Finist

Redis based Finite State Machine.

Description

Finist is a finite state machine that is defined and persisted in Redis.

Community

Meet us on IRC: #lesscode on freenode.net.

Related projects

• Finist implemented in Lua
• Finist implemented in Ruby
• Finist implemented in Rust

Getting started

Install Redis. On most platforms it's as easy as grabbing the sources, running make and then putting the redis-server binary in the PATH.

Once you have it installed, you can execute redis-server and it will run on localhost:6379 by default. Check the redis.conf file that comes with the sources if you want to change some settings.

Usage

Finist requires a Redis Python Redis compatible client. To make things easier, redic is listed as a runtime dependency so the examples in this document will work.

from finist import Finist
import redis
redis_conn = redis.Redis()

# Initialize with a Redis client, the name of the machine and the
# initial state. In this example, the machine is called "order" and
# the initial status is "pending". The Redis client is connected to
# the default host (127.0.0.1:6379).
machine = Finist(redis_conn, "order", "pending")

# Available transitions are defined with the `on` method
# `machine.on(<event>, <initial_state>, <final_state>)`
machine.on("approve", "pending", "approved")
machine.on("cancel", "pending", "cancelled")
machine.on("cancel", "approved", "cancelled")
machine.on("reset", "cancelled", "pending")

Now that the possible transitions are defined, we can check the current state:

machine.state()
# => "pending"

And we can trigger an event:

machine.trigger("approve")
# => ["approved", true]

The trigger method returns an array of two values: the first represents the current state, and the second represents whether a transition occurred.

Here's what happens if an event doesn't cause a transition:

machine.trigger("reset")
# => ["approved", false]

Here's a convenient way to use this flag:

state, changed = machine.trigger("reset")

if changed
  print "State changed to %s\n" %state

If you need to remove all the transitions for a given event, you can use rm:

machine.rm("reset")

Note that every change is persisted in Redis.

Representation

Each event is represented as a hash in Redis, and its field/value pairs are the possible transitions.

For the FSM described in the examples above, the keys are laid out as follows:

# Current state
finist:order (string)

# Transitions for event `approve`
finist:order:approve (hash)
    pending   -> approved

# Transitions for event `cancel`
finist:order:cancel (hash)
    pending   -> cancelled
    approved  -> cancelled

# Transitions for event `reset`
finist:order:reset (hash)
    cancelled -> pending

Installation

$ python setup.py install
or 
$ pip install finist