Workflow Functional Specification

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Overview

I recently built a typical workflow-based application, bug-tracker, and decided against using the acs-workflow package that I myself built. That's not a good recommendation. We need to fix that.

Goals

The goal is to implement a workflow package that:

• Is ideally suited for at least 3 use-cases: Bug-tracker, CMS-style publication process, and simple approval.
• Gives people a usable UI.
• Can be used entirely through a clean Tcl API
• Doesn't require people to learn Petri Nets
• Is much easier for developers to use in their applications

Gripes with the current acs-workflow:

• Engine is in PL/SQL, not in Tcl, which makes it hard to write callbacks.
• Petri net is just too complicated for people to learn how to use, and there are too many ways for them to mess up. The primary benefit is parallel routing, which I've never actually come across any applications that seriously needed.
• The UI sucks, and it's really hard to use workflow without using the interface.
• Graphviz is actually not terribly great at displaying workflow nets, as it tries to fit everything into a circle, whereas most of the time with workflow what you really want is to have it appear as a nice sequence of events with some loops back here and there.
• It's too restrictive and inflexible. It's hard to change your mind and go back, or to manually bash the case into a certain state.
• Also, it was never finished. Unfortunately, finishing it would be a tremendous amount of work for limited benefit.
• Data model has some big issues: Workflows are object types. Workflows aren't tied to packages, and the context idea isn't working very well. It's based on petri nets.

Finite State Machine

Take bug-tracker as an example. The bug-tracker workflow and user interface can be defined as:

Roles

• Submitter
• Assignee

States

• Open
• Resolved
• Closed

Actions

• Resolve: Enabled in states open and resolved; changes state to resolved.
• Close: Enabled in state resolved; changes state to closed.
• Reopen: Enabled in states resolved, closed; changes state to open.
• Edit: Enabled in all states; no changes to state.
• Comment: Enabled in all states; no changes to state.
• Reassign: Enabled in states open, resolved; no changes to state.

I've finally come to the realization that we'll be better off in the short to medium term with just a well-functioning implementation of a finite state machine based workflow module. In general, a workflow consists of a finite set of states, and a finite set of actions. Each action has a set of states in which it's enabled, or it can be always enabled in all states. And each action can cause the workflow case to move into a new state, or it can leave the state unaltered.

Note that the ability to have an action enabled in more than one state is a convenience, and not part of the mathematical model of finite state machines. Likeways with actions that don't change the state. But it's mighty convenient, as you've seen illustrated by the bug-tracker example above.

Workflows

A workflow is a set of roles, actions, and states, and their relations.

A workflow is associated with an object, which would typically be one of the following:

1. A package type: This is the default workflow for the bug-tracker package.
2. A package instance: This is the default workflow for a particular instance of the bug-tracker package.
3. A single case: Future versions could allow you to customize your workflow for a particular bug or content story.

There's also a short_name, so you can easily distinguish between multiple workflows for the same package, e.g., one for handling the bug, and another for approving creation of new versions or components in the bug-tracker.

A workflow is also associated with an object type. The reason for this is that assignments will frequently depend on attributes of the specific object for the case. In bug-tracker, for example, the default assignee for a bug will be the maintainer of the component in which the bug has been found. The bug-tracker will provide one or more assignment service contract implementations, which, given the bug_id will give you the component maintainer, or the project maintainer. These can be used to set up automatic assignment through a nice web-based user interface.

When you create a new workflow case for a specific object, we will check that this object descends from the object type for which the workflow is for. If your workflow is general enough to work for all object types, then you can simply associate it with the common ancestor of all objects, 'acs_object'.

When you create a new instance of the bug-tracker, we would make a copy of the default bug-tracker workflow for your particular package, so that you can make local changes to the workflow, to the assignments, etc.

A workflow can have side-effects, which fire when any action is triggered on that workflow. These fire after the specific actions. See more under action side-effects. These are declared as a standard "Action_SideEffect" service contract implementation.

Another service contract on the workflow level is the activity log entry title formatting contract. Using a side-effect callback, you can store additional key/value pairs in the activity log. You can use the title formatting service contract to pull these out, along with any other data you like, and use them to format the title of the log entry for display.

Roles

A workflow has a set of roles. For bug-tracker, this is Submitter, and Assignee. More complex bug-tracker workflows, could add Triager and Tester. For a typical pulication workflow, you'd have Author, Editor, and Publisher. Normally, you'd always include an 'Administrator' role.

Each role is associated with one or more actions in the workflow. The assignee is assigned to the 'Resolve' action, but also has permission to perform the Edit, Comment and Reassign actions. The submitter is assigned to the 'Close' action, but also has permission to 'Reopen', 'Edit', 'Comment', and possibly 'Reassign'.

The idea behind introducing roles is that you do not want to go through the bother of assigning each action individually, when normally they are grouped together.

Then, as the workflow case unfolds, people are given roles--you will be the submitter, you will be the assignee. Roles can get reassigned at any time.

Default Assignment

The tricky part, however, is the rules saying who should be assigned by default, or who can be assigned to this role. First, let's look at how the default assignees can be determined.

1. If you only have one publisher, then you simply want to assign the Publisher role to that publisher always. That's called a static assignment, and the information about that current assignee is kept in the workflow data model.
2. The Submitter role in bug-tracker, you want to assign to whoever opened the bug, namely the object creation user.
3. The Assignee role in bug-tracker is given to the maintainer of the component in which the bug was found. This is an example of a completely application-specific assignment, one which is only relevant for bug-tracker bug objects, because we need to know the particular bug-tracker data model for this to work.

These different options are supplied by programmers as implementations of a particular service contract (see below under service contracts).

In the definition of a workflow, you can select an ordered list of default assignment methods Each will be tried in the order you specify. The first to return a non-empty list of assignees is the one which will be used, and the rest won't get called. So for example you can say "first try component maintainer, and if non is specified, use the project maintainer".

The workflow package will supply a few standard implementations:

• Creation user: Assign to the user who created the given object.
• Static assignee: Use the static assignment from the workflow definition.

Default assignment is done in a lazy fashion, in that we don't try to find the default assignees until we need to. We need to the first time an action assigned to that role is enabled. This allows your default assignment to depend on things that happened in prior tasks.

Reassignment

Now, let's look at what happens when you want to reassign a role to someone else. the

1. If you want to reassign the role, the user interface offers a pick-list of the users and groups which you're most likely to want to reassign the role to. Who they are will depend on the particular application. One common idea is to display the users who are currently assigned to this role in other cases.
2. If the desired person or group wasn't in the pick-list, you can search. The search is conducted among all the users who could possibly be assigned to this role, which, again, will depend on the application. It could be all registered users on the site, it could be all members of the nearest surrounding subsite, it could be all members of a particular named group, or it could be some other calculation based on the application.

A couple of default implementations will be supplied by the workflow package. For the pick-list:

• Current assigness: Returns the list of parties who are currently assigned to this role in this workflow (for example, all the current assignees in this bug-tracker instance).

For the search query:

• Registered users: No limitation, search among all registered users. Simply returns a query name for "cc_users".
• Nearest subsite members: Limit to members of the nearest subsite above the current package.
• Static allowed assignment: The users defined as the allowed parties in the workflow_role_allowed_parties table.

Actions

In order to determine who are supposed to perform an action, and who are allowed to perform the action, we let you specify these three things for each action:

• Assigned role(s): People who are mapped to this role will be assigned to this action, e.g., the submitter is assigned to the Close action once the bug is resolved. When you're assigned to something, you're expected to go and do something about it.
• Allowed role(s): People who are mapped to this role will have the permission to perform this action, e.g., the submitter is allowed to Reopen the bug once it's resolved, but not assigned to it. She's only assigned to "Close".
• Privileges: People who have these privileges on the object pointed to by workflow_case.object_id (e.g. the bug object for bug-tracker) will also have permission to do perform this action. Same as above, but allows for using permissions to grant 'feedback', 'write', and 'admin', for example.

Actions can also have side-effects, which simply means that whenever an action is triggered, one or more specified service contract implementations will get executed. These side-effects are executed after all other updates, both to the case object, and to the workflow tables, have been completed.

States

This is specific to the FSM-model. A workflow has a finite set of states, for example "open", "resolved", and "closed". A case will always be in exactly one such state. When you perform an action, the workflow can be pushed into a new state.

There will be one initial state, which the workflow will start out in. This will be the first state according to the sort order from workflow_fsm_states

States have almost no information associated with them, they're simply used to govern which actions are available.

Cases

A case is the term for a workflow in action. A case always revolves around a specific object. and we currently only allow one case for one object. That is, you can only have one workflow in process for one object.

The case holds information about the current state, the current assignments, and an activity log over everything that happens on the case.

Data Model

//--------------------//
//  Workflow level    //
//--------------------//

create table workflows (
  workflow_id             integer ... primary key, references acs_objects
  short_name              varchar ...
  pretty_name             varchar ...
  object_id               integer ... references acs_objects
  -- object_id points to either a package type, 
  -- package instance, or single workflow case
  object_type             varchar ... references acs_object_types
  -- which object type (or its subtypes) is this workflow designed for
  unique (object_id, short_name)
);

create table workflow_callbacks (
  workflow_id             integer ... references workflows
  acs_sc_impl_id          integer ... references acs_sc_impls
  sort_order              integer ...
    
  constraint ...
  primary key (workflow_id, acs_sc_impl_id)
);

create table workflow_roles (
  role_id                 integer ... primary key
  workflow_id             integer ... references workflows
  short_name              varchar ...
  pretty_name             varchar ...
  
);

create table workflow_role_default_parties (
  role_id                 integer ... references workflow_roles
  party_id                integer ... references parties

  constraint ... 
  primary key (role_id, party_id)
);

create table workflow_role_allowed_parties (
  role_id                 integer ... references workflow_roles
  party_id                integer ... references parties

  constraint ... 
  primary key (role_id, party_id)
);

create table workflow_role_callbacks (
  role_id                 integer ... references workflow_roles
  acs_sc_impl_id          integer ... references acs_sc_impls
  -- this can be an implementation of any of the three assignment
  -- service contracts: DefaultAssignee, AssigneePickList, or 
  -- AssigneeSubQuery
  sort_order              integer ...
  
  constraint ...
  primary key (role_id, acs_sc_impl_id)
);

create table workflow_actions (
  action_id               integer ... primary key
  workflow_id             integer ... references workflows
  sort_order              integer ...
  short_name              varchar ...
  pretty_name             varchar ...
  pretty_past_tense       varchar ...
  assigned_role           integer ... references workflow_roles
);

create table workflow_action_allowed_roles (
  action_id               integer ... references workflow_actions
  role_id                 integer ... references workflow_roles
);

create table workflow_action_privileges (
  action_id               integer ... references workflow_actions
  privilege               varchar ... references acs_privileges
);

create table workflow_action_callbacks (
  action_id               integer ... references workflow_actions
  acs_sc_impl_id          integer ... references acs_sc_impls
  sort_order              integer ...
    
  constraint ...
  primary key (action_id, acs_sc_impl_id)
);

//  Finite State Machine model //

create table workflow_fsm_states (
  state_id                integer ... primary key
  workflow_id             integer ... references workflows
  sort_order              integer ...
  short_name              varchar ...
  pretty_name             varchar ...
);

create table workflow_fsm_actions (
  action_id               integer ... primary key ... references workflow_actions
  new_state               integer ... references workflow_fsm_states (can be null)
);

create table workflow_fsm_action_enabled_in_states (
  action_id               integer ... references workflow_fsm_actions
  state_id                integer ... references workflow_fsm_states
);

create table workflow_fsm (
  workflow_id             integer ... primary key, references workflows
  initial_state           integer ... references workflow_fsm_states
);

//--------------------//
//  Case level        //
//--------------------//

create table workflow_cases (
  case_id                 integer ... primary key
  workflow_id             integer ... references workflows
  object_id               integer ... references acs_objects ... unique
  -- the object which this case is about, e.g. object_id of the bug
);

create table workflow_case_log (
  entry_id                integer ... primary key
  case_id                 integer ... references workflow_cases
  action_id               integer ... references workflow_actions
  user_id                 integer ... references users
  action_date             timestamp not null default now(),
  comment                 text ...
  comment_format          varchar ...
);

create table workflow_case_log_data (
  entry_id                integer ... references workflow_case_log
  key                     varchar
  value                   varchar

  constraint ...
  primary key (entry_id, key)
);

create table workflow_case_role_assigned_parties (
  case_id                 integer ... references workflow_cases
  role_id                 integer ... references workflow_roles
  party_id                integer ... references parties

  constraint ...
  primary key (case_id, role_id, party_id)
);


//  Finite State Machine model //

create table workflow_case_fsm (
  case_id                 integer ... references workflow_cases
  current_state           integer ... references workflow_fsm_states
);

workflow.Role_DefaultAssignees:
  GetObjectType -> string
  GetPrettyName -> string
  GetAssignees (case_id, object_id, role_id) -> { list of party_id }

workflow.Role_AssigneePickList
  GetObjectType -> string
  GetPrettyName -> string
  GetPickList (case_id, object_id, role_id) -> { list of party_id }

workflow.Role_AssigneeSubQuery
  GetObjectType -> string
  GetPrettyName -> string
  GetSubQueryName (case_id, object_id, role_id) -> { subquery_name { bind variable list } }

workflow.Action_SideEffect
  GetObjectType -> string
  GetPrettyName -> string
  DoSideEffect (case_id, object_id, action_id, entry_id) -> (none)

workflow.ActivityLog_FormatTitle
  GetObjectType -> string
  GetPrettyName -> string
  GetTitle (entry_id) -> title

The GetObjectType method is used for the service contract implementation to tell which object types it is valid for. For example, a DefaultAssignee implementation can look at a bug, find out which component it is found in, then look up the component definition to find the default maintainer. This implementation, though, is only valid for objects of type 'bt_bugs', or any descendants thereof. Thus, this is what the GetObjectType call would return for this implementation. If your implementation is valid for any ACS Object, then simply return 'acs_object', as this is the mother of all objects.

The GetPrettyName method will be run through a localization filter, meaning that any occurrence of the #message-key# notation will be replaced with a message catalog lookup for the current domain.

The AssigneeQuery service contract probably needs a little explanation. You're supposed to supply a valid subquery, which will select the columns party_id, name, email, and screen_name (nulls are okay) of all the parties that a role can possibly be assigned to. A simple version could simply be "cc_users". Another would be:

select u.user_id as party_id, 
       u.first_names || ' ' || u.last_names as name,
       u.email,
       u.screen_name
from   cc_users u
where  (some condition)

This would then typically be used like this:

select distinct 
       q.party_id, 
       q.name || ' (' || u.email || ')' as name_and_email
from   (your subquery goes here) q
where  upper(coalesce(q.name, '') || 
          q.email || ' ' || 
          coalesce(q.screen_name, '')) 
          like upper('%'||:value||'%')
order  by name_and_email

Now, one little caveat is that you have to return the query dispatcher query name, not the actual query. The query name will then get passed to db_map to produce the actual subquery.

Workflow will supply these service contract implementations by default:

workflow.Role_DefaultAssignee

• Creation user: Returns the creation_user of the given object.
• Status assignee: Returns the contents of the workflow_role_default_parties table.

workflow.Role_AssigneePickList

• Current assignees: Returns the list of parties who are currently assigned to this role in some case in this workflow.
• Static allowed assignees: Search through the contents of the workflow_role_allowed_parties table.

workflow.Role_AssigneeSubQuery

• Registered users: Search through all registered users.
• Static allowed assignees: Search through the contents of the workflow_role_allowed_parties table.

Notifications

You can sign up for notifications at several levels:

1. Notify me of all actions to which I'm assigned. You don't have to manually go sign up for these notifications, but you should be able to change the delivery method and frequency.
2. Notify me of all activity on ...
   1. Any case where I'm assigned to some role.
   2. A particular case (one bug-tracker bug)
   3. All cases in the particular workflow (entire bug-tracker project)

You should always receive at most one notification per activity. They're sent out in the order in which they're listed here, and if you get the first, you won't get the second, third or fourth; if you get the second, you won't get the third or fourth, etc.

A special case is that the first notification isn't optional. You don't have to manually go sign up for those notifications, and you can't turn them off entirely. You can still change the delivery method and the frequency, though.

In order to implement this, we need to make three fairly trivial enhancements to the notifications package.

1. We need to be able to pass on the list of already notified users from one call of notification::new to the next. So notification::new needs to take a parameter like -already_notified and to not notify those again, and likewise, to return the list of users notified by the given notification.
2. We need to be able to limit notifications to only a subset of the subscribed base. If you have a subscription on "any case where I'm assigned to some role", that's a dynamic relationship. So the call to notification::new would take as a parameter the list of people who are assigned to some role on this particular case. Only people who are subscribed and on that list will get notified. I can't think of a good name for such a parameter, perhaps -positive_list.
3. Finally, we need to force people on the positive list above to get notifications even though they don't currently have a subscription. This could be a -force:boolean parameter which works in conjunction with the positive list, so that people on the positive list who aren't subscribers get a default email/instant subscription automatically. They can then go back and change their delivery method and frequency later.

Workflow API

API for Defining Workflows

You can define it using a Tcl interface:

set workflow_id [workflow::new \
    -short_name "bug"
    -pretty_name "Bug" \
    -object_id [package::object_id "bug-tracker"] \
    -object_type "bt_bug" \
    -callbacks { bug-tracker.FormatLogTitle } 

#####
#
# Roles
#
#####

workflow::role::add $workflow_id \
    -short_name "submitter" \
    -pretty_name "Submitter" \
    -callbacks { workflow.CreationUser }

workflow::role::add $workflow_id \
    -short_name "assignee" \
    -pretty_name "Assignee" \
    -callbacks {
                bug-tracker.ComponentMaintainer
                bug-tracker.ProjectMaintainer
            }

#####
#
# States
#
#####

workflow::fsm::state::add $workflow_id \
    -short_name "open" \
    -pretty_name "Open" \
    
workflow::fsm::state::add $workflow_id \
    -short_name "resolved" \
    -pretty_name "Resolved"
    
workflow::fsm::state::add $workflow_id \
    -short_name "closed" \
    -pretty_name "Closed"

#####
#
# Actions
#
#####

workflow::fsm::action::add $workflow_id \
    -short_name "comment" \
    -pretty_name "Comment" \
    -pretty_past_tense "Commented" \
    -allowed_roles { submitter assignee } \
    -privileges { feedback }

workflow::fsm::action::add $workflow_id \
    -short_name "edit" \
    -pretty_name "Edit" \
    -pretty_past_tense "Edited" \
    -allowed_roles { submitter assignee } \
    -privileges { write }

workflow::fsm::action::add $workflow_id \
    -short_name "resolve" \
    -pretty_name "Resolve" \
    -pretty_past_tense "Resolved" \
    -assigned_roles { assignee } \
    -enabled_states { open resolved } \
    -new_state "resolved" \
    -privileges { write } \
    -callbacks { bug-tracker.CaptureResolutionCode }

workflow::fsm::action::add $workflow_id \
    -short_name "close" \
    -pretty_name "Close" \
    -pretty_past_tense "Closed" \
    -assigned_roles { submitter } \
    -enabled_states { resolved } \
    -new_state "closed" \
    -privileges { write }

workflow::fsm::action::add $workflow_id \
    -short_name "reopen" \
    -pretty_name "Reopen" \
    -pretty_past_tense "Closed" \
    -allowed_roles { submitter } \
    -enabled_states { resolved closed } \
    -new_state "open" \
    -privileges { write }

Alternatively, we could have an ad_form/ad_page_contract style spec as well:

set workflow {
     roles {
         submitter {
             pretty_name "Submitter"
             callbacks { 
                 workflow.CreationUser 
             }
         }
         assignee {
             pretty_name "Assignee"
             callbacks {
                 bug-tracker.ComponentMaintainer
                 bug-tracker.ProjectMaintainer
             }
         }
     }
     states {
         open {
             pretty_name "Open"
         }
         resolved {
             pretty_name "Resolved"
         }
         closed {
             pretty_name "Closed"
         }
     }
     actions {
         comment {
             pretty_name "Comment"
             pretty_past_tense "Commented"
             allowed_roles { submitter assignee }
             privileges { feedback }
         }
         edit {
             pretty_name "Edit"
             pretty_past_tense "Edited"
             allowed_roles { submitter assignee }
             privileges { write }
         }
         resolve {
             pretty_name "Resolve"
             pretty_past_tense "Resolved"
             assigned_roles { assignee }
             enabled_states { open resolved }
             new_state "resolved"
             privileges { write }
             callbacks { bug-tracker.CaptureResolutionCode }
         }
         close {
             pretty_name "Close"
             pretty_past_tense "Closed"
             assigned_roles { submitter }
             enabled_states { resolved }
             new_state "closed"
             privileges { write }
         }
         reopen {
             pretty_name "Reopen"
             pretty_past_tense "Closed"
             allowed_roles { submitter }
             enabled_states { resolved closed }
             new_state "open"
             privileges { write }
         }
     }
 }

set workflow_id [workflow::new \
    -short_name "bug" \
    -pretty_name "Bug" \
    -object_id [package::object_id "bug-tracker"] \
    -object_type "bt_bug" \
    -callbacks { bug-tracker.FormatLogTitle } \
    -workflow $workflow]

API for Starting a Case

set bug_id [bug_tracker::bug::new ...]

workflow::case::new \
    -workflow_id [workflow::get_id -object_id [ad_conn package_id] -short_name "bug"] \
    -object_id $bug_id

API for the Form Page

The intended user interface for a workflow-based application is similar to the bug-tracker. The form is shown in display-only mode, with buttons corresponding to actions along the bottom (e.g. Comment, Edit, Resolve, Close).

case::get_case_id(object_id, short_name) -> case_id

Find the case_id from object_id and workflow short_name.

case::get_user_roles(case_id, user_id) -> { list of roles }

Find out which roles the current user has wrt the current object.

case::get_enabled_actions(case_id, user_id) -> { list of { label name } }

The actions currently enabled in this state.

case::get_user_actions(case_id, user_id) -> { list of { label name } }

The enabled actions which the current user has permission to perform.

case::action::get_editable_fields(case_id, action) -> { list of field names }

Which fields should we edit, depending on the current action. NOTE! We probably won't be able to support this in the first version.

case::state::get_hidden_fields(case_id) -> { list of field names }

Which fields should we hide, depending on the state. NOTE! We probably won't be able to support this in the first version.

case::action::available_p(case_id, user_id, action_id) -> (boolean)

Is this action enabled and allowed for this user?

case::action::new_state(case_id, action_id) -> (state_id)

The new state which the case will have after this action has been performed (if action doesn't change state, returns the current state again.

case::action::execute(case_id, action_id, comment, comment_format) -> (state_id)

Perform the action, updating the workflow state, etc. This should be called from inside a db_transaction where the case object has just been updated.

Here's what the form page would look like:

ad_page_contract { ... } {
    bug_id:integer,notnull  
}

# Setup return_url, user_id, etc.
...

# Current action, blank for display mode
set action [form get_action bug]

# Check permissions
workflow::case::require_permission -object_id $bug_id -action $action

# Create the form
form create bug \
    -mode display \
    -actions [workflow::case::get_actions -object_id $object_id -action $action] \
    -cancel_url $return_url

element create ...

# Valid submission: Update
if { [form is_valid bug] } {
    bug_tracker::bug::edit \
        -bug_id $bug_id \
        ...

    ad_returnredirect $return_url
    ad_script_abort
}

# Non-valid submission: Either request or error form
if { ![form is_valid bug] } {
    bug_tracker::bug::get -bug_id $bug_id -array bug

    set bug(status) [workflow::action::new_state -object_id $object_id -action $action]

    # Hide elements that should be hidden
    foreach element [workflow::state::get_hidden_fields -object_id $object_id] {
        element set_properties bug $element -widget hidden
    }
    
    # Set element values
    ...
    # - if [form is_request] then set all
    # - otherwise only set elements in display-mode  

    # Page title, context bar, filters, etc.
    ...
}

Future Extensions

• Implement metadata spec and integrate with that so you can pick which fields in your form to view/edit/hide depending on state and action.

Nice-to-haves that aren't entirely pie-in-the-sky include:

• User interface components that can generate a user interface like bug-tracker's, i.e. buttons below the form showing the actions that you can take, the resolution entries, the sub-status codes, etc.
• Pluggable models, for example, finite-state machines, petri-nets, dependency graphs. A service-contract-based interface allows you to plug in a new model.
• Integration with a task-list application to maintain the user's one task list (synchronization with Palm, etc.).
• Integration with calendar, so deadlines show up there.

Appendix A. Pluggable Models

I've looked into pluggable models before, and it's not too complicated. The trick is that you have four areas where the generic workflow framework/engine will interface with the plugin model:

• All workflow models have some definition of 'state'. For finite state machines, it's simply the name of the state, you're currently in: The structure here is a value from an enumeration. A petri net has as its state a list of tokens, each of which is currently in a particular place. A dependency graph model has as its state the list of tasks that have been completed. The workflow engine must provide an API for the pluggable model to access, manipulate, and store its state, but need not know anything about the internals of the state or how it's manipulated.
• All workflow models has some elements that go into its workflow specification: FSMs have states and transitions; a transition is an arc from one state to another. Petri nets have places and transitions, and it has arcs that point from a place to a transition, or a transition to a state. Dependency graphcs has tasks and dependencies, where a dependency goes from one task to another task.
• All workflow models has some concept of actions (tasks, transitions). An action has some precondition for when it's enabled, i.e., for when a user can or should perform this action. This is a function of the state. And actions also cause a well-defined change to the state, i.e., we move to a different state, tokens are consumed from some places and produced on others, etc. This is a function of the state, and also produces a new state.

These are the interaction points between a generic workflow engine, and its specific model implementations.

Appendix B. Fix or Rewrite

Should we discard workflow and rewrite, or should we try to incrementally improve what's there?

In general, you should be weary of rewriting if:

• You have many users of your software who'll want to upgrade, because they'll be annoyed by small changes to how things work.
• You have a different set of people implementing it the second time than you had the first time.

Neither of these are the case here. We don't have any significant users of workflow, and we have access to the same people (person) who did the original implementation to implement it again.

Besides, the planned changes are so big that there would be no code left untouched.

• Switch to FSMs instead of Petri Nets, which obliviates the engine and most of the admin UI
• Discard Graphviz for admin UI
• Current UI not using form builder/ad_form
• Current data model not using acs-kernel properly, e.g., a new workflow is an object type.

Hence, we've concluded that a rewrite is in fact the most productive strategy.