a backing process**.
  Since GenServer can be a potential bottleneck in a system, for that reason I think it's best
  to leave process management to users of the library.
- **No before/after callbacks**. Callback allow you to add side effects, but side effects violate
  Single Responsibility Principle, and that can bring surprises to your codebase
  (e.g: "How come everytime this transition happens, X happens?"). Gearbox nudges you to keep domain-logic
  callbacks close to your contexts/domain events. Gearbox still ships with a `guard_transition/3` callback,
  as that is intrinsic to state machines.
- Gearbox does not ship with a `Phoenix Dashboard` view.
  A really cool and great concept, but more often than not it is not needed and the added dependency
  can prove more trouble than worth.

For a more detailed documentation, checkout the [Gearbox's HexDoc](https://hexdocs.pm/gearbox).

## Installation

Get the latest version from [Hex](https://hex.pm/packages/gearbox)

```elixir
def deps do
  [
    {:gearbox, "~> 0.3.1"}
  ]
end
```

## Usage

Gearbox's main API is `Gearbox.transition/3`. There's a `bang!` variant available too.

### Example

`Gearbox.transitions(%Order{}, PaymentMachine, "paid")`

- **First Argument** - an Elixir map, can be a struct or a non-struct.
- **Second Argument** - a State Machine, read on to find out how to create a state machine.
- **Third Argument** - the desired next state.

Here's how to create a state machine:

```elixir
defmodule PaymentMachine do
  use Gearbox,
    field: :status, # used to retrieve the state of the given struct. Defaults to `:state`
    states: ~w(pending_payment paid refunded), # list of finite states in the state machine
    initial: "pending_payment", # initial state of the struct, if struct has `nil` state to begin with. Defaults to the first item of `:states`
    transitions: %{
      "pending_payment" => "paid",
      "paid" => "refunded",
    } # a map of possible transitions from `current_state` to `next_state`. `*` wildcard is allowed to indicate any states.
end
```

## Rationale

Gearbox operates on the philosophy that it acts purely as a functional state machine, wherein
it does not care where your state is store (e.g: Ecto, GenServer), all Gearbox does is to help you
ensure state transitions happen the way you expect it to.

In most cases like for example `Order`, it is very likely that you don't need a process for that.
Just get the record out of the database, run it through Gearbox machine, then persist it back to database.

In some rare cases where you need to have a stateful state machine, for example a traffic light
that has an internal timer to shift from `red` (30s) -> `green` (30s) -> `yellow` (5s) -> `red`,
you are better off to use an `Agent`/`GenServer` where you have better control over backpressuring/
business logics.

As of now, Gearbox does not provide a way to create `events/actions` in a state machine.
This is because Gearbox is not a domain/context wrapper, events and actions that can
trigger a state change should reside closer to your contexts, therefore I urge users to
group these events as domain events (contexts), rather than state machine events.

Gearbox previously shipped with `before_transition/3` and `after_transition/3` in `0.1.0`,
but after some discussions I have decided to take a deliberate decision to **remove** callbacks.
This is because callbacks by nature, allow you to add side effects, but side effects violate
**Single Responsibility Principle**, and callbacks can often bring unintended surprises
to your codebase (e.g: "How come everytime this transition happens, X happens?").

Therefore, Gearbox nudges you to keep domain/business-logic callbacks close to your contexts/domain events.
Gearbox still ships with a `guard_transition/3` callback, as that is intrinsic to state machines.

## Features
Below lists a couple of features that Gearbox currently have.

### State Transitions
The core of Gearbox. Allows you to transition a state from one to another (managed by your own machine).

```elixir
defmodule Commerce do
  def pay(user, order) do
    # ...
    # Your payment logic
    {:ok, updated_order} = Gearbox.transition(order, PaymentMachine, "paid")
    # ...
  end
end
```

There's also a `bang!` variant of transition, `Gearbox.transition!/3`, so you can rewrite your code to like so:

```elixir
defmodule Commerce do
  def pay(user, order) do
    # ...
    # Your payment logic
    order
    |> Gearbox.transition!(PaymentMachine, "paid")
    |> Repo.insert!
  end
end
```

### Guard Transitions
Guard transitions enforces a condition to be passed before a transitions is committed.

A transition is halted if the function returns `{:halt, reason}`, it continues otherwise.
The reason giving in `{:halt, reason}` will then propagate up to `Gearbox.transition/3`
as `{:error, reason}`.

```elixir
# You can add condition check on both `from` and `to` states.
def guard_transition(struct, _from, _to) do
  case :rand.uniform() do
    val when val >= 0.5 ->
      # You can return anything
    _ ->
      {:halt, "You have been snapped."}
  end
end
```

> **Note** that guard transitions will only be run if transition is valid.

### Ecto Support
If your project uses Ecto, you can use the `Gearbox.Ecto` module to create a changeset based on the outcome of the transition. The original struct will not be modified.  
A successful transition will return a changeset with the change applied, and an unsuccessful one will return a changeset with the appropriate error message.

````elixir
# similar to the above example
def pay(user, order) do
  with {:ok, changeset} <- Gearbox.Ecto.transition_changeset(order, PaymentMachine, "paid")
  do
    Repo.update(changeset)
  else
    {:error, changeset} ->
      # handle error here
  end
end
````

## Contributions
Contributions are very welcomed, but please first [open an issue](https://github.com/edisonywh/gearbox/issues/new) so we can align and discuss before any development begins.

## License

View [License](https://github.com/edisonywh/gearbox/blob/master/LICENSE)