Rails Magazine - Issue 3

22
22
Workflow Definition
A workflow is an abstract representation of real work
undertaken by a single or complex group of individuals
or mechanisms. It describes a system of activities enabled
through a collection of resources and information flows.
Workflows are primarily used to achieve some form of processing
intentions, such as data processing.
There are two main forms of workflows: sequential and
state-machine. Sequential workflows are predictable. They
utilize the rules and conditions we provide at the beginning
to progress a process forward. The workflow is in control of
the process.
A state-machine workflow is the reverse. It is driven by
external events to its completion. We define the states and
required transitions between those states. The workflow sits
and waits for an external event to occur before transitioning
to one of the defined states. The decision making process happens
externally outside of the workflow – there is a structure
to be followed still like a sequential workflow but control is
passed to its external environment.
What is a state machine?
The diagram below is a simple example of a state machine.
It has three main properties: states, events and transitions.
Event
Workflow solutions using AASM by Chee Yeo
Workflow solutions using AASM
by Chee Yeo
A state represents a particular situation. The state machine
above has two main states – ‘Power On’ and ‘Power Off ’. The
machine will be in one of these two states at any point in
time.
There is only one main event in the state machine above
– a button click. An event describes an external stimulus or
input into the system. In this instance, a button click will
either ‘power on’ the state machine or power it off and both
states shown above respond to the same event.
A transition moves the state machine from one state to
the next. In the diagram above, following a button click, if
the machine is in a ‘power on’ state it will transition to the
‘power off ’ state and vice versa. Not all transitions move the
state machine to a new state – it is possible to loop back to the
same state.
A state machine does not just store states and events
alone. It can also execute code when an event arrives. In the
example above, the state machine can be designed to control
electricity flow when it transitions to a new state following a
button click event.
State Machine in Rails
Sometimes you need to track the state of a model in an
application and the transitions between a set of pre-defined
states which are triggered based on certain events. Or you
might need to express the business logic of your application
using a workflow to illustrate the flow of state changes based
on its events.
For example, you might have an application
that allows the user to upload files
for processing and you need to provide
some form of mechanism to track the file
while it is added to a queue for processing
and to provide some form of feedback to
inform the user of the status of the outcome
(success or an error) and the relevant
action to take based on the outcome (if an
error occurred, inform the user).
This can be modelled by adding a ‘state’
or ‘status’ column in your model table and
updating it as you go along. However, it
becomes tedious and error prone when
multiple states are involved.
The acts_as_state_machine (AASM) plugin converts any
model into a Finite State Machine capable of remembering
which state it is in at any point in time and the transitions it
is allowed to go through between those states and the actions
that are to be triggered when a specific state is reached.