mooremachine

mooremachine

Short version

This is a framework for organising your async node.js code as Moore Finite State Machines. FSMs can be easier to reason about and debug than implicit state kept in callbacks and objects, leading to more correct code.

License and contributing

MPL v2.0

Contributions should be made through Gerrit -- see CONTRIBUTING.md.

Introduction

It's widely known that if you want to sequence some series of asynchronous actions in node, you should use a library like vasync or async to do so -- they let you define a series of callbacks to be run in order, like this:

async.series([
    function (cb) { thing.once('something', cb) },
    function (cb) { anotherthing.get(key, function () { cb(null, blah); }); }
]);

This lets you define sequential actions with asynchronous functions. However, if you need more complex logic within this structure, this becomes rapidly limiting -- it is difficult, for example, to create a loop. You have to improvise one by nesting some form of loop within a series call and a second layer of callbacks.

Another problem comes if you want to define multiple ways to return from one of these functions in the async series -- e.g. if there is an error and result path that are separate:

    function (cb) {
        thing.once('error', cb);
        thing.once('success', function (result) { cb(null, result); });
    }

While one such additional path is manageable, things quickly become very complex.

Instead, let us think of each of the callbacks in such an async sequence as being states of a finite state machine. With async.series we are limited to defining only edges that progress forwards through the list. If, instead, we could define whatever edges we like, we could construct conditional logic and loops across async boundaries. If we had some way to "gang" the callbacks set up in each state together, they could all be disconnected at state exit and avoid the need for complex logic to deal with out-of-state events.

This library provides a framework for dealing with just such an async finite state machine.

Moore machines

A Moore machine (as opposed to a Mealy machine) is an FSM whose outputs depend solely on the present state, and not any other inputs. They are considered to be a simpler approach than the Mealy machine, and easier to reason about.

In our analogy, of course, our state machine does not have distinct outputs (since really we are using it to run arbitrary code). If we consider the FSM's "outputs" as the total set of side-effects it has on the program's state, however, we can interpret a Moore machine as being an FSM where code only runs on the entry to a new state, and all other events can only serve to cause state transitions.

Example

In this example we'll create an FSM called ThingFSM. It's a typical network client, which wants to make a TCP connection to something and talk to it. It also wants to delay/backoff and retry on failure.

var mod_mooremachine = require('mooremachine');
var mod_util = require('util');
var mod_net = require('net');

function ThingFSM() {
    this.tf_sock = undefined;
    this.tf_lastError = undefined;
    mod_mooremachine.FSM.call(this, 'stopped');
}
mod_util.inherits(ThingFSM, mod_mooremachine.FSM);

ThingFSM.prototype.state_stopped = function (S) {
    S.on(this, 'startAsserted', function () {
        S.gotoState('connecting');
    });
};

ThingFSM.prototype.state_connecting = function (S) {
    var self = this;
    this.tf_sock = mod_net.connect(...);
    S.on(this.tf_sock, 'connect', function () {
        S.gotoState('connected');
    });
    S.on(this.tf_sock, 'error', function (err) {
        self.tf_lastError = err;
        S.gotoState('error');
    });
};

ThingFSM.prototype.state_error = function (S) {
    var self = this;
    if (this.tf_sock !== undefined)
        this.tf_sock.destroy();
    this.tf_sock = undefined;
    /* Print an error, do something, check # of retries... */

    /* Retry the connection in 5 seconds */
    S.timeout(5000, function () {
        S.gotoState('connecting');
    });
};

ThingFSM.prototype.state_connected = function (S) {
    /* ... */
};

API

Inheriting from FSM

Implementations of a state machine should inherit from mod_mooremachine.FSM, using mod_util.inherits. The only compulsory methods that the subprototype must implement are the state callbacks.

mod_mooremachine.FSM(initialState)

Constructor. Must be called by the constructor of the subprototype.

Parameters:

• initialState: String, name of the initial state the FSM will enter at startup

FSM#state_name(stateHandle)

State entry functions. These run exactly once, at entry to the new state. They should take any actions associated with the state and set up any callbacks that can cause transition out of it.

The stateHandle argument is a handle giving access to functions that should be used to set up events that can lead to a state transition. It provides replacements for EventEmitter#on, setTimeout, and other mechanisms for async event handling, which are automatically torn down as soon as the FSM leaves its current state. This prevents erroneous state transitions from a dangling callback left behind by a previous state.

It is permissible to call stateHandle.gotoState() immediately within the state_ function.

Caution should be used when emitting events or making synchronous calls within a state_ function -- if it is possible for the handler of the event or callee to call back into the FSM or emit an event itself that may cause the FSM to transition, then the results of this occurring synchronously within the state entry function may be undesirable. It is highly recommended to emit any events within a setImmediate() callback.

Parameters:

• stateHandle, an Object, instance of mod_mooremachine.FSMStateHandle

FSM#allStateEvent(name)

Adds an "all-state event". Should be called in the constructor for an FSM subclass. Any registered all-state event must have a handler registered on it after any state transition. This allows you to enforce that particular events must be handled in every state of the FSM.

Parameters:

• name: String, name of the event

FSM#getState()

Returns a String, full current state of the FSM (including sub-state).

FSM#isInState(state)

Tests whether the FSM is in the given state, or any sub-state of it.

Parameters:

• state: String, state to test for

Returns a Boolean.

State handles

FSMStateHandle#gotoState(state)

Transitions the FSM into the given new state. Can only be called once per state handle.

FSMStateHandle#on(emitter, event, cb)

Works like EventEmitter#on: equivalent to emitter.on(event, cb) but registers the callback for removal as soon as the FSM moves out of the current state.

FSMStateHandle#timeout(timeoutMs, cb)

Equivalent to setTimeout(cb, timeoutMs), but registers the timer for clearing as soon as the FSM moves out of the current state.

Returns: the timer handle.

FSMStateHandle#callback(cb)

Wraps an arbitrary callback function in such a way that calling it once the FSM has left the current state is a no-op.

FSMStateHandle#interval(intervalMs, cb)

Equivalent to setInterval(cb, intervalMs), but registers the timer for clearing as soon as the FSM moves out of the current state.

FSMStateHandle#validTransitions(possibleStates)

Should be called from a state entry function. Sets the list of valid transitions that are possible out of the current state. Any attempt to transition the FSM out of the current state to a state not on this list (using gotoState()) will throw an error.

Parameters:

• possibleStates: Array of String, names of valid states

FSMStateHandle#gotoState(state)

Causes the FSM to enter the given new state.

Parameters:

• state: String, state to enter

Sub-states

It is possible to create a "sub-state" with mooremachine FSMs, which "inherits from" its parent state. For example:

ThingFSM.prototype.state_connected = function (S) {
    S.on(this.tf_sock, 'close', function () {
        S.gotoState('closed');
    });
    if (workAvailable)
        S.gotoState('connected.busy');
    else
        S.gotoState('connected.idle');
};

ThingFSM.prototype.state_connected.busy = function (S) {
    this.tf_sock.ref();
    /* ... */
    S.on(this.tf_work, 'finished', function () {
        S.gotoState('connected');
    });
};

ThingFSM.prototype.state_connected.idle = function (S) {
    this.tf_sock.unref();
    S.on(this, 'workAvailable', function () {
        S.gotoState('connected.busy');
    });
};

All event handlers that are set up in the 'connected' state entry function are kept when entering 'connected.busy' or 'connected.idle'. When changing from 'connected.busy' to 'connected.idle', the handlers set up in that sub-state are torn down, but those originating from 'connected' are kept.

While in a sub-state of 'connected', fsm.isInState('connected') will continue to evaluate to true. Separate 'stateChanged' events will be emitted for each sub-state entered.

Once a handle is used to transition to an unrelated state (e.g. 'closed' in the example), all handlers are torn down (from both the parent state and sub-state) as usual before entering the new state.