Simple Workflows With ApprovaFlow and Stateless

This is the second in a series of posts for ApprovaFlow, an alternative to Windows Workflow written in C# and JSON.Net. Source code for this post is here.

Last time we laid out out goals for a simple workflow engine, ApprovaFlow, with the following objectives:
• Model a workflow in a clear format that is readable by both developer and business user. One set of verbiage for all parties.
•. Allow the state of a workflow to be peristed as an integer, string. Quicky fetch state of a workflow.
•. Create pre and post nprocessing methods that can enforce enforce rules or carry out actions when completing a workflow task.
•. Introduce new functionality while isolating the impact of the new changes. New components should not break old ones
•.Communicate to the client with a standard set of objects. In other words, your solution domain will not change how the user interface will gather data from the user.
•. Use one. aspx page to processes user input for any type of workflow.
•. Provide ability to roll your own customizations to the front end or backend of your application.

The fulcrum point of all we have set out to do with ApprovaFlow is a state machine that will present a state and accept answers supplied by the users. One of Sensei’s misgivings about Windows Workflow is that it is such a behemoth when all you want to implement is a state machine.
Stateless, created Nicholas Blumhardt, is a shining example of adhering to the rule of “necessary and sufficient”. By using Generics Stateless allows you to create a state machine where the State and Trigger can be represented by an integer, string double, enum – say this sounds like it fulfills our goal:

•. Allow the state of a workflow to be persisted as an integer, string. Quicky fetch state of a workflow.
Stateless constructs a state machine with the following syntax:

var statemachine =
       new StateMachine(TState currentState);

For our discussion we will create a state machine that will process a request for promotion workflow. We’ll use:

var statemachine =
       new StateMachine(string currentstate);

This could very easily take the form of

<int, int>

and will depend on your preferences. Regardless of your choice, if the current state is represent by a primitive like int or string, you can just fetch that from a database or a repository and now your state machine is loaded with the current state. Contrast that with WF where you have multiple projects and confusing nomenclature to learn. Stateless just stays out of our way.
Let’s lay out our request for promotion workflow. Here is our state machine represented in English:

Step: Request Promotion Form
  Answer => Complete
  Next Step => Manager Review

Step: Manager Review
  Answer => Deny
  Next Step => Promotion Denied
  Answer => Request Info
  Next Step => Request Promotion Form
  Answer => Approve
  Next Step => Vice President Approve

Step: Vice President Approve
  Answer => Deny
  Next Step => Promotion Denied
  Answer => Manager Justify
  Next Step => Manager Review
  Answer => Approve
  Next Step => Promoted

Step: Promotion Denied
Step: Promoted

Remember the goal Model a workflow in a clear format that is readable by both developer and business user. One set of verbiage for all parties? We are very close to achieving that goal. If we substitute “Step” with “State” and “Answer” with “Trigger”, then we have a model that matches how Stateless configures a state machine:

var statemachine = new StateMachine(startState);

//  Request Promo form states
statemachine.Configure("RequestPromotionForm")
               .Permit("Complete", "ManagerReview");

//  Manager Review states
statemachine.Configure("ManagerReview")
               .Permit("RequestInfo", "RequestPromotionForm")
               .Permit("Deny", "PromotionDenied")
               .Permit("Approve", "VicePresidentApprove");

Clearly you will not show the code to your business partners or end users, but a simple chart like this should not make anyone’s eyes glaze over:

State: Request Promotion Form
  Trigger => Complete
  Target State => Manager Review

Before we move on you may want to study the test in the file SimpleStateless.cs. Here configuring the state machine and advancing from state to state is laid out for you:

//  Request Promo form states
statemachine.Configure("RequestPromotionForm")
                    .Permit("Complete", "ManagerReview");

//  Manager Review states
statemachine.Configure("ManagerReview")
                     .Permit("RequestInfo", "RequestPromotionForm")
                     .Permit("Deny", "PromotionDenied")
                     .Permit("Approve", "VicePresidentApprove");

//  Vice President state configuration
statemachine.Configure("VicePresidentApprove")
                      .Permit("ManagerJustify", "ManagerReview")
                      .Permit("Deny", "PromotionDenied")
                      .Permit("Approve", "Promoted");

//  Tests
Assert.AreEqual(startState, statemachine.State);

//  Move to next state
statemachine.Fire("Complete");
Assert.IsTrue(statemachine.IsInState("ManagerReview"));

statemachine.Fire("Deny");
Assert.IsTrue(statemachine.IsInState("PromotionDenied"));

The next question that comes to mind is how to represent the various States, Triggers and State configurations as data. Our mission on this project is to adhere to simplicity. One way to represent a Stateless state machine is with JSON:

{WorkflowType : "RequestPromotion",
  States : [{Name : "RequestPromotionForm" ; DisplayName : "Request Promotion Form"}
    {Name : "ManagerReview", DisplayName : "Manager Review"},
    {Name : "VicePresidentApprove", DisplayName : "Vice President Approve"},
    {Name : "PromotionDenied", DisplayName : "Promotion Denied"},
    {Name : "Promoted", DisplayName : "Promoted"}
    ],
  Triggers : [{Name : "Complete", DisplayName : "Complete"},
     {Name : "Approve", DisplayName : "Approve"},
     {Name : "RequestInfo", DisplayName : "Request Info"},
     {Name : "ManagerJustify", DisplayName : "Manager Justify"},
     {Name : "Deny", DisplayName : "Deny"}
  ],
StateConfigs : [{State : "RequestPromotionForm", Trigger : "Complete", TargetState : "ManagerReview"},
     {State : "ManagerReview", Trigger : "RequestInfo", TargetState : "RequestPromotionForm"},
     {State : "ManagerReview", Trigger : "Deny", TargetState : "PromotionDenied"},
     {State : "ManagerReview", Trigger : "Approve", TargetState : "VicePresidentApprove"},
     {State : "VicePresidentApprove", Trigger : "ManagerJustify", TargetState : "ManagerApprove"},
     {State : "VicePresidentApprove", Trigger : "Deny", TargetState : "PromotionDenied"},
     {State : "VicePresidentApprove", Trigger : "Approve", TargetState : "Promoted"}
  ]
}

As you can see we are storing all States and all Triggers with their display names. This will allow you some flexibility with UI screens and reports. Each rule for transitioning a state to another is stored in the StateConfigs node. Here we are simply representing our chart that we created above as JSON.

Since we have a standard way of representing a workflow with JSON de-serializing this definition to objects is straight forward. Here are the corresponding classes that define a state machine:

public class WorkflowDefinition
{
        public string WorkflowType { get; set; }
        public List States { get; set; }
        public List Triggers { get; set; }
        public List StateConfigs { get; set; }

        public WorkflowDefinition() { }
}

public class State
{
        public string Name { get; set; }
        public string DisplayName { get; set; }
}

public class Trigger
{
        public string Name { get; set; }
        public string DisplayName { get; set; }

        public Trigger() { }
}
public class StateConfig
{
        public string State { get; set; }
        public string Trigger { get; set; }
        public string TargetState { get; set; }

        public StateConfig() { }
}

We’ll close out this post with an example that will de-serialize our state machine definition and allow us to respond to the triggers that we supply. Basically it will be a rudimentary workflow. RequestionPromotion.cs will be the workflow processor. The method Configure is where we will perform the de-serialization, and the process is quite straight forward:

1. Deserialize the States
2. Deserialize the Triggers
3. Deserialize the StateConfigs that contain the transitions from state to state
4. For every StateConfig, configure the state machine.

Here’s the code:

public void Configure()
{
    Enforce.That((string.IsNullOrEmpty(source) == false),
                            "RequestPromotion.Configure - source is null");

    string json = GetJson(source);

    var workflowDefintion = JsonConvert.DeserializeObject(json);

    Enforce.That((string.IsNullOrEmpty(startState) == false),
                            "RequestPromotion.Configure - startStep is null");

    this.stateMachine = new StateMachine(startState);

    //  Get a distinct list of states with a trigger from state configuration
    //  "State => Trigger => TargetState
    var states = workflowDefintion.StateConfigs.AsQueryable()
                                    .Select(x => x.State)
                                    .Distinct()
                                    .Select(x => x)
                                    .ToList();

    //  Assing triggers to states
    states.ForEach(state =>
    {
        var triggers = workflowDefintion.StateConfigs.AsQueryable()
                                   .Where(config => config.State == state)
                                   .Select(config => new { Trigger = config.Trigger, TargeState = config.TargetState })
                                   .ToList();

        triggers.ForEach(trig =>
        {
            this.stateMachine.Configure(state).Permit(trig.Trigger, trig.TargeState);
        });
    });
}

And we advance the workflow with this method:

public void ProgressToNextState(string trigger)
{
Enforce.That((string.IsNullOrEmpty(trigger) == false),
"RequestPromotion.ProgressToNextState – trigger is null");

this.stateMachine.Fire(trigger);
}

The class RequestPromotionTests.cs illustrates how this works.

We we have seen how we can fulfill the objectives laid out for ApprovaFlow and have covered a significant part of the functionality that Stateless will provide for our workflow engine.   Here is the source code.