Introduction

Flowr is based on two key concepts: rules and flow graphs. A flow graph is the dynamic state machine that the user can create and is stored in the database. Multiple instances of states can then be active for any given flow graph. One or more sets of rules is defined by the programmer which limits the progression of possible states in a flow graph.

Rules are defined by subclassing the Rule class and filling in the “children” attribute. Cycles are allowed, so Rule instances can point to each other or themselves. Once a hierarchy of Rule classes have been defined, a RuleSet can be created and stored in the database. Flow objects are created with the GUI and describe the flow through a state machine with the state flow graph being a subset of those defined by the collection of Rule objects that were registered in the RuleSet instance.

The State object is an instantiation of a traversal of the state machine represented by a Flow. Once there are State objects for a Flow, the Flow can no longer be edited.

Definitions:

  • Rule – a base class for rules that the programmer defines which

    specifies what other Rule objects can be connected to and what happens when a state is entered and exited

  • RuleSet – a registery for the collections of Rule subclasses

  • Flow – user defined state machine based on a RuleSet

    instance

  • State – an instance of a state machine and its current state

Example

A user defines the following Rule subclasses:

class A(Rule):
    children = [B, C]

class B(Rule):
    children = []

class C(Rule):
    children = [D, E]
    multiple_paths = True

class D(Rule):
    children = []

class E(Rule):
    children = []

A RuleSet object is created using the factory and passing in the single starting point of our allowed flows:

RuleSet.factory('My Rules', A)

Using these rules, you could create some flows:

  • A -> B
  • A -> C -> D
  • A -> C -> D or E

You would not be able to create:

  • A -> D
  • A -> B -> C

The first of these is not allowed because “D” is not a direct child of “A” in the Rule definitions. The second is not allowed because “B” has no allowed children.