Mechanus (v0.1.0)

Mechanus is a system for defining and running, event-driven state machines.

A state machine consists of a set of states. Each state is associated with a list of entry actions, a list of exit actions, and a transition table. Actions are application-specific and opaque to Mechanus. Transition tables are indexed by events, which are either sent to Mechanus or the result of an action.

State machines are described in a simple DSL. State machines are run by first creating an instance from a DSL-template, then sending events to that instance. Events cause state transitions, which may cause actions to be fired.

In the standard D&D cosmology, Mechanus is the plane of existence associated with the Lawful Neutral alignment. Mechanus is home to a race of sentient automatons known as Modrons.

Concepts

• State machine: an abstract description of some process
• Modron: a running instance of a state machine
• Event: a message sent to some Modron
• Transition: a state change in some Modron caused by some event
• Action: an effect associated with a transition
• Mid: ModronID
• MSL: Modron Specification Language, the DSL used to describe state machines

MSL

Whitespace may be used at will. Comments begin with a '%'. MSL filenames should end in '.mrl'.

An MSL program is a sequence of state declarations. A state declaration is a state name optionally followed by entry actions, exit actions, and transitions. Actions is a list of names of the corresponding erlang callback-modules. Actions are evaluated in order left to right with each result merged with the previous. Transitions are described as an event name, followed by an arrow, followed by a state name. State names are upper case, event and action names are lower case.

Example:

START: go -> FIRST_STATE.

FIRST_STATE:
  EntryActions my_action.

  event_1 -> SECOND_STATE,
  event_2 -> THIRD_STATE.

SECOND_STATE:
  ExitActions your_action.
  event_3 -> THIRD_STATE.

THIRD_STATE: . %halting state

More formally, the keywords are ':' and '.' to delimit states, 'EntryActions' or 'ExitActions' and '.' to delimit action lists, and '->' and ',' to delimit transitions.

See mechanus_parser.yrl for a formal grammar.

Event scheduling

Events are either injected (returned by an action) or sent (via send/2). Events are either immediate (valid_from = 0) or future (valid_from > 0). Events are processed in validity order (in particular, immediate events are always processed before future events). Ties are broken as follows. Injected events are processed before sent events. If event A was injected/sent before event B, event A will be processed before event B.

TODO

• Allow loops in FSMs.