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ApprovaFlow: A Quick Primer On RavenDB December 15, 2011

Posted by ActiveEngine Sensei in .Net, .Net Development, ActiveEngine, Approvaflow, ASP.Net, Business Processes, JSON.Net, Open Source, RavenDB, Workflow.
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This post focuses on getting started with RavenDB, so we’ll set aside our focus on workflows for a bit.  It’s included in the ApprovaFlow series as it is an important part of the workflow framework we’re building.  To follow along you might want to get the source code.

RavenDB is a document database that provides a flexible means for storing object graphs.  As you’ll see a document database presents you with a different set of challenges than you are normally presented when using a traditional relational database.

The storage “unit” in RavenDB is a schema-less JSON document.  This takes the form of:  Because you are working with documents you now have the flexibility to define documents differently; that is, you can support variations to your data without have to re-craft your data model each time you want to add a new property to a class.  You can adopt a “star” pattern for SQL as depicted here, but querying can become difficult.  Raven excels in this situation and one such sweet spot is:

Dynamic Entities, such as user-customizable entities, entities with a large number of optional fields, etc. – Raven’s schema free nature means that you don’t have to fight a relational model to implement it.

Installing and Running RavenDB

The compiled binaries are easy to install.  Download the latest build and extract the files to a share.  Note that in order to run the console you are required to install Silverlight.  To start the server, navigate to the folder[] and double click “Start.cmd”.  You will see a screen similar to this one once the server is up and running:

The console will launch it self and will resemble this:

How To Start Developing

In Visual Studio, reference Raven with Raven.Client.Lightweight.  For CRUD operations and querying this will be all that you will need.

First you will need to connect to the document store.  It is recommended that you do this once per application.  That is accomplished with

var documentStore = new DocumentStore {Url = "http://localhost:8080"};
documentStore.Initialize();

Procedures are carried out using the Unit of Work pattern, and in general you will be using these type of blocks:

using(var session = documentStore.OpenSession())
{
   //... Do some work
}

RavenDB will work with Plain Old C# Objects and only requires an Id property of type string.  An identity key is generated for Id during this session.  If were were to create multiple steps we would have identities created in succession.  A full discussion of the alternatives to the Id property is here.

Creating a document from your POCOs’ object graphs is very straight forward:

public class Person
{
    public string FirstName { get; set; }
public string LastName { get; set; }
public string Id { get; set; }
public int DepartmentId { get; set; }
    // ...
}

var person = new Person();

using(var session = documentStore.OpenSession())
{
   session.Store(person);
   session.SaveChanges();
}

Fetching a document can be accomplished in two manners:  by Id or with a LINQ query.  Here’s how to get a document by id:

string person = "Person/1";  //  Raven will have auto-generated a value for us.
using(var session = documentStore.OpenSession())
{
   var fetchedPerson = session.Load<Person>(personId);
   //Do some more work
}

You’ll note that there is no casting or conversion required as Raven will determine the object type and populate the properties for you.

There are naturally cases where you want to query for documents based on attributes other than the Id. Best practices guides that we should create static indexes on our documents as these will offer the best performance. RavenDB also has a dynamic index feature that learns from queries fired at the server and over time these dynamic indexes are memorialized.

For your first bout with RavenDB you can simply query the documents with LINQ.   The test code takes advantage of the dynamic feature.  Later you will want to create indexes based on how you most likely will retrieve the documents.  This is different that a traditional RDMS solution, where the data is optimized for querying.  A document database is NOT.

Continuing with our example of Person documents we would use:

int departmentId = 139;

using(var session = documentStore.OpenSession())
{
   var people = session.Query<Person>()
                          .Where(x => x.DepartmentId == departmentId)
                          .ToList();
}

In the source code for this post there are more examples of querying.

Debugging, Troubleshooting and Dealing with Frustration

Given that this is something new and an open source project you may find yourself searching for help and more guidelines.  One thing to avail yourself of while troubleshooting is the fact that RavenDB has REST interface and you can validate your assumptions – or worse, confirm your errors – by using curl from the command line.  For example, to create a document via http you issue:

curl -X POST http://localhost:8080/docs -d "{ FirstName: 'Bob', LastName: 'Smith', Address: '5 Elm St' }"

Each action that takes place on the RavenDB server is displayed in a log on the server console app.  Sensei had to resort to this technique when troubleshooting some issues when he first started.  This StackOverflow question details the travails.

Another area that threw Sensei for a loop at first was the nature of the RavenDB writing and maintaining indexes.  In short, indexing is a background process, and Raven is designed to be “eventually consistent”.  That means that there can be a latency between when a change is submitted, saved, and indexed in the repository so that it can be fetched via queries.  When running tests from NUnit this code did not operate as expected, yet the console reported that the document was created:

session.Store(teamMember);

int posttestCount = session.Query<TeamMember>()
                .Count();

According to the documentation you can overcome this inconsistency by declaring that you are willing to wait until RavenDB has completed its current write operation.   This code will get you the expected results:

int posttestCount = session.Query<TeamMember>()
              .Customize(x => x.WaitForNonStaleResults())
              .Count();

Depending on the number of tests you write you may wish to run RavenDB in Embedded mode for faster results.  This might prove useful for automated testing and builds.  The source code provided in this post does NOT use embedded mode; rather, you have need your server running as this gives you the opportunity to inspect documents and acclimate yourself to the database.

There is much more that you can do with RavenDB, such as creating indexes across documents, assign security to individual documents, and much more.  This primer should be enough to get you started.  Next post we’ll see how RavenDB will fit into the ApprovaFlow framework.  Grab the source, play around and get ready for the next exciting episode.

 

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ApprovaFlow: Where We At? December 3, 2011

Posted by ActiveEngine Sensei in .Net, ActiveEngine, Approvaflow, ASP.Net, RavenDB, Workflow.
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It’s been a while, and as usual Sensei has started something with such bravado and discovered that life offers more bluster and pounding than even he can anticipate.  Hopefully you haven’t given up on the series, ’cause Sensei hasn’t.  Hell, ApprovaFlow is constantly on the forefront, even though it appears that he’s taken a good powder.

Let’s recap our goals and talk about philosophy and direction.  In this long silence a few additional considerations have taken precedence, and this is a good opportunity to assess goals and re-align the development efforts.  In the end ApprovaFlow will:

 Model a workflow in a clear format that is readable by both developer and business user. One set of verbiage for all parties.  Discussed in Simple Workflows With ApprovaFlow and Stateless.

  Allow the state of a workflow to be peristed as an integer, string, etc.  Quickly fetch state of a workflow.  Discussed in Simple Workflows With ApprovaFlow and Stateless.

 Create pre and post processing methods that can enforce enforce rules or carry out actions when completing a workflow task.  Discussed inApprovaFlow:  Using the Pipe and Filter Pattern to Build a Workflow Processor

• 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 astute members of the audience will no doubt say “What about the technical objectives, like how are you going to store all the workflow data?  In flat files?  Will you give me alternatives for storage?  How will I create the workflows, by using Notepad?” Indeed, Sensei has pondered these issues as well and has accumulated a fair amount failed experiments with some being quite interesting.   Given time these little experiments may become posts as well, since there are interesting things to learn from these failures.

What ApprovaFlow Will Need To Provide:  Workflow Storage

The biggest issue is storage.  The point of using Stateless was that we wanted flexibility.  Recall that the state of our state machine can be represented with a mere integer or string.  Makes it pretty easy to store this in a database, or a document.  While you could map the Step and Workflow class to tables in SQL our domain is using JSON so it makes sense to gravitate to a storage solution that will easily support that format.  ApprovaFlow will use RavenDB as the document database, but will provide the opportunity for you to use a different solution if you wish.  You’ll find that RavenDB quite readily provides a document storage format for our workflows that is quite elegant.

As an aside, Sensei experimented with a great alternative to the NoSQL solutions called Sis0DB.  This open project provides you that ability to store you object graphs in SQL Server.  Time permitting Sensei will share some of his adventures with you regarding this neat project.

What ApprovaFlow Will Need to Provide:  Authorization of Actions

While Sensei was off in the weeds learning about RavenDB he discovered that Ayende created a fantastic mechanism for authorizing user actions on documents.  This authorization of activity can be a granular as denying / allowing updates to occur based on an operation.

Since we want to adhere to principles of flexibility the Authorization features will be implemented as a plug-in, so if you wish to roll your own mechanisms to govern workflow approvals you will be free to do so.

What ApprovaFlow Will Need to Provide:  Admin Tools

Yep.  Sensei is sick of using Notepad to create JSON documents as well.  We want to be able to create the states, the triggers and the target states and save.  We’ll want to assign the filters to specific states and save.  No more text fiddling. Period.  As Sensei is thinking about this, it seems that another pipeline can be created for administration.  Luckily we have a plug-in architecture so this should be rather straight forward.

Summing It All Up

These are really important things to consider, and as much as Sensei hates changing goals in mid stream the capabilities discussed above can make life much easier while implementing a workflow system.  In making the decision to use RavenDB the thought that “a storage solution should not shape the solution domain” kept raising its ugly maw.  But, so what.   We want to finish something, and admittedly this has been a challenge – just look at the lag between posts if you need a reminder.  If Sensei decided to include an IOC container just to remain “loosely” coupled to document storage we’ll get no where.  Would you really want to read those posts?  How boring.  Besides, Sensei doesn’t know how to do all that stuf – gonna stick to the stuff he thinks he knows.  Or at least the stuff he can fake.

ApprovaFlow: Create A Plugin System And Reduce Deployment Headaches June 29, 2011

Posted by ActiveEngine Sensei in .Net, ActiveEngine, Approvaflow, ASP.Net, Problem Solving, Workflow.
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This is the fourth 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 on ApprovaFlow

In the previous post we discussed how the Pipe and Filter pattern facilitated a robust mechanism for executing tasks prior and after a transition is completed by the workflow state machine.  This accomplished our third goal and to date we have completed:

Model a workflow in a clear format that is readable by both developer and business user. One set of verbiage for all parties.  Discussed in Simple Workflows With ApprovaFlow and Stateless.

•  Allow the state of a workflow to be peristed as an integer, string, etc.  Quickly fetch state of a workflow.  Discussed in Simple Workflows With ApprovaFlow and Stateless.

•  Create pre and post processing methods that can enforce enforce rules or carry out actions when completing a workflow task.  Discussed in ApprovaFlow:  Using the Pipe and Filter Pattern to Build a Workflow Processor

These goals remain:

• 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.

It’s the Small Changes After You Go Live That Upset You

The goal we’ll focus on next is Introduce new functionality while isolating the impact of the new changes. New components should not break old ones, as it’s the small upsetters that lurk around the corner that your users will think up that will keep you in the constant redeployment cycle. If we implement a plug-in system, then we can prevent the new features from breaking the current production system. Implementing these changes in isolation will lead to faster testing, validation and happier users.

We lucked out as our implementation of the Pipe And Filter pattern forced us to create objects with finite functionality.  If you recall each step in our workflow chain was implemented as a filter derived from FilterBase and this lends itself nicely to creating plug-ins.  The Pipe and Filter pattern forces us to have a filter for each unique action we wish to carry out.  To save data we have a SaveData filter, to validate that a user can supply a Trigger we have the ValidateUserTrigger, and so on.

“Great, Sensei, but aren’t we still constrained by the fact that we have to recompile and deploy any time we add new filters?  And, if I have to do that, why bother with the pattern in the first place?”

Well, we can easily reduce the need for re-deploying the application through the use of a plugin system where we read assemblies from a share and interrogate them by searching for a particular object type on application start up.  Each new feature will be a new filter.  This means you will be working with a small project that references ApprovaFlow to create new filters without disturbing the existing architecture.   We’ll also create a manifest of approved plug-ins so that we can control what is used and institute a little security since we wouldn’t want any plugin to be introduced surreptitiously.

Plug-in Implementation

The class FilterRegistry will perform the process of reading a share, fetching the object with type FilterBase, and register these components just like we do with our system components.  There are a few additions since the last version, as we now need to read and store the manifest for later comparison with the plug-ins.  The new method ReadManifest takes care of this new task:

private void ReadManifest()
{
  string manifestSource = ConfigurationManager.AppSettings["ManifestSource"].ToString();

  Enforce.That(string.IsNullOrEmpty(manifestSource) == false,
          "FilterRegistry.ReadManifest - ManifestSource can not be null");

  var fileInfo = new FileInfo(manifestSource);

  if (fileInfo.Exists == false)
  {
    throw new ApplicationException("RequestPromotion.Configure - File not found");
  }

  StreamReader sr = fileInfo.OpenText();
  string json = sr.ReadToEnd();
  sr.Close();

  this.approvedFilters = JsonConvert.DeserializeObject>>(json);
}

The manifest is merely a serialized list of FilterDefinitions. This is de-serialized into a list of approved filters.With the approved list the method LoadPlugin performs the action of reading the share and matching the FullName of the object type between the manifest entries and the methods in the assembly file:

public void LoadPlugIn(string source)
{
  Enforce.That(string.IsNullOrEmpty(source) == false,
             "PlugInLoader.Load - source can not be null");

  AppDomain appDomain = AppDomain.CurrentDomain;
  var assembly = Assembly.LoadFrom(source);

  var types = assembly.GetTypes().ToList();

  types.ForEach(type =>
  {
    var registerFilterDef = new FilterDefinition();

    //  Is type from assembly registered?
    registerFilterDef = this.approvedFilters.Where(app => app.TypeFullName == type.FullName)
                                   .SingleOrDefault();

    if (registerFilterDef != null)
    {
      object obj = Activator.CreateInstance(type);
      var filterDef = new FilterDefinition();
      filterDef.Name = obj.ToString();
      filterDef.FilterCategory = registerFilterDef.FilterCategory;
      filterDef.FilterType = type;
      filterDef.TypeFullName = type.FullName;
      filterDef.Filter = AddCreateFilter(filterDef);

      this.systemFilters.Add(filterDef);
     }
  });
}

That’s it. We can now control what assemblies are included in our plug-in system.  Later we’ll create a tool that will help us create the manifest so we do not have to managed it by hand.

What We Can Do with this New Functionality

Let’s turn to our sample workflow to see what possibilities we can develop.  The test CanPromoteRedShirtOffLandingParty from the class WorkflowScenarios displays the capability of our workflow.  First lets review our workflow scenario.  We have created a workflow for the Starship Enterprise to allow members of a landing party to request to be left out of the mission.  Basically there is only one way to get out of landing party duty and that is if Kirk says it’s okay.  Here are the workflow’s State, Trigger and Target State combinations:

State Trigger Target State
RequestPromotionForm Complete FirstOfficerReview
FirstOfficerReview RequestInfo RequestPromotionForm
FirstOfficerReview Deny PromotionDenied
FirstOfficerReview Approve CaptainApproval
CaptainApproval OfficerJustify FirstOfficerReview
CaptainApproval Deny PromotionDenied
CaptainApproval Approve PromotedOffLandingParty

Recalling the plots from Star Trek, there were times that the medical officer could declare the commanding officer unfit for duty. Since the Enterprise was originally equipped with our workflow, we want to make just a small addition – not a modification – and give McCoy the ability to allow a red shirt to opt out of the landing party duty.

Here’s where our plugin system comes in handy.  Instead of adding more states and or branches to our workflow we’ll check for certain conditions when Kirk makes his decisions, and execute actions.  In order to help out McCoy the following filter is created in a separate project:

public class CaptainUnfitForCommandFilter : FilterBase
{
  protected override Step Process(Step input)
  {
    if(input.CanProcess & input.State == "CaptainApproval")
    {
      bool kirkInfected = (bool)input.Parameters["KirkInfected"];

      if(kirkInfected & input.Answer == "Deny")
      {
        input.Parameters.Add("MedicalOverride", true);
        input.Parameters.Add("StarfleetEmail", true);
        input.ErrorList.Add("Medical Override of Command");
        input.CanProcess = false;
      }
    }

    return input;
  }
}

This plug-in is simple: check that the state is CaptainApproval and when the answer was “Deny” and Kirk has been infected, set the MedicalOverride flag and send Starfleet an email.

The class WorkflowScenarioTest.cs has the method CanAllowMcCoyToIssueUnfitForDuty() that demonstrates how the workflow will execute. We simply add the name of the plug-in to our list of post transition filters:

string postFilterNames = "MorePlugins.TransporterRepairFilter;Plugins.CaptainUnfitForCommandFilter;SaveDataFilter;";

This portion of code uses the plug-in:

//  Captain Kirt denies request, but McCoy issues unfit for command
parameters.Add("KirkInfected", true);

step.Answer = "Deny";
step.AnsweredBy = "Kirk";
step.Participants = "Kirk";
step.State = newState;

processor = new WorkflowProcessor(step, filterRegistry, workflow);
newState = processor.ConfigurePipeline(preFilterNames, postFilterNames)
  .ConfigureStateMachine()
  .ProcessAnswer()
  .GetCurrentState();

//  Medical override issued and email to Starfleet generated
bool medicalOverride = (bool)parameters["MedicalOverride"];
bool emailSent = (bool)parameters["StarfleetEmail"];

Assert.IsTrue(medicalOverride);
Assert.IsTrue(emailSent);

Now you don’t have to hesitate with paranoia each time you need introduce a variation into your workflows. No more small upsetters lurking around the corner. Plus you can deliver these changes faster to your biggest fan, your customer. Source code is here.   Run through the tests and experiment for your self.

ApprovaFlow: Using the Pipe and Filter Pattern to Build a Workflow Processor May 17, 2011

Posted by ActiveEngine Sensei in .Net, ActiveEngine, Approvaflow, ASP.Net, C#, Stateless, Workflow.
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This is the third entry 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.

What We’ve Accomplished Thus Far

In the last post we discussed how Stateless makes creating a lean workflow engine possible, and we saw that we were able to achieve two of our overall goals for ApprovaFlow.  Here’s what we accomplished:

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, etc.  Quickly fetch state of a workflow.

So we have these goals left:

•. Create pre and post processing 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.

Our next goal will be Create pre and post processing methods that can enforce enforce rules or carry out actions when completing a workflow task.  We’ll use the Pipe and Filter Pattern to simplify the processing, and we’ll see that this approach not only streamlines how you handle variation in tasks, but also provides a clean method for extending our application abilities.


The advantage of breaking down the activities of a process is that you can create a series of inter-changeable actions.  There may be some cases where you want to re-order the order of operations at runtime and you can do so easily when the actions are individual components.

Before we proceed applying the Pipe and Filter pattern to our solution, we need to establish some nomenclature for our workflow processing.  The following chart lays out the vocab we’ll use for the rest of series.

Term Definition
State A stage of a workflow.
Trigger A message that tells the workflow how to change states.  If the state is “Phone Ringing” and the trigger is “Answer Phone” the new state for the phone would be “Off hook”.
StateConfig A StateConfig defines a pathway or transition from one state to another.  It is comprised of a State, the Trigger and the Target State.
Step A Step contains the workflow’s current State.  In the course of your workflow you may have many of the same type of steps differentiated by date and time.  In other words, when you workflow has looping capability, the workflow step for a state may be issued many times.
Answer The Step asks a question, waiting for the user response.  The answer the user provides is the trigger that will start the transition from one state to another.  The Answer becomes the Trigger that will change the State.
Workflow A series of Steps compromised of States, Triggers and their respective transition expressed as a series of State Configs.  Think of this as a definition of a process.
Workflow Instance The Workflow Instance is a running workflow.  The Steps of the Workflow Instance are governed by how the Steps are defined by a Workflow.

Essentially a framework for providing an extensible workflow system boils down to answering the following questions asked in this order:
• Is the user authorized to provide an Answer to trigger a change to the step’s State?
• Is a special data set required for this particular State that is not part of the Step properties?
• Is the data provided from the user sufficient / valid for triggering a transition in the Workflow Step’s State?
• Are there actions to be performed such as saving special data?
• Can the system execute custom actions based on the State’s Trigger?

This looks very similar to the Pipe and Filter pattern.  Every time a workflow processes a trigger, the questions we asked above must be answered.  Each question could be considered a filter in the pipe and filter scenario.

The five questions above become the basis for our workflow processor components.  For this post we’ll assume that all data will be simply fetched then saved with no special processing.  We’ll also assume that a Workflow Step is considered to be valid when the following elements are correctly supplied:

public bool IsValidForWorkflowTransition()
{
  return this.Enforce("Step", true)
                 .When("AnsweredBy", Janga.Validation.Compare.NotEqual, string.Empty)
                 .When("Answer", Janga.Validation.Compare.NotEqual, string.Empty)
                 .When("State", Janga.Validation.Compare.NotEqual, string.Empty)
                 .When("WorkflowInstanceId", Janga.Validation.Compare.NotEqual, string.Empty)
                 .IsValid;
}

public bool IsUserValidParticipant()
{
  return this.Enforce("Step", true)
    .When("Participants", Janga.Validation.Compare.Contains, this.AnsweredBy)
    .IsValid;
}

Our Workflow Processor will function in accordance with the Pipe and Filter pattern where no matter what type of workflow instance we wish to process, the questions that we listed above will be answered.  Later we will return to discuss points of where the workflow can execute actions respective to the workflow’s definition.

Workflow Processor Code In Depth

Well, how do we configure a Workflow Processor?  In other words, we want to process an actual workflow, but how will we know the workflow type and what to do?  Some of configuration steps were previewed in Simple Workflows With ApprovaFlow and Stateless and the same principles apply here with the Configure method.  Collect the States, the Triggers and the StateConfigs, load them into Stateless along with the current state and you are ready to accept or reject the Trigger for the next State.  The Workflow Processor will conduct these steps and here is the code:

public WorkflowProcessor ConfigureStateMachine()
        {
            Enforce.That(string.IsNullOrEmpty(this.step.State) == false,
                            "WorkflowProcessor.Confgiure - step.State can not be empty");

            this.stateMachine = new StateMachine(this.step.State);

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

            //  Assing triggers to states
            states.ForEach(state =>
            {
                var triggers = this.workflow.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);
                });
            });

            return this;
        }

The Workflow Processor will need to know the current state of a workflow instance, the answer supplied, who supplied the answer, as well as any parameters that the filters will need fetching special data. This will be contained in the class Step.cs:

#region Properties

        public string WorkflowInstanceId { get; set; }
        public string WorkflowId { get; set; }
        public string StepId { get; set; }
        public string State { get; set; }
        public string PreviousState { get; set; }
        public string Answer { get; set; }
        public DateTime Created { get; set; }
        public string AnsweredBy { get; set; }
        public string Participants { get; set; }

        public List ErrorList;
        public bool CanProcess { get; set; }
        public IDictionary Parameters { get; set; }

        #endregion

        #region Constructors

        public Step()
            : this(string.Empty, string.Empty, string.Empty, string.Empty,
                   string.Empty, new DateTime(), string.Empty, string.Empty,
                    new Dictionary())
        { }

        public Step(string workflowInstanceId, string stepId, string state, string previousState,
                        string answer, DateTime created, string answeredBy, string participants,
                        Dictionary parameters)
        {
            this.WorkflowInstanceId = workflowInstanceId;
            this.StepId = stepId;
            this.State = state;
            this.PreviousState = previousState;
            this.Answer = answer;
            this.Created = created;
            this.AnsweredBy = answeredBy;
            this.Participants = participants;

            this.ErrorList = new List();
            this.Parameters = parameters;
        }

        #endregion

Our goal with the Workflow Processor is to accept the users answer, process actions, and create the next Step base on the new State all in one pass.  We will create a pipeline of actions that will always be invoked.  Each action or “filter” will be a component that performs and individual task, such as determining if the step is answered by the correct user.  Each filter will point to the subsequent filter in the pipeline, and the succession of the filters can change easily if we see fit.  All that is needed is to add the filters to the pipeline in the order we want.  Here is the class schema for the Pipe and Filter processing:

We’ll quickly find that the information regarding whether the result of an action or the condition of a Step will need to be accessible to each of the filters.  The class Step is the natural place to store this information, so we will include a property CanProcess to indicate that a filter should be invoked, as well a List<string> to act as an error log.  This log can be passed back to the client to communicate any errors to the user.  Note that the Step class has the Dictionary property named “Parameters” that allows a filter to pass data on to next filter in the sequence.

Setting Up the Pipeline

The sequence of filter execution is controlled by the order that the filters are registered.  The class Pipeline is responsible for registering and executing the chain of filters.  Here is the method Register that accepts a filter and retains it for future processing:

We also record the name of the filter so that we may interrogate the pipeline should we want to know if a filter has already been registered.

Pipeline.Register returns a reference to itself, so we can chain together commands fluently:

pipeline.Register(new ValidParticipantFilter())
                      .Register(new SaveDataFilter());

The class FilterBase is the foundation of our filter components.  As stated earlier, each component will point the subsequent filter in the filter chain.  You’ll note that the class also has a Register method.  This takes on the task of point the current filter to the next, and this method is called by the Pipeline as it registers all of the filters. Here is FilterBase:

public abstract class FilterBase : IFilter
{
  private IFilter next;

  protected abstract T Process(T input);

  public T Execute(T input)
  {
    T val = Process(input);

     if (this.next != null)
    {
      val = this.next.Execute(val);
    }

    return val;
  }

  public void Register(IFilter filter)
  {
    if (this.next == null)
    {
      this.next = filter;
    }
    else
    {
      this.next.Register(filter);
    }
  }
}

The method Execute accepts input of type T, and in the Workflow Processor instance this will Step.  Basically the Execute method is a wrapper, as we call the abstract method Process.  Process will be overridden in each filter, and this will contain the logic specific to the tasks that will be performed.  The code for a filter is quite simple:

public class ValidParticipantFilter : FilterBase
{
  protected override Step Process(Step input)
  {
    if (input.CanProcess)
    {
      input.Parameters["ValidFired"] = true;
      input.Parameters["FilterOrder"] += "ValidParticipantFilter;";

      input.CanProcess = input.IsUserValidParticipant();

    if(input.CanProcess == false)
    {
      input.ErrorList.Add("Invalid Pariticipant - " + input.AnsweredBy);
    }
  }

  return input;
}

Here we check to see if we can process, then perform specific actions if appropriate.  Given that the filters have no knowledge of each other, we can see that they can be executed in any order.  In other words you could have a Pipeline that had filters Step1, Step2, Step3 and you could configure a different pipeline to execute Step3, Step1, and Step2.

FilterRegistry Organizes Your Filters

Because we want to be able to use our filters in different successions we’ll need to keep a registry of what is available to use and provide the ability to look up or query different filters depending on our processing needs.  This registry will be created on application start up and will contain all objects of type FilterBase.  Later we’ll add the ability for the registry to load assemblies from a share, so that you can add other filters as simple plugins.  Information about each filter retained in a class FilterDefinition, and the FilterRegistry is merely a glorified List of the FilterDefintions. When we want to create a pipeline of filters we will want to instantiate new copies. Using Expressions we can create Functions that will be stored with with our definition for each filter type.  Here is FilterDefinition:

public class FilterDefinition
{
  public string Name { get; set; }
  public string FilterCategory { get; set; }
  public Type FilterType { get; set; }
  public Func> Filter{get; set;}

  public FilterDefinition() { }
}

We’ll invoke the compiled delegate at runtime to create our filter.  The method AddCreateFilter handles this:

private Func> AddCreateFilter(FilterDefinition filterDef)
        {
            var body = Expression.MemberInit(Expression.New(filterDef.FilterType));
            return Expression.Lambda>>(body, null).Compile();
        }

FilterRegistry is meant to be run once at start up so that all filters are registered and ready to use. You can imagine how slow it could become if every time you process a Workflow Step that you must interrogate all the assemblies.

Once you FilterRegistry has all assemblies registered you can query and create new combinations with the method GetFilters:

public IEnumerable> GetFilters(string filterNames)
        {
            Enforce.That(string.IsNullOrEmpty(filterNames) == false,
                            "FilterRegistry.GetFilters - filterNames can not be null");

            var returnFilters = new List>();
            var names = filterNames.Split(';').ToList();

            names.ForEach(name =>
            {
                var filter = this.filters.Where(x => x.Name == name)
                                           .SingleOrDefault();

                if (filter != null)
                {
                    returnFilters.Add(filter.Filter.Invoke());
                }
            });

            return returnFilters;
        }

Pipeline can accept a list of filters along with the string that represents the order of execution.  The method RegisterFrom accepts a reference to the FilterRegistry along with the names of the filters you want to use.

In the case of the Workflow Processor, we need to divide our filters into pre-trigger and post-trigger activities. Referring back to our 5 questions that our processor asks, question 1 – 3 must be answered before we attempt to transition the Workflow State, while steps 4-5 must be answered after the transition has succeeded. The method ConfigurePipeline in WorkflowProcessor.cs accomplishes this task:

public WorkflowProcessor ConfigurePipeline(string preProcessFilterNames, string postProcessFilterNames)
        {
            Enforce.That(string.IsNullOrEmpty(preProcessFilterNames) == false,
                            "WorkflowProcessor.Configure - preProcessFilterNames can not be null");

            Enforce.That(string.IsNullOrEmpty(postProcessFilterNames) == false,
                            "WorkflowProcessor.Configure - postProcessFilterNames can not be null");

            var actionWrapper = new ActionWrapperFilter(this.ExecuteTriggerFilter);

            this.pipeline.RegisterFromList(preProcessFilterNames, this.filterRegistry)
                            .Register(actionWrapper)
                            .RegisterFromList(postProcessFilterNames, this.filterRegistry);

            return this;
        }

Putting It all Together

A lot of talk and theory, so how does this all fit together?  The test class WorkflowScenarioTests illustrates how our processor works.  We are creating a workflow that implements the process for a Red Shirt requesting a promotion off a landing party.  You may recall that the dude wearing the red shirt usually got killed with in the first few minutes of Star Trek, so this workflow will help those poor saps get off the death list.  The configuration for the Workflow is contained within the file RedShirtPromotion.json.  There are a few simple rules that we want to enforce with the Workflow.  For one, Spock must review the Red Shirt request, but Kirk will have the final say.

Here is a sample from the class WorkflowScenarioTests.cs:

  string source = @"F:\vs10dev\ApprovaFlowSimpleWorkflowProcessor\TestSuite\TestData\RedShirtPromotion.json";
            string preFilterNames = "FetchDataFilter;ValidParticipantFilter;";
            string postFilterNames = "SaveDataFilter";

            var workflow = DeserializeWorkflow(source);

            var parameters = new Dictionary();
            parameters.Add("FilterOrder", string.Empty);
            parameters.Add("FetchDataFired", false);
            parameters.Add("SaveDataFired", false);
            parameters.Add("ValidFired", false);

            var step = new Step("13", "12", "RequestPromotionForm", "",
                                    "Complete", DateTime.Now, "RedShirtGuy", "Data;RedShirtGuy",
                                    parameters);
            step.CanProcess = true;

            var filterRegistry = new FilterRegistry();

            var processor = new WorkflowProcessor(step, filterRegistry, workflow);
            string newState = processor.ConfigurePipeline(preFilterNames, postFilterNames)
                        .ConfigureStateMachine()
                        .ProcessAnswer()
                        .GetCurrentState();

            Assert.AreEqual("FirstOfficerReview", newState);

Study the tests.  We’ve covered a lot together and admittedly there is a lot swallow in this post.  In our next episode we’ll look at how to the Pipe and Filter pattern can help us with extending our workflow processor’s capability without causing us a lot of pain.  Here’s the source code.  Enjoy and check back soon for our next installment.  Sensei will let you take it on out with this groovy theme (click play).

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