simple finite state machine implementation
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Codeship Status for ryanfaerman/fsm GoDoc

FSM provides a lightweight finite state machine for Golang. It runs allows any number of transition checks you'd like the it runs them in parallel. It's tested and benchmarked too.


go get


package main

import (


type Thing struct {
    State fsm.State

    // our machine cache
    machine *fsm.Machine

// Add methods to comply with the fsm.Stater interface
func (t *Thing) CurrentState() fsm.State { return t.State }
func (t *Thing) SetState(s fsm.State)    { t.State = s }

// A helpful function that lets us apply arbitrary rulesets to this
// instances state machine without reallocating the machine. While not
// required, it's something I like to have.
func (t *Thing) Apply(r *fsm.Ruleset) *fsm.Machine {
    if t.machine == nil {
        t.machine = &fsm.Machine{Subject: t}

    t.machine.Rules = r
    return t.machine

func main() {
    var err error

    some_thing := Thing{State: "pending"} // Our subject

    // Establish some rules for our FSM
    rules := fsm.Ruleset{}
    rules.AddTransition(fsm.T{"pending", "started"})
    rules.AddTransition(fsm.T{"started", "finished"})

    machine := some_thing.Apply(&rules)

    err = machine.Transition("started")

    if err != nil {


Note: FSM makes no effort to determine the default state for any ruleset. That's your job.

The Apply(r *fsm.Ruleset) *fsm.Machine method is absolutely optional. I like having it though. It solves a pretty common problem I usually have when working with permissions - some users aren't allowed to transition between certain states.

Since the rules are applied to the the subject (through the machine) I can have a simple lookup to determine the ruleset that the subject has to follow for a given user. As a result, I rarely need to use any complicated guards but I can if need be. I leave the lookup and the maintaining of independent rulesets as an exercise of the user.


Golang makes it easy enough to benchmark things... why not do a few general benchmarks?

$ go test -bench=.
BenchmarkRulesetParallelGuarding      100000   13163 ns/op
BenchmarkRulesetTransitionPermitted  1000000    1805 ns/op
BenchmarkRulesetTransitionDenied    10000000     284 ns/op
BenchmarkRulesetRuleForbids          1000000    1717 ns/op
ok 14.864s

I think that's pretty good. So why do I go through the trouble of running the guards in parallel? Consider the benchmarks below:

$ go test -bench=.
BenchmarkRulesetParallelGuarding    100000       13261 ns/op
ok 1.525s

$ go test -bench=.
BenchmarkRulesetSerialGuarding         1  1003140956 ns/op
ok 1.007s

For the Parallel vs Serial benchmarks I had a guard that slept for 1 second. While I don't imagine most guards will take that long, the point remains true. Some guards will be comparatively slow -- they'll be accessing the database or consulting with some other outside service -- and why not get an answer back as soon as possible?