I'm working on a really simple workflow for my bug tracker. I want
filed bugs to start in an UNSTARTED status. From there, they can go to
STARTED.
From STARTED, bugs can go to FINISHED or ABANDONED.
I know I can easily hard-code this stuff into some if-clauses, but I
expect to need to add a lot more statuses over time and a lot more
relationships.
This seems like a crude state diagram. So, has anyone on this list done
similar work?
I've got a similar workflow in one of my applications at work. I
use a database to persist the available states along with their
transitions. Both the states themselves and the transitions have
various attributes to them to facilitate my workflows. The
following schema should work in most databases such as sqlite
(included with Python2.5+) with minor syntactic tweaks (such as
auto-increment)
CREATE TABLE States (
id INT PRIMARY KEY AUTOINCREMENT,
description VARCHAR(50),
other_attributes ...
);
CREATE TABLE Transitions (
id INT PRIMARY KEY AUTOINCREMENT,
from_state INT NOT NULL REFERENCES States(id),
to_state INT NOT NULL REFERENCES States(id),
other_attributes ...
);
INSERT INTO States (description) VALUES
('Started'),
('Finished'),
('Abandoned');
INSERT INTO Transitions (from_state, to_state) VALUES
(1, 2), -- Started -> Finished
(1, 3), -- Started -> Abandoned
(3, 2); -- Abandoned -> Finished
You can then query states you can transition to:
SELECT s1.description, s2.description, t.other_attributes
FROM Transitions t
INNER JOIN States s1
ON s1.id = t.from_id
INNER JOIN States s2
ON s2.id = t.to_id
-- WHERE t.from_id = @current_id
ORDER BY s1.description, s2.description
You can easily add more state rows as well as transition rows to
the tables. Then use the other_attributes of either the
transition or the state to control the workflows (we have
transition attributes for things like "notifies account-manager",
"notifies customer", "number of days that must pass from initial
workflow-start", "number of days that must pass from most recent
status change", "does this transition happen manually or
automatically via an overnight script", etc.) The workflow
processing code then looks at these attributes for the transition
that's being attempted, and behaves accordingly.
Hope this gives you some ideas. If you want to read more, the
magic google-words are "finite state automaton" (FSA) or "finite
state machine" (FSM)[1]
-tkc
[1]
http://en.wikipedia.org/wiki/Finite_state_machine
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