Redis based Finite State Machine
Redis based Finite State Machine.
Finist is a finite state machine that is defined and persisted in
Redis.
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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.
Finist requires a Redic compatible client. To make things
easier, redic is listed as a runtime dependency so the examples
in this document will work.
require "finist"
# 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.new(Redic.new, "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
printf("State changed to %s\n", state)
end
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.
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
$ gem install finist