soypat / go-maquina

Small finite-state machine library for Go

go-maquina

Create delightfully simple finite-state machines in Go. Inspired by stateless.

If you wish to build state machines that are extremely maintainable and stand the test of time you have come to the right place.

Highlights

• Labelled everything: Label your states, triggers, guard clauses and callbacks so that...
  • You can visualize state machines as a DOT generated graph. See examples below!
  • Deep introspection into what is going on in your state machine.
• Decent performance, no allocations: see benchmark below.

Maquina is the spanish word for machine. It is pronounced maa-kee-nuh, much like machina from the Latin calque deus-ex-machina.

Benchmark

Benchmarked below is the time it takes for a transition to complete when no callbacks or guard clauses are in place.

$ go test -test.bench=. -benchmem
goos: linux
goarch: amd64
pkg: github.com/soypat/go-maquina
cpu: 12th Gen Intel(R) Core(TM) i5-12400F
BenchmarkHyper-12       31407175                37.38 ns/op            0 B/op        0 allocs/op
PASS
ok      github.com/soypat/go-maquina    2.192s

Code organization

• maquina.go contains internal logic for the state machine such as the fire() functions triggered by a state transition.

• state.go contains most of the user visible exported methods on State type.

• statemachine.go contains code relevant to the State manager StateMachine.

Toll booth example

const (
	passageCost                      = 10.00
	defaultPay                       = 0.0
	payUp            maquina.Trigger = "customer pays"
	customerAdvances maquina.Trigger = "customer advances"
)
var (
	tollClosed = maquina.NewState("toll barrier closed", defaultPay)
	tollOpen   = maquina.NewState("toll barrier open", defaultPay)
	guardPay   = maquina.NewGuard("payment check", func(ctx context.Context, pay float64) error {
		if pay < passageCost {
			// Barrier remains closed unless customer pays up
			return fmt.Errorf("customer underpaid with $%.2f", pay)
		}
		return nil
	})
)

tollClosed.Permit(payUp, tollOpen, guardPay)
tollOpen.Permit(customerAdvances, tollClosed)
SM := maquina.NewStateMachine(tollClosed)
for i := 0; i < 5; i++ {
	pay := 2 * passageCost * rand.Float64()
	err := SM.FireBg(payUp, pay)
	if err != nil {
		fmt.Println(err)
	} else {
		fmt.Printf("customer paid $%.2f, let them pass!\n", pay)
		SM.FireBg(customerAdvances, 0)
	}
}

The code above outputs:

customer paid $12.09, let them pass!
customer paid $18.81, let them pass!
customer paid $13.29, let them pass!
guard clause failed: customer underpaid with $8.75
guard clause failed: customer underpaid with $8.49

Algorithmic trading graph

The code below outputs the following DOT graph code. Note how parent/super states can be crafted. Entry/Exit callbacks will be triggered on a superstate when entering/exiting a substate from outside/within the super state.

	getStock := func() string {
		return string([]byte{byte(rand.Intn(26)) + 'A', byte(rand.Intn(26)) + 'A', byte(rand.Intn(26)) + 'A'})
	}
	type tradeState struct {
		targetStock   string
		quoteReceived time.Time
	}
	type transition = maquina.Transition[*tradeState]

	const (
		trigRequestQuote     = "request quote"
		trigExecute          = "execute"
		trigExecuteFail      = "execute failed"
		trigCancel           = "cancel"
		trigQuoteReceived    = "quote received"
		trigExecuteConfirmed = "execute confirmed"
	)
	var (
		stateWaitingOnQuote = maquina.NewState("waiting on quote", &tradeState{})
		stateReadyToOperate = maquina.NewState("ready to operate", &tradeState{})
		stateIdle           = maquina.NewState("idle", &tradeState{})
		stateExecuting      = maquina.NewState("executing", &tradeState{})
		stateCritical       = maquina.NewState("critical section", &tradeState{})

		fringeStockSelect = maquina.NewFringeCallback("stock select", func(_ context.Context, _ transition, state *tradeState) {
			state.targetStock = getStock()
		})

		fringeStockClear = maquina.NewFringeCallback("stock clear", func(_ context.Context, _ transition, state *tradeState) {
			state.targetStock = ""
		})

		guardQuoteStale = maquina.NewGuard("quote stale", func(ctx context.Context, state *tradeState) error {
			const staleQuoteTimeout = 10 * time.Minute
			elapsed := time.Since(state.quoteReceived)
			if elapsed > staleQuoteTimeout || elapsed < 1 { // Sanity check included.
				return errors.New("quote is stale: " + elapsed.String() + " elapsed")
			}
			return nil
		})
	)

	stateIdle.Permit(trigRequestQuote, stateWaitingOnQuote)
	stateIdle.OnExitThrough(trigRequestQuote, fringeStockSelect)
	stateIdle.OnEntry(fringeStockClear)

	stateWaitingOnQuote.Permit(trigCancel, stateIdle)
	stateWaitingOnQuote.Permit(trigQuoteReceived, stateReadyToOperate)

	stateReadyToOperate.Permit(trigExecute, stateExecuting, guardQuoteStale)
	stateReadyToOperate.Permit(trigCancel, stateIdle)

	stateExecuting.Permit(trigExecuteConfirmed, stateIdle)
	stateExecuting.Permit(trigExecuteFail, stateReadyToOperate)

	// Mark critical section as a superstate.
	stateCritical.LinkSubstates(stateWaitingOnQuote, stateReadyToOperate, stateExecuting)

	sm := maquina.NewStateMachine(stateIdle)
	var buf bytes.Buffer
	maquina.WriteDOT2(&buf, sm)
	fmt.Println(buf.String())

3D Printer graphviz example

The code below outputs the following DOT graph code:

type printerState struct {
	x, y, z int
}
// Declaration of triggers. These are actions.
// In the example of a 3D printer one could think of them
// as buttons exposed to the end user.
const (
	trigHome      maquina.Trigger = "home"
	trigCalibrate maquina.Trigger = "calibrate"
	trigStop      maquina.Trigger = "stop"
)
var (
	// stateSingleton contains the state of the printer at all times.
	// It is a singleton and is shared by all states.
	stateSingleton   = &printerState{}
	stateIdleHome    = maquina.NewState("idle at home", stateSingleton)
	stateIdle        = maquina.NewState("idle", stateSingleton)
	stateCalibrating = maquina.NewState("calibrating", stateSingleton)
	stateGoingHome   = maquina.NewState("going home", stateSingleton)
	// guardNotAtHome is a guard clause that checks if the printer is at home position.
	guardNotAtHome = maquina.NewGuard("not at home", func(ctx context.Context, state *printerState) error {
		if state.x != 0 || state.y != 0 || state.z != 0 {
			return fmt.Errorf("not at home")
		}
		return nil
	})
)
// Declare Calibration and Stop transitions. These would be the actions taken
// when user presses CALIBRATE or STOP button.
stateIdleHome.Permit(trigCalibrate, stateCalibrating)
stateIdle.Permit(trigCalibrate, stateCalibrating, guardNotAtHome)
// Special case of STOP while home: we stay at home.
stateIdleHome.Permit(trigStop, stateIdleHome)

// Declare home transitions. These would be the actions taken when a user presses
// the HOME button, as an example.
stateCalibrating.Permit(trigHome, stateGoingHome)
stateIdle.Permit(trigHome, stateGoingHome)
stateGoingHome.Permit(trigHome, stateIdleHome, guardNotAtHome)
sm := maquina.NewStateMachine(stateIdleHome)
// In the case of stopping we go to Idle state since we are not
// guaranteed to be at home position.
sm.AlwaysPermit(trigStop, stateIdle)
var buf bytes.Buffer
maquina.WriteDOT(&buf, sm)
fmt.Println(buf.String())
// With the code below one can also output a PNG file with the graph:
// One must have graphviz installed and in the path: `sudo apt install graphviz`
//
//  cmd := exec.Command("dot", "-Tpng", "-o ", "3dprinter.png")
//  cmd.Stdin = &buf
//  cmd.Run()

Hyper connected state diagram

A toy example of 8 states, all of them connected to illustrate capabilities of go-maquina when coupled to graphviz (code is below):

const n = 8
hyperStates := make([]maquina.State[int], n)
for i := 0; i < n; i++ {
	hyperStates[i] = *maquina.NewState("S"+strconv.Itoa(i), i)
	for j := i - 1; j >= 0; j-- {
		trigger := maquina.Trigger("T" + strconv.Itoa(i) + "→" + strconv.Itoa(j))
		hyperStates[i].Permit(trigger, &hyperStates[j])
	}
}
for i := 0; i < n; i++ {
	for j := i + 1; j < n; j++ {
		trigger := maquina.Trigger("T" + strconv.Itoa(i) + "→" + strconv.Itoa(j))
		hyperStates[i].Permit(trigger, &hyperStates[j])
	}
}
sourceState := maquina.NewState("source", 0)
sourceState.Permit("goto S0", &hyperStates[0])
failsafeState := maquina.NewState("sink failsafe", -1)
sm := maquina.NewStateMachine(sourceState)
sm.AlwaysPermit("goto failsafe", failsafeState)
var buf bytes.Buffer
maquina.WriteDOT(&buf, sm)
cmd := exec.Command("dot", "-Tpng", "-o", "hyper-states.png")
cmd.Stdin = &buf
cmd.Run()

go-maquina's Issues

add graphviz example