View Source Gearbox behaviour (Gearbox v0.3.4)
Gearbox is a functional state machine with an easy-to-use API, inspired by both Fsm and 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, and also took inspiration from Fsm.
Gearbox is very similar to 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.
rationale
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.
options
Options
:field
- used to retrieve the state of the given struct. Defaults to:state
:states
- list of finite states in the state machine:initial
- initial state of the struct, if struct hasnil
state to begin with. Defaults to the first item of:states
:transitions
- a map of possible transitions fromcurrent_state
tonext_state
.*
wildcard is allowed to indicate any states.
example
Example
defmodule Gearbox.Order do
defstruct items: [], total: 0, status: nil
end
defmodule Gearbox.OrderMachine do
use Gearbox,
field: :status,
states: ~w(pending_payment cancelled paid pending_collection refunded fulfilled),
initial: "pending_payment",
transitions: %{
"pending_payment" => ~w(cancelled paid),
"paid" => ~w(pending_collection refunded),
}
end
iex> alias Gearbox.Order # Your struct
iex> alias Gearbox.OrderMachine # Your machine
iex> Gearbox.transition(%Order{}, OrderMachine, "paid")
{:ok, %Gearbox.Order{items: [], status: "paid", total: 0}}
ecto-example
Ecto Example
{:ok, order} = Gearbox.transition(%Order{status: "pending_payment"}, OrderMachine, "paid")
Repo.insert!(order)
# or even
%Order{status: "pending_payment"}
|> Gearbox.transition!(OrderMachine, "paid")
|> Repo.insert!()
Link to this section Summary
Callbacks
Add guard conditions before transitioning.
Functions
Transition a struct or map to a given state. If transition is invalid, an InvalidTransitionError
exception is raised.
Transition a struct or a map to a given state.
Checks if the transition should be made.
Mainly for internal use to support transition/3
and other methods performing transition actions.
Link to this section Types
Link to this section Callbacks
@callback guard_transition(struct :: any(), from :: state(), to :: state()) :: {:halt, any()} | any()
Add guard conditions before transitioning.
The function receives struct as the first argument, the current state as the second argument, and the desired state as the last argument.
You can guard on both from
and to
states, e.g:
- Every time %Order{} transits out of
pending
, do X - Every time %Order{} transits into
paid
, do Y
If this function returns a {:halt, reason}
, execution of the transition will halt.
Any other things will allow the transition to go through.
Note: This hook only gets triggered if the transition is valid.
Link to this section Functions
@spec transition!(struct :: struct() | map(), machine :: any(), next_state :: state()) :: struct() | map()
Transition a struct or map to a given state. If transition is invalid, an InvalidTransitionError
exception is raised.
Uses Gearbox.transition/3
under the hood.
@spec transition(struct :: struct() | map(), machine :: any(), next_state :: state()) :: {:ok, struct() | map()} | {:error, String.t()}
Transition a struct or a map to a given state.
Returns an {:ok, updated_struct_or_map}
or {:error, message}
tuple.
@spec validate_transition( struct :: struct() | map(), machine :: any(), next_state :: state() ) :: {:ok, any()} | {:error, String.t()}
Checks if the transition should be made.
Mainly for internal use to support transition/3
and other methods performing transition actions.
Returns a tuple containing the outcome:
{:ok, nil}
if it can transition or,{:error, reason}
if transition cannot be made.