Fsmx
A Finite-state machine implementation in Elixir, with opt-in Ecto friendliness.
Highlights:
- Plays nicely with both bare Elixir structs and Ecto changesets
- Ability to wrap transitions inside an Ecto.Multi for atomic updates
- Guides you in the right direction when it comes to side effects
Installation
Add fsmx to your list of dependencies in mix.exs:
def deps do
[
{:fsmx, "~> 0.2.0"}
]
endUsage
Simple state machine
defmodule App.StateMachine do
defstruct [:state, :data]
use Fsmx.Struct, transitions: %{
"one" => ["two", "three"],
"two" => ["three", "four"],
"three" => "four",
"four" => :*, # can transition to any state
"*" => ["five"] # can transition from any state to "five"
}
endUse it via the Fsmx.transition/2 function:
struct = %App.StateMachine{state: "one", data: nil}
Fsmx.transition(struct, "two")
# {:ok, %App.StateMachine{state: "two"}}
Fsmx.transition(struct, "four")
# {:error, "invalid transition from one to four"}Callbacks before transitions
You can implement a before_transition/3 callback to mutate the struct when before a transition happens.
You only need to pattern-match on the scenarios you want to catch. No need to add a catch-all/do-nothing function at the
end (the library already does that for you).
defmodule App.StateMachine do
# ...
def before_transition(struct, "two", _destination_state) do
{:ok, %{struct | data: %{foo: :bar}}}
end
endUsage:
struct = %App.StateMachine{state: "two", data: nil}
Fsmx.transition(struct, "three")
# {:ok, %App.StateMachine{state: "three", data: %{foo: :bar}}Validating transitions
The same before_transition/3 callback can be used to add custom validation logic, by returning an {:error, _} tuple
when needed:
defmodule App.StateMachine do
# ...
def before_transition(%{data: nil}, _initial_state, "four") do
{:error, "cannot reach state four without data"}
end
endUsage:
struct = %App.StateMachine{state: "two", data: nil}
Fsmx.transition(struct, "four")
# {:error, "cannot reach state four without data"}Decoupling logic from data
Since logic can grow a lot, and fall out of scope in your structs/schemas, it's often useful to separate all that business logic into a separate module:
defmodule App.StateMachine do
defstruct [:state]
use Fsmx.Struct, fsm: App.BusinessLogic
end
defmodule App.BusinessLogic do
use Fsmx.Fsm, transitions: %{
"one" => ["two", "three"],
"two" => ["three", "four"],
"three" => "four"
}
# callbacks go here now
def before_transition(struct, "two", _destination_state) do
{:ok, %{struct | data: %{foo: :bar}}}
end
def before_transition(%{data: nil}, _initial_state, "four") do
{:error, "cannot reach state four without data"}
end
endEcto support
Support for Ecto is built in, as long as ecto is in your mix.exs dependencies. With it, you get the ability to
define state machines using Ecto schemas, and the Fsmx.Ecto module:
defmodule App.StateMachineSchema do
use Ecto.Schema
schema "state_machine" do
field :state, :string, default: "one"
field :data, :map
end
use Fsmx.Struct, transitions: %{
"one" => ["two", "three"],
"two" => ["three", "four"],
"three" => "four"
}
endYou can then mutate your state machine in one of two ways:
1. Transition changesets
Returns a changeset that mutates the :state field (or {:error, _} if the transition is invalid).
{:ok, schema} = %App.StateMachineSchema{state: "one"} |> Repo.insert()
Fsmx.transition_changeset(schema, "two")
# #Ecto.Changeset<changes: %{state: "two"}>You can customize the changeset function, and again pattern match on specific transitions, and additional params:
defmodule App.StateMachineSchema do
# ...
# only include sent data on transitions from "one" to "two"
def transition_changeset(changeset, "one", "two", params) do
# changeset already includes a :state field change
changeset
|> cast(params, [:data])
|> validate_required([:data])
end
Usage:
{:ok, schema} = %App.StateMachineSchema{state: "one"} |> Repo.insert()
Fsmx.transition_changeset(schema, "two", %{"data"=> %{foo: :bar}})
# #Ecto.Changeset<changes: %{state: "two", data: %{foo: :bar}>2. Transition with Ecto.Multi
Note: Please read a note on side effects first. Your future self will thank you.
If a state transition is part of a larger operation, and you want to guarantee atomicity of the whole operation, you can
plug a state transition into an Ecto.Multi. The same changeset seen above will be used here:
{:ok, schema} = %App.StateMachineSchema{state: "one"} |> Repo.insert()
Ecto.Multi.new()
|> Fsmx.transition_multi(schema, "transition-id", "two", %{"data" => %{foo: :bar}})
|> Repo.transaction()When using Ecto.Multi, you also get an additional after_transition_multi/3 callback, where you can append additional
operations the resulting transaction, such as dealing with side effects (but again, please know that side effects are
tricky)
defmodule App.StateMachineSchema do
def after_transition_multi(schema, _from, "four") do
Mailer.notify_admin(schema)
|> Bamboo.deliver_later()
{:ok, nil}
end
endNote that after_transition_multi/3 callbacks still run inside the database transaction, so be careful with expensive
operations. In this example Bamboo.deliver_later/1 (from the awesome Bamboo package) doesn't spend time sending the actual email, it just spawns a task to do it asynchronously.
A note on side effects
Side effects are tricky. Database transactions are meant to guarantee atomicity, but side effects often touch beyond the database. Sending emails when a task is complete is a straight-forward example.
When you run side effects within an Ecto.Multi you need to be aware that, should the transaction later be rolled
back, there's no way to un-send that email.
If the side effect is the last operation within your Ecto.Multi, you're probably 99% fine, which works for a lot of cases.
But if you have more complex transactions, or if you do need 99.9999% consistency guarantees (because, let's face
it, 100% is a pipe dream), then this simple library might not be for you.
Consider looking at Sage, for instance.
# this is *probably* fine
Ecto.Multi.new()
|> Fsmx.transition_multi(schema, "transition-id", "two", %{"data" => %{foo: :bar}})
|> Repo.transaction()
# this is dangerous, because your transition callback
# will run before the whole database transaction has run
Ecto.Multi.new()
|> Fsmx.transition_multi(schema, "transition-id", "two", %{"data" => %{foo: :bar}})
|> Ecto.Multi.update(:update, a_very_unreliable_changeset())
|> Repo.transaction()Contributing
Feel free to contribute. Either by opening an issue, a Pull Request, or contacting the team directly
If you found a bug, please open an issue. You can also open a PR for bugs or new features. PRs will be reviewed and subject to our style guide and linters.
About
Fsmx is maintained by Subvisual.
