Journey 4: Extending and Enhancing the Orders and Registrations Bounded Context

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Changes to the bounded context | Patterns and concepts | Implementation details | Impact on testing | A journey into code comprehension | Summary

Further exploration of the Orders and Registrations bounded context.

"I see that it is by no means useless to travel, if a man wants to see something new." Jules Verne, Around the World in Eighty Days

Changes to the bounded context

The previous chapter described the Orders and Registrations bounded context in some detail. This chapter describes some changes that the team made in this bounded context during the second stage of our CQRS journey.

The specific topics described in this chapter include:

• Improvements to the way message correlation works with the RegistrationProcessManager class. This illustrates how aggregate instances within the bounded context can interact in a complex manner.
• Implementing a record locator to enable a registrant to retrieve an order that she saved during a previous session. This illustrates adding some additional logic to the write side that enables you to locate an aggregate instance without knowing its unique ID.
• Adding a countdown timer to the UI to enable a registrant to track how much longer they have to complete an order. This illustrates enhancements to the write side to support the display of rich information in the UI.
• Supporting orders for multiple seat types simultaneously. For example, a registrant requests five seats for a preconference event and eight seats for the full conference. This requires more complex business logic on the write side.
• CQRS command validation. This illustrates how to make use of the model validation feature in MVC to validate your CQRS commands before you send them to the domain.

The Contoso Conference Management System described in this chapter is not the final version of the system. This guidance describes a journey, so some of the design decisions and implementation details change in later steps in the journey. These changes are described in subsequent chapters.

Working definitions for this chapter

This chapter uses a number of terms, which we will describe next. For more detail, and possible alternative definitions, see Chapter 4, "A CQRS and ES Deep Dive," in the Reference Guide.

Command.A command is a request for the system to perform an action that changes the state of the system. Commands are imperatives; for example, MakeSeatReservation. In this bounded context, commands originate from either the UI as a result of a user initiating a request, or from a process manager when the process manager is directing an aggregate to perform an action.

A single recipient processes a command. A command bus transports commands that command handlers then dispatch to aggregates. Sending a command is an asynchronous operation with no return value.

Event. An event, such as OrderConfirmed, describes something that has happened in the system, typically as a result of a command. Aggregates in the domain model raise events.

Multiple subscribers can handle a specific event. Aggregates publish events to an event bus; handlers register for specific types of events on the event bus and then deliver the events to the subscriber. In this bounded context, the only subscriber is a process manager.

Process manager. In this bounded context, a process manager is a class that coordinates the behavior of the aggregates in the domain. A process manager subscribes to the events that the aggregates raise, and then follows a simple set of rules to determine which command or commands to send. The process manager does not contain any business logic, only logic to determine the next command to send. The process manager is implemented as a state machine, so when the process manager responds to an event, it can change its internal state in addition to sending a new command.

The process manager in this bounded context can receive commands as well as subscribe to events.

Our process manager is an implementation of the Process Manager pattern defined on pages 312 to 321 in the book Enterprise Integration Patterns: Designing, Building, and Deploying Messaging Solutions by Gregor Hohpe and Bobby Woolf (Addison-Wesley Professional, 2003).

User stories

This chapter discusses the implementation of two user stories in addition to describing some changes and enhancements to the Orders and Registrations bounded context.

Implement a login using a record locator

When a registrant creates an order for seats at a conference, the system generates a five-character order access code and sends it to the registrant by email. The registrant can use her email address and the order access code on the conference web site as a record locator to retrieve the order from the system at a later date. The registrant may wish to retrieve the order to review it, or to complete the registration process by assigning attendees to seats.

Carlos Says:
	From the business perspective it was important for us to be as user-friendly as possible: we don't want to block or unnecessarily burden anyone who is trying to register for a conference. Therefore, we have no requirement for a user to create an account in the system prior to registration, especially since users must enter most of their information in a standard checkout process anyway.</td>

Tell the registrant how much time remains to complete an order

When a registrant creates an order, the system reserves the seats requested by the registrant until the order is complete or the reservations expire. To complete an order, the registrant must submit her details, such as name and email address, and make a successful payment.

To help the registrant, the system displays a countdown timer to inform her how much time remains to complete the order before the seat reservations expire.

Enable a registrant to create an order that includes multiple seat types

When a registrant creates an order, she may request different numbers of different seat types. For example, a registrant may request five seats for the full conference and three seats for the preconference workshop.

Architecture

The application is designed to deploy to MicrosoftAzure. At this stage in the journey, the application consists of a web role that contains the ASP.NET MVC web application and a worker role that contains the message handlers and domain objects. The application uses Azure SQL Database (SQL Database) instances for data storage, both on the write side and the read side. The application uses the Azure Service Bus to provide its messaging infrastructure. Figure 1 shows this high-level architecture.

Figure 1

Contoso Conference Management System high-level architecture

While you are exploring and testing the solution, you can run it locally, either using the Azure compute emulator or by running the MVC web application directly and running a console application that hosts the handlers and domain objects. When you run the application locally, you can use a local SQL Server Express database instead of SQL Database, and use a simple messaging infrastructure implemented in a SQL Server Express database.

For more information about the options for running the application, see Appendix 1, "Release Notes."

Patterns and concepts

This section describes some of the key areas of the application that the team visited during this stage of the journey and introduces some of the challenges met by the team when we addressed these areas.

Record locators

The system uses access codes instead of passwords so the registrant is not forced to set up an account with the system. Many registrants may use the system only once, so there is no need to create a permanent account with a user ID and a password.

The system needs to be able to retrieve order information quickly based on the registrant's email address and access code. To provide a minimum level of security, the access codes that the system generates should not be predictable, and the order information that registrants can retrieve should not contain any sensitive information.

Querying the read side

The previous chapter focused on the write-side model and implementation; in this chapter we'll explore the read-side implementation in more detail. In particular, we'll explain how we implemented the read model and the querying mechanism from the MVC controllers.

In this initial exploration of the CQRS pattern, the team decided to use SQL views in the database as the underlying source of the data queried by the MVC controllers on the read side. To minimize the work that the queries on the read side must perform, these SQL views provide a denormalized version of the data. These views currently exist in the same database as the normalized tables that the write model uses.

Jana Says:
	The team will split the database into two and explore options for pushing changes from the normalized write side to the denormalized read side in a later stage of the journey. For an example of using Azure blob storage instead of SQL tables for storing the read-side data, see the <strong>SeatAssignmentsViewModelGenerator</strong> class.</td>

Storing denormalized views in a database

One common option for storing the read-side data is to use a set of relational database tables to hold the denormalized views. You should optimize the read side for fast reads, so there is typically no benefit in storing normalized data because this will require complex queries to construct the data for the client. This implies that goals for the read side should be to keep the queries as simple as possible, and to structure the tables in the database in such a way that they can be read quickly and efficiently.

Gary Says:
	Application scalability and a responsive UI are often explicit goals when people choose to implement the CQRS pattern. Optimizing the read side to provide fast responses to queries while keeping resource utilization low will help you to achieve these goals.</td>

Jana Says:
	A normalized database schema can fail to provide adequate response times because of the excessive table JOIN operations. Despite advances in relational database technology, a JOIN operation is still very expensive compared to a single-table read.</td>

An important area for consideration is the interface whereby a client such as an MVC controller action submits a query to the read-side model.

Figure 2

The read side storing data in a relational database

In Figure 2, a client, such as an MVC controller action, invokes a method on the ViewRepository class to request the data it needs. The ViewRepository class in turn runs a query against the denormalized data in the database.

Jana Says:
	The Repository pattern mediates between the domain and data mapping layers using a collection-like interface for accessing domain objects. For more info see Martin Fowler, Catalog of Patterns of Enterprise Application Architecture, <a href="http://martinfowler.com/eaacatalog/repository.html">Repository</a>.</td>

The team at Contoso evaluated two approaches to implementing the ViewRepository class: using the IQueryable interface and using non-generic data access objects (DAOs).

Using the IQueryable interface

One approach to consider for the ViewRepository class is to have it return an IQueryable instance that enables the client to use language-integrated query (LINQ) to specify its query. It is very easy to return an IQueryable instance from many ORMs such as Entity Framework or NHibernate. The following code snippet illustrates how the client can submit such queries.

var ordersummary = repository.Query<OrderSummary>().Where(LINQ query to retrieve order summary);
var orderdetails = repository.Query<OrderDetails>().Where(LINQ query to retrieve order details);

This approach has a number of advantages:

Simplicity

• This approach uses a thin abstraction layer over the underlying database. Many ORMs support this approach and it minimizes the amount of code that you must write.
• You only need to define a single repository and a single Query method.
• You don't need a separate query object. On the read side, the queries should be simple because you have already denormalized the data from the write side to support the read-side clients.
• You can make use of Language-Integrated Query (LINQ) to provide support for features such as filtering, paging, and sorting on the client.

Testability

• You can use LINQ to Objects for mocking.

Markus Says:
	In the RI, using Entity Framework, we didn't need to write any code at all to expose the IQueryable instance. We also had just a single ViewRepository class.

There are possible objections to this approach including that:

• It is not easy to replace the data store with a non-relational database that does not expose an IQueryable object. However, you can choose to implement the read model differently in each bounded context using an approach that is appropriate to that bounded context.
• The client might abuse the IQueryable interface by performing operations that can be done more efficiently as a part of the denormalization process. You should ensure that the denormalized data fully meets the requirements of the clients.
• Using the IQueryable interface hides the queries away. However, since you denormalize the data from the write side, the queries against the relational database tables are unlikely to be complex.
• It's hard to know if your integration tests cover all the different uses of the Query method.

Using non-generic DAOs

An alternative approach is to have the ViewRepository expose custom Find and Get methods, as shown in the following code snippets.

var ordersummary = dao.FindAllSummarizedOrders(userId);
var orderdetails = dao.GetOrderDetails(orderId);

You could also choose to use different DAO classes. This would make it easier to access different data sources.

var ordersummary = OrderSummaryDAO.FindAll(userId);
var orderdetails = OrderDetailsDAO.Get(orderId);

This approach has a number of advantages:

Simplicity

• Dependencies are clearer for the client. For example, the client references an explicit IOrderSummaryDAO instance rather than a generic IViewRepository instance.
• For the majority of queries, there are only one or two predefined ways to access the object. Different queries typically return different projections.

Flexibility

• The Get and Find methods hide details such as the partitioning of the data store and the data access methods such as an object relational mapping (ORM) or executing SQL code explicitly. This makes it easier to change these choices in the future.
• The Get and Find methods could use an ORM, LINQ, and the IQueryable interface behind the scenes to get the data from the data store. This is a choice that you could make on a method-by-method basis.

Performance

• You can easily optimize the queries that the Find and Get methods run.
• The data access layer executes all queries. There is no risk that the client MVC controller action tries to run complex and inefficient LINQ queries against the data source.

Testability

• It is easier to specify unit tests for the Find and Get methods than to create suitable unit tests for the range of possible LINQ queries that a client could specify.

Maintainability

• All of the queries are defined in the same location, the DAO classes, making it easier to modify the system consistently.

Possible objections to this approach include:

• Using the IQueryable interface makes it much easier to use grids that support features such as paging, filtering, and sorting in the UI. However, if the developers are aware of this downside and are committed to delivering a task-based UI, then this should not be an issue.

The team decided to adopt the second approach because of the clarity it brings to the code; in this context, we did not see any significant advantage in the simplicity of the first approach. For examples, see the ConferenceDao and OrderDao classes in the Registration project.

Making information about partially fulfilled orders available to the read side

The UI displays data about orders that it obtains by querying the model on the read side. Part of the data that the UI displays to the registrant is information about partially fulfilled orders: for each seat type in the order, the number of seats requested and the number of seats that are available. This is temporary data that the system only uses while the registrant is creating the order using the UI; the business only needs to store information about seats that were actually purchased, not the difference between what the registrant requested and what the registrant purchased.

The consequence of this is that the information about how many seats the registrant requested only needs to exist in the model on the read side.

Jana Says:
	You can't store this information in an HTTP session because the registrant may leave the site between requesting the seats and completing the order.

A further consequence is that the underlying storage on the read side cannot be simple SQL views because it includes data that is not stored in the underlying table storage on the write side. Therefore, you must pass this information to the read side using events.

Figure 3 shows all the commands and events that the Order and SeatsAvailability aggregates use and how the Order aggregate pushes changes to the read side by raising events.

Figure 3

The new architecture of the reservation process

The OrderViewModelGenerator class handles the OrderPlaced, OrderUpdated, OrderPartiallyReserved, OrderRegistrantAssigned, and OrderReservationCompleted events and uses DraftOrder and DraftOrderItem instances to persist changes to the view tables.

Gary Says:
	If you look ahead to Chapter 5, "Preparing for the V1 Release," you'll see that the team extended the use of events and migrated the Orders and Registrations bounded context to use event sourcing.

CQRS command validation

When you implement the write model, you should try to ensure that commands very rarely fail. This gives the best user experience, and makes it much easier to implement the asynchronous behavior in your application.

One approach, adopted by the team, is to use the model validation features in ASP.NET MVC.

You should be careful to distinguish between errors and business failures. Examples of errors include:

• A message is not delivered due to a failure in the messaging infrastructure.
• Data is not persisted due to a connectivity problem with the database.

In many cases, especially in the cloud, you can handle these errors by retrying the operation.

Markus Says:

The Transient Fault Handling Application Block from Microsoft patterns & practices is designed to make it easier to implement consistent retry behavior for any transient faults. It comes with a set of built-in detection strategies for Azure SQL Database, Azure storage, Azure Caching, and Azure Service Bus, and it also allows you to define your own strategies. Similarly, it comes with a set of handy built-in retry policies and supports custom ones. For more information, see The Transient Fault Handling Application Block.

A business failure should have a predetermined business response. For example:

• If the system cannot reserve a seat because there are no seats left, then it should add the request to a wait list.
• If a credit card payment fails, the user should be given the chance to either try a different card, or set up payment by invoice.

Gary Says:
	Your domain experts should help you to identify possible business failures and determine the way that you handle them: either using an automated process or manually.

The countdown timer and the read model

The countdown timer that displays how much time remains to complete the order to the registrant is part of the business data in the system, and not just a part of the infrastructure. When a registrant creates an order and reserves seats, the countdown begins. The countdown continues, even if the registrant leaves the conference website. The UI must be able to display the correct countdown value if the registrant returns to the site; therefore, the reservation expiry time is a part of the data that is available from the read model.

Implementation details

This section describes some of the significant features of the implementation of the Orders and Registrations bounded context. You may find it useful to have a copy of the code so you can follow along. You can download a copy from the Download center, or check the evolution of the code in the repository on GitHub: https://github.com/mspnp/cqrs-journey-code.

The order access code record locator

A registrant may need to retrieve an order, either to view it, or to complete the assignment of attendees to seats. This may happen in a different web session, so the registrant must supply some information to locate the previously saved order.

The following code sample shows how the Order class generates a new five-character order access code that is persisted as part of the Order instance.

public string AccessCode { get; set; }

protected Order()
{
    ...
    this.AccessCode = HandleGenerator.Generate(5);
}

To retrieve an Order instance, a registrant must provide her email address and the order access code. The system will use these two items to locate the correct order. This logic is part of the read side.

The following code sample from the OrderController class in the web application shows how the MVC controller submits the query to the read side using the LocateOrder method to discover the unique OrderId value. This Find action passes the OrderId value to a Display action that displays the order information to the registrant.

[HttpPost]
public ActionResult Find(string email, string accessCode)
{
    var orderId = orderDao.LocateOrder(email, accessCode);

    if (!orderId.HasValue)
    {
        return RedirectToAction("Find", new { conferenceCode = this.ConferenceCode });
    }

    return RedirectToAction("Display", new { conferenceCode = this.ConferenceCode, orderId = orderId.Value });
}

The countdown timer

When a registrant creates an order and makes a seat reservation, those seats are reserved for a fixed period of time. The RegistrationProcessManager instance, which forwards the reservation from the SeatsAvailability aggregate, passes the time that the reservation expires to the Order aggregate. The following code sample shows how the Order aggregate receives and stores the reservation expiry time.

public DateTime? ReservationExpirationDate { get; private set; }

public void MarkAsReserved(DateTime expirationDate, IEnumerable<SeatQuantity> seats)
{
    ...

    this.ReservationExpirationDate = expirationDate;
    this.Items.Clear();
    this.Items.AddRange(seats.Select(seat => new OrderItem(seat.SeatType, seat.Quantity)));
}

Markus Says:
	The ReservationExpirationDate is initially set in the Order constructor to a time 15 minutes after the Order is instantiated. The RegistrationProcessManager class may revise this time based on when the reservations are actually made. It is this time that the process manager sends to the Order aggregate in the MarkSeatsAsReserved command.

When the RegistrationProcessManager sends the MarkSeatsAsReserved command to the Order aggregate with the expiry time that the UI will display, it also sends a command to itself to initiate the process of releasing the reserved seats. This ExpireRegistrationProcess command is held for the expiry duration plus a buffer of five minutes. This buffer ensures that time differences between the servers don't cause the RegistrationProcessManager class to release the reserved seats before the timer in the UI counts down to zero. In the following code sample from the RegistrationProcessManager class, the UI uses the Expiration property in the MarkSeatsAsReserved command to display the countdown timer, and the Delay property in the ExpireRegistrationProcess command determines when the reserved seats are released.

public void Handle(SeatsReserved message)
{
    if (this.State == ProcessState.AwaitingReservationConfirmation)
    {
        var expirationTime = this.ReservationAutoExpiration.Value;
        this.State = ProcessState.ReservationConfirmationReceived;

        if (this.ExpirationCommandId == Guid.Empty)
        {
            var bufferTime = TimeSpan.FromMinutes(5);

            var expirationCommand = new ExpireRegistrationProcess { ProcessId = this.Id };
            this.ExpirationCommandId = expirationCommand.Id;

            this.AddCommand(new Envelope<ICommand>(expirationCommand)
            {
                Delay = expirationTime.Subtract(DateTime.UtcNow).Add(bufferTime),
            });
        }


        this.AddCommand(new MarkSeatsAsReserved
        {
            OrderId = this.OrderId,
            Seats = message.ReservationDetails.ToList(),
            Expiration = expirationTime,
        });
    }

    ...
}

The MVC RegistrationController class retrieves the order information on the read side. The DraftOrder class includes the reservation expiry time that the controller passes to the view using the ViewBag class, as shown in the following code sample.

[HttpGet]
public ActionResult SpecifyRegistrantDetails(string conferenceCode, Guid orderId)
{
   var repo = this.repositoryFactory();
   using (repo as IDisposable)
   {
       var draftOrder = repo.Find<DraftOrder>(orderId);
       var conference = repo.Query<Conference>()
           .Where(c => c.Code == conferenceCode)
           .FirstOrDefault();

       this.ViewBag.ConferenceName = conference.Name;
       this.ViewBag.ConferenceCode = conference.Code;
       this.ViewBag.ExpirationDateUTCMilliseconds = 
         draftOrder.BookingExpirationDate.HasValue ? 
         ((draftOrder.BookingExpirationDate.Value.Ticks - EpochTicks) / 10000L) : 0L;
       this.ViewBag.OrderId = orderId;

       return View(new AssignRegistrantDetails { OrderId = orderId });
   }
}

The MVC view then uses JavaScript to display an animated countdown timer.

Using ASP.NET MVC validation for commands

You should try to ensure that any commands that the MVC controllers in your application send to the write model will succeed. You can use the features in MVC to validate the commands on both the client side and server side before sending them to the write model.

Markus Says:
	Client-side validation is primarily a convenience to the user in that it avoids the need for round trips to the server to help the user complete a form correctly. You still need to implement server-side validation to ensure that the data is validated before it is forwarded to the write model.

The following code sample shows the AssignRegistrantDetails command class that uses DataAnnotations to specify the validation requirements; in this example, the requirement is that the FirstName, LastName, and Email fields are not empty.

using System;
using System.ComponentModel.DataAnnotations;
using Common;

public class AssignRegistrantDetails : ICommand
{
    public AssignRegistrantDetails()
    {
        this.Id = Guid.NewGuid();
    }

    public Guid Id { get; private set; }

    public Guid OrderId { get; set; }

    [Required(AllowEmptyStrings = false)]
    public string FirstName { get; set; }

    [Required(AllowEmptyStrings = false)]
    public string LastName { get; set; }

    [Required(AllowEmptyStrings = false)]
    public string Email { get; set; }
}

The MVC view uses this command class as its model class. The following code sample from the SpecifyRegistrantDetails.cshtml file shows how the model is populated.

@model Registration.Commands.AssignRegistrantDetails

...

<div class="editor-label">@Html.LabelFor(model => model.FirstName)</div><div class="editor-field">@Html.EditorFor(model => model.FirstName)</div>
<div class="editor-label">@Html.LabelFor(model => model.LastName)</div><div class="editor-field">@Html.EditorFor(model => model.LastName)</div>
<div class="editor-label">@Html.LabelFor(model => model.Email)</div><div class="editor-field">@Html.EditorFor(model => model.Email)</div>

The Web.config file configures the client-side validation based on the DataAnnotations attributes, as shown in the following snippet.

<appSettings>
    ...
    <add key="ClientValidationEnabled" value="true" />
    <add key="UnobtrusiveJavaScriptEnabled" value="true" />
</appSettings>

The server-side validation occurs in the controller before it sends the command. The following code sample from the RegistrationController class shows how the controller uses the IsValid property to validate the command. Remember that this example uses an instance of the command as the model.

[HttpPost]
public ActionResult SpecifyRegistrantDetails(string conferenceCode, Guid orderId, AssignRegistrantDetails command)
{
    if (!ModelState.IsValid)
    {
        return SpecifyRegistrantDetails(conferenceCode, orderId);
    }

    this.commandBus.Send(command);

    return RedirectToAction("SpecifyPaymentDetails", new { conferenceCode = conferenceCode, orderId = orderId });
}

For an additional example, see the RegisterToConference command and the StartRegistration action in the RegistrationController class.

For more information, see Models and Validation in ASP.NET MVC on MSDN.

Pushing changes to the read side

Some information about orders only needs to exist on the read side. In particular, the information about partially fulfilled orders is only used in the UI and is not part of the business information persisted by the domain model on the write side.

This means that the system can't use SQL views as the underlying storage mechanism on the read side because views cannot contain data that does not exist in the tables that they are based on.

The system stores the denormalized order data in a SQL Database instance in two tables: the OrdersView and OrderItemsView tables. The OrderItemsView table includes the RequestedSeats column that contains data that only exists on the read side.

Column	Description
OrderId	A unique identifier for the Order
ReservationExpirationDate	The time when the seat reservations expire
StateValue	The state of the Order: Created, PartiallyReserved, ReservationCompleted, Rejected, Confirmed
RegistrantEmail	The email address of the Registrant
AccessCode	The Access Code that the Registrant can use to access the Order

OrdersView Table descriptions

Column	Description
OrderItemId	A unique identifier for the Order Item
SeatType	The type of seat requested
RequestedSeats	The number of seats requested
ReservedSeats	The number of seats reserved
OrderID	The OrderId in the parent OrdersView table

OrderItemsView Table descriptions

To populate these tables in the read model, the read side handles events raised by the write side and uses them to write to these tables. See Figure 3 above for more details.

The OrderViewModelGenerator class handles these events and updates the read-side repository.

public class OrderViewModelGenerator :
    IEventHandler<OrderPlaced>, IEventHandler<OrderUpdated>,
    IEventHandler<OrderPartiallyReserved>, IEventHandler<OrderReservationCompleted>,
    IEventHandler<OrderRegistrantAssigned>
{
    private readonly Func<ConferenceRegistrationDbContext> contextFactory;

    public OrderViewModelGenerator(Func<ConferenceRegistrationDbContext> contextFactory)
    {
        this.contextFactory = contextFactory;
    }

    public void Handle(OrderPlaced @event)
    {
        using (var context = this.contextFactory.Invoke())
        {
            var dto = new DraftOrder(@event.SourceId, DraftOrder.States.Created)
            {
                AccessCode = @event.AccessCode,
            };
            dto.Lines.AddRange(@event.Seats.Select(seat => new DraftOrderItem(seat.SeatType, seat.Quantity)));

            context.Save(dto);
        }
    }

    public void Handle(OrderRegistrantAssigned @event)
    {
        ...
    }

    public void Handle(OrderUpdated @event)
    {
        ...
    }

    public void Handle(OrderPartiallyReserved @event)
    {
        ...
    }

    public void Handle(OrderReservationCompleted @event)
    {
        ...
    }

    ...
}

The following code sample shows the ConferenceRegistrationDbContext class.

public class ConferenceRegistrationDbContext : DbContext
{
    ...

    public T Find<T>(Guid id) where T : class
    {
        return this.Set<T>().Find(id);
    }

    public IQueryable<T> Query<T>() where T : class
    {
        return this.Set<T>();
    }

    public void Save<T>(T entity) where T : class
    {
        var entry = this.Entry(entity);

        if (entry.State == System.Data.EntityState.Detached)
            this.Set<T>().Add(entity);

        this.SaveChanges();
    }
}

Jana Says:
	Notice that this ConferenceRegistrationDbContext in the read side includes a Save method to persist the changes sent from the write side and handled by the OrderViewModelGenerator handler class.

Querying the read side

The following code sample shows a nongeneric DAO class that the MVC controllers use to query for conference information on the read side. It wraps the ConferenceRegistrationDbContext class shown previously.

public class ConferenceDao : IConferenceDao
{
    private readonly Func<ConferenceRegistrationDbContext> contextFactory;

    public ConferenceDao(Func<ConferenceRegistrationDbContext> contextFactory)
    {
        this.contextFactory = contextFactory;
    }

    public ConferenceDetails GetConferenceDetails(string conferenceCode)
    {
        using (var context = this.contextFactory.Invoke())
        {
            return context
                .Query<Conference>()
                .Where(dto => dto.Code == conferenceCode)
                .Select(x => new ConferenceDetails { Id = x.Id, Code = x.Code, Name = x.Name, Description = x.Description, StartDate = x.StartDate })
                .FirstOrDefault();
        }
    }

    public ConferenceAlias GetConferenceAlias(string conferenceCode)
    {
        ...
    }

    public IList<SeatType> GetPublishedSeatTypes(Guid conferenceId)
    {
        ...
    }
}

Jana Says:
	Notice how this ConferenceDao class contains only methods that return data. It is used by the MVC controllers to retrieve data to display in the UI.

Refactoring the SeatsAvailability aggregate

In the first stage of our CQRS journey, the domain included a ConferenceSeatsAvailabilty aggregate root class that modeled the number of seats remaining for a conference. In this stage of the journey, the team replaced the ConferenceSeatsAvailabilty aggregate with a SeatsAvailability aggregate to reflect the fact that there may be multiple seat types available at a particular conference; for example, full conference seats, pre-conference workshop seats, and cocktail party seats. Figure 4 shows the new SeatsAvailability aggregate and its constituent classes.

Figure 4

The SeatsAvailability aggregate and its associated commands and events

This aggregate now models the following facts:

• There may be multiple seat types at a conference.
• There may be different numbers of seats available for each seat type.

The domain now includes a SeatQuantity value type that you can use to represent a quantity of a particular seat type.

Previously, the aggregate raised either a ReservationAccepted or a ReservationRejected event, depending on whether there were sufficient seats. Now the aggregate raises a SeatsReserved event that reports how many seats of a particular type it could reserve. This means that the number of seats reserved may not match the number of seats requested; this information is passed back to the UI for the registrant to make a decision on how to proceed with the registration.

The AddSeats method

You may have noticed in Figure 3 that the SeatsAvailability aggregate includes an AddSeats method with no corresponding command. The AddSeats method adjusts the total number of available seats of a given type. The business customer is responsible for making any such adjustments, and does this in the Conference Management bounded context. The Conference Management bounded context raises an event whenever the total number of available seats changes. The SeatsAvailability class then handles the event when its handler invokes the AddSeats method.

Impact on testing

This section discusses some of the testing issues addressed during this stage of the journey.

Acceptance tests and the domain expert

In Chapter 3, "Orders and Registrations Bounded Context," you saw some of the UI mockups that the developers and the domain expert worked on together to refine some of the functional requirements for the system. One of the planned uses for these UI mockups was to form the basis of a set of acceptance tests for the system.

The team had the following goals for their acceptance testing approach:

• The acceptance tests should be expressed clearly and unambiguously in a format that the domain expert could understand.
• It should be possible to execute the acceptance tests automatically.

To achieve these goals, the domain expert paired with a member of the test team and used SpecFlow to specify the core acceptance tests.

Defining acceptance tests using SpecFlow features

The first step in defining acceptance tests using SpecFlow is to define the acceptance tests using the SpecFlow notation. These tests are saved as feature files in a Visual Studio project. The following code sample from the ConferenceConfiguration.feature file in the Features\UserInterface\Views\Management folder shows an acceptance test for the Conference Management bounded context. A typical SpecFlow test scenario consists of a collection of Given, When, and Then statements. Some of these statements include the data that the test uses.

Markus Says:
	In fact, SpecFlow feature files use the Gherkin language—a domain specific language (DSL) created especially for behavior descriptions.

Feature:  Conference configuration scenarios for creating and editing Conference settings
    In order to create or update a Conference configuration
    As a Business Customer
    I want to be able to create or update a Conference and set its properties


Background: 
Given the Business Customer selected the Create Conference option

Scenario: An existing unpublished Conference is selected and published
Given this conference information
| Owner         | Email                    | Name      | Description                             | Slug   | Start      | End        |
| William Flash | william@fabrikam.com | CQRS2012P | CQRS summit 2012 conference (Published) | random | 05/02/2012 | 05/12/2012 |
And the Business Customer proceeds to create the Conference
When the Business Customer proceeds to publish the Conference
Then the state of the Conference changes to Published

Scenario: An existing Conference is edited and updated
Given an existing published conference with this information
| Owner         | Email                    | Name      | Description                            | Slug   | Start      | End        |
| William Flash | william@fabrikam.com | CQRS2012U | CQRS summit 2012 conference (Original) | random | 05/02/2012 | 05/12/2012 |
And the Business Customer proceeds to edit the existing settings with this information
| Description                           |
| CQRS summit 2012 conference (Updated) |
When the Business Customer proceeds to save the changes
Then this information appears in the Conference settings
| Description                           |
| CQRS summit 2012 conference (Updated) |

...

Carlos Says:
	I found these acceptance tests were a great way for me to clarify my definitions of the expected behavior of the system to the developers.

For additional examples, see the Conference.AcceptanceTests Visual Studio solution file included with the downloadable source.

Making the tests executable

An acceptance test in a feature file is not directly executable; you must provide some plumbing code to bridge the gap between the SpecFlow feature file and your application.

For examples of implementations, see the classes in the Steps folder in the Conference.Specflow project in the Conference.AcceptanceTests solution.

These step implementations use two different approaches.

The first approach runs the test by simulating a user of the system. It does this by driving a web browser directly using the WatiN open source library. The advantages of this approach are that it exercises the system in exactly the same way that a real user would interact with the system and that it is simple to implement initially. However, these tests are fragile and will require a considerable maintenance effort to keep them up to date as the UI and system change. The following code sample shows an example of this approach, defining some of the Given, When, and Then steps from the feature file shown previously. SpecFlow uses the Given, When, and Then attributes to link the steps to the clauses in the feature file and to pass parameter values to step methods:

public class ConferenceConfigurationSteps : StepDefinition
{
    ...

    [Given(@"the Business Customer proceeds to edit the existing settings with this information")]
    public void GivenTheBusinessCustomerProceedToEditTheExistingSettignsWithThisInformation(Table table)
    {
        Browser.Click(Constants.UI.EditConferenceId);
        PopulateConferenceInformation(table);
    }

    [Given(@"an existing published conference with this information")]
    public void GivenAnExistingPublishedConferenceWithThisInformation(Table table)
    {
        ExistingConferenceWithThisInformation(table, true);
    }

    private void ExistingConferenceWithThisInformation(Table table, bool publish)
    {
        NavigateToCreateConferenceOption();
        PopulateConferenceInformation(table, true);
        CreateTheConference();
        if(publish) PublishTheConference();

        ScenarioContext.Current.Set(table.Rows[0]["Email"], Constants.EmailSessionKey);
        ScenarioContext.Current.Set(Browser.FindText(Slug.FindBy), Constants.AccessCodeSessionKey);
    }

    ...

    [When(@"the Business Customer proceeds to save the changes")]
    public void WhenTheBusinessCustomerProceedToSaveTheChanges()
    {
        Browser.Click(Constants.UI.UpdateConferenceId);
    }

    ...

    [Then(@"this information appears in the Conference settings")]
    public void ThenThisInformationIsShowUpInTheConferenceSettings(Table table)
    {
        Assert.True(Browser.SafeContainsText(table.Rows[0][0]),
                        string.Format("The following text was not found on the page: {0}", table.Rows[0][0]));
    }

    private void PublishTheConference()
    {
        Browser.Click(Constants.UI.PublishConferenceId);
    }

    private void CreateTheConference()
    {
        ScenarioContext.Current.Browser().Click(Constants.UI.CreateConferenceId);
    }

    private void NavigateToCreateConferenceOption()
    {
        // Navigate to Registration page
        Browser.GoTo(Constants.ConferenceManagementCreatePage);
    }

    private void PopulateConferenceInformation(Table table, bool create = false)
    {
        var row = table.Rows[0];

        if (create)
        {
            Browser.SetInput("OwnerName", row["Owner"]);
            Browser.SetInput("OwnerEmail", row["Email"]);
            Browser.SetInput("name", row["Email"], "ConfirmEmail");
            Browser.SetInput("Slug", Slug.CreateNew().Value);
        }

        Browser.SetInput("Tagline", Constants.UI.TagLine);
        Browser.SetInput("Location", Constants.UI.Location);
        Browser.SetInput("TwitterSearch", Constants.UI.TwitterSearch);

        if (row.ContainsKey("Name")) Browser.SetInput("Name", row["Name"]);
        if (row.ContainsKey("Description")) Browser.SetInput("Description", row["Description"]);
        if (row.ContainsKey("Start")) Browser.SetInput("StartDate", row["Start"]);
        if (row.ContainsKey("End")) Browser.SetInput("EndDate", row["End"]);
    }
}

You can see how this approach simulates clicking on, and entering text into, UI elements in the web browser.

The second approach is to implement the tests by interacting with the MVC controller classes. In the longer-term, this approach will be less fragile at the cost of an initially more complex implementation that requires some knowledge of the internal implementation of the system. The following code samples show an example of this approach.

First, an example scenario from the SelfRegistrationEndToEndWithControllers.feature file in the Features\UserInterface\Controllers\Registration project folder:

Scenario: End to end Registration implemented using controllers
    Given the Registrant proceeds to make the Reservation
    And these Order Items should be reserved
    | seat type                 | quantity |
    | General admission         | 1        |
    | Additional cocktail party | 1        |
    And these Order Items should not be reserved
    | seat type     |
    | CQRS Workshop |
    And the Registrant enters these details
    | first name | last name | email address            |
    | William    | Flash     | william@fabrikam.com |
    And the Registrant proceeds to Checkout:Payment
    When the Registrant proceeds to confirm the payment
    Then the Order should be created with the following Order Items
    | seat type                 | quantity |
    | General admission         | 1        |
    | Additional cocktail party | 1        |
    And the Registrant assigns these seats
    | seat type                 | first name | last name | email address       |
    | General admission         | William    | Flash     | William@fabrikam.com   |
    | Additional cocktail party | Jim        | Corbin   | Jim@litwareinc.com     |
    And these seats are assigned
    | seat type                 | quantity |
    | General admission         | 1        |
    | Additional cocktail party | 1        |

Second, some of the step implementations from the SelfRegistrationEndToEndWithControllersSteps class:

[Given(@"the Registrant proceeds to make the Reservation")]
public void GivenTheRegistrantProceedToMakeTheReservation()
{
    var redirect = registrationController.StartRegistration(
        registration, registrationController.ViewBag.OrderVersion) as RedirectToRouteResult;

    Assert.NotNull(redirect);

    // Perform external redirection
    var timeout =  DateTime.Now.Add(Constants.UI.WaitTimeout);

    while (DateTime.Now < timeout && registrationViewModel == null)
    {
        //ReservationUnknown
        var result = registrationController.SpecifyRegistrantAndPaymentDetails(
            (Guid)redirect.RouteValues["orderId"], registrationController.ViewBag.OrderVersion);

        Assert.IsNotType<RedirectToRouteResult>(result);
        registrationViewModel = RegistrationHelper.GetModel<RegistrationViewModel>(result);
    }

    Assert.False(registrationViewModel == null, "Could not make the reservation and get the RegistrationViewModel");
}

...

[When(@"the Registrant proceeds to confirm the payment")]
public void WhenTheRegistrantProceedToConfirmThePayment()
{
    using (var paymentController = RegistrationHelper.GetPaymentController())
    {
        paymentController.ThirdPartyProcessorPaymentAccepted(
            conferenceInfo.Slug, (Guid) routeValues["paymentId"], " ");
    }
}

...

[Then(@"the Order should be created with the following Order Items")]
public void ThenTheOrderShouldBeCreatedWithTheFollowingOrderItems(Table table)
{
    draftOrder = RegistrationHelper.GetModel<DraftOrder>(registrationController.ThankYou(registrationViewModel.Order.OrderId));
    Assert.NotNull(draftOrder);

    foreach (var row in table.Rows)
    {
        var orderItem = draftOrder.Lines.FirstOrDefault(
            l => l.SeatType == conferenceInfo.Seats.First(s => s.Description == row["seat type"]).Id);

        Assert.NotNull(orderItem);
        Assert.Equal(Int32.Parse(row["quantity"]), orderItem.ReservedSeats);
    }
}

You can see how this approach uses the RegistrationController MVC class directly.

The team chose to implement these steps as xUnit.net tests. To run these tests within Visual Studio, you can use any of the test runners supported by xUnit.net such as ReSharper, CodeRush, or TestDriven.NET.

Jana Says:
	Remember that these acceptance tests are not the only tests performed on the system. The main solution includes comprehensive unit and integration tests, and the test team also performed exploratory and performance testing on the application.

Using tests to help developers understand message flows

A common comment about implementations that use the CQRS pattern or that use messaging extensively is the difficulty in understanding how all of the different pieces of the application fit together through sending and receiving commands and events. You can help someone to understand your code base through appropriately designed unit tests.

Consider this first example of a unit test for the Order aggregate:

public class given_placed_order
{
    ...

    private Order sut;

    public given_placed_order()
    {
        this.sut = new Order(
            OrderId, new[] 
            {
                new OrderPlaced 
                { 
                    ConferenceId = ConferenceId,
                    Seats = new[] { new SeatQuantity(SeatTypeId, 5) },
                    ReservationAutoExpiration = DateTime.UtcNow
                }
            });
    }

    [Fact]
    public void when_updating_seats_then_updates_order_with_new_seats()
    {
        this.sut.UpdateSeats(new[] { new OrderItem(SeatTypeId, 20) });

        var @event = (OrderUpdated)sut.Events.Single();
        Assert.Equal(OrderId, @event.SourceId);
        Assert.Equal(1, @event.Seats.Count());
        Assert.Equal(20, @event.Seats.ElementAt(0).Quantity);
    }

    ...
}

This unit test creates an Order instance and directly invokes the UpdateSeats method. It does not provide any information to the person reading the test code about the command or event that causes this method to be invoked.

Now consider this second example that performs the same test, but in this case by sending a command:

public class given_placed_order
{
    ...

    private EventSourcingTestHelper<Order> sut;

    public given_placed_order()
    {
        this.sut = new EventSourcingTestHelper<Order>();
        this.sut.Setup(new OrderCommandHandler(sut.Repository, pricingService.Object));

        this.sut.Given(
                new OrderPlaced 
                { 
                    SourceId = OrderId,
                    ConferenceId = ConferenceId,
                    Seats = new[] { new SeatQuantity(SeatTypeId, 5) },
                    ReservationAutoExpiration = DateTime.UtcNow
                });
    }

    [Fact]
    public void when_updating_seats_then_updates_order_with_new_seats()
    {
        this.sut.When(new RegisterToConference { ConferenceId = ConferenceId, OrderId = OrderId, Seats = new[] { new SeatQuantity(SeatTypeId, 20) }});

        var @event = sut.ThenHasSingle<OrderUpdated>();
        Assert.Equal(OrderId, @event.SourceId);
        Assert.Equal(1, @event.Seats.Count());
        Assert.Equal(20, @event.Seats.ElementAt(0).Quantity);
    }

    ...
}

This example uses a helper class that enables you to send a command to the Order instance. Now someone reading the test can see that when you send a RegisterToConference command, you expect to see an OrderUpdated event.

A journey into code comprehension

A tale of pain, relief, and learning by Josh Elster

This section describes the journey taken by Josh Elster, a member of the CQRS Advisory Board, as he explored the source code of the Contoso Conference Management System.

Testing is important

I've once believed that well-factored applications are easy to comprehend, no matter how large or broad the codebase. Any time I had a problem understanding how some feature of an application behaved, the fault would lie with the code and not in me.

Never let your ego get in the way of common sense.

Truth was, up until a certain point in my career, I simply hadn't had exposure to a large, well-factored codebase. I wouldn't have known what one looked like if it walked up and hit me in the face. Thankfully, as I got more experienced reading code, I learned to recognize the difference.

My first action after cloning the Conference code was to skim the tests. After a perusal of the integration and unit test suites in the Conference Visual Studio solution, I focused my attention on the Conference.AcceptanceTests Visual Studio solution that contains the SpecFlow acceptance tests. Other members of the project team had done some initial work on the .feature files, which worked out nicely for me since I wasn't familiar with the details of the business rules. Implementing step bindings for these features would be an excellent way to both contribute to the project and learn about how the system worked.

Domain tests

My goal then was to take a feature file looking something like this:

    Feature: Self Registrant scenarios for making a Reservation for a Conference site with all Order Items initially available
    In order to reserve Seats for a conference
    As an Attendee
    I want to be able to select an Order Item from one or many of the available Order Items and make a Reservation

    Background: 
    Given the list of the available Order Items for the CQRS Summit 2012 conference with the slug code SelfRegFull
    | seat type                 | rate | quota |
    | General admission         | $199 | 100   |
    | CQRS Workshop             | $500 | 100   |
    | Additional cocktail party | $50  | 100   |
    And the selected Order Items
    | seat type                 | quantity |
    | General admission         | 1        |
    | CQRS Workshop             | 1        |
    | Additional cocktail party | 1        |

    Scenario: All the Order Items are available and all get reserved
    When the Registrant proceeds to make the Reservation     
    Then the Reservation is confirmed for all the selected Order Items
    And these Order Items should be reserved
        | seat type                 |
        | General admission         |
        | CQRS Workshop             |
        | Additional cocktail party |
    And the total should read $749
    And the countdown started

And bind it to code that either performs an action, creates expectations, or makes assertions:

    [Given(@"the '(.*)' site conference")]
    public void GivenAConferenceNamed(string conference)
    {
        ...
    }

All at a level just below the UI, but above (and beyond) infrastructure concerns. Testing is tightly focused on the behavior of the overall solution domain, which is why I'll call these types of tests Domain Tests. Other terms such as behavior-driven development (BDD) can be used to describe this style of testing.

Jana Says:
	These "below the UI" tests are also known as subcutaneous tests, (see Meszaros, G., Melnik, G., Acceptance Test Engineering Guide).

It may seem a little redundant to rewrite application logic already implemented on the website, but there are a number of reasons why it is worth the time:

• You aren't interested (for these purposes) in testing how the website or any other piece of infrastructure behaves; you're only interested in the domain. Unit and integration-level tests will validate the correct functioning of that code, so there's no need to duplicate those tests.
• When iterating stories with product owners, spending time on pure UI concerns can slow down the feedback cycle, reducing the quality and usefulness of feedback.
• Discussing a feature in more abstract terms can lead to a better understanding of the problem that the business is trying to solve, given the sometimes large mismatches between the vocabularies used by different people when they discuss technological issues.
• Obstacles encountered in implementing the testing logic can help improve the system's overall design quality. Difficulty in separating infrastructure code from application logic is generally regarded as a smell.

The architecture for the Contoso Conference Management System is loosely coupled, utilizing messages to transfer commands and events to interested parties. Commands are routed to a single handler via a command bus, while events are routed to their 0...N handlers via an event bus. A bus isn't tied to any specific technology as far as consuming applications are concerned, allowing arbitrary implementations to be created and used throughout the system in a manner transparent to users.

Another bonus when it comes to behavioral testing of a loosely coupled message architecture is related to the fact that BDD (or similarly styled) tests do not involve themselves with the inner workings of application code. They only care about the observable behavior of the application under test. This means that for the SpecFlow tests, we need only concern ourselves with publishing some commands to the bus and examining the outward results by asserting expected message traffic and payloads against the actual traffic/data.

The other side of the coin

With all of the pixels I just spent describing how awesome and easy things are, where's the pain? The pain is in comprehending what goes on in a system. The loose coupling of the architecture has a wicked flip side; techniques such as Inversion of Control and Dependency Injection hinder code readability by their very nature, since one can never be sure what concrete class is being injected at a particular point without examining the container's initialization closely. In the journey code, the IProcess interface marks classes representing long-running business processes (also known as sagas or process managers) responsible for coordinating business logic between different aggregates. In order to maintain the integrity, idempotency, and transactionality of the system's data and state, processes leave the actual publishing of their issued commands to the individual persistence repository's implementation. Since IoC and DI containers hide these types of details from consumers, it and other properties of the system create a bit of difficulty when it comes to answering seemingly trivial questions such as:

• Who issues or issued a particular command or event?
• What class handles a particular command or event?
• Where are processes or aggregates created or persisted?
• When is a command sent in relation to other commands or events?
• Why does the system behave the way it does?
• How does the application's state change as a result of a particular command?

Because the application's dependencies are so loose, many traditional tools and methods of code analysis become either less useful or completely useless.

Let's take an example of this and work out some heuristics involved in answering these questions. We'll use as an example the RegistrationProcessManager.

1. Open the RegistrationProcessManager.cs file, noting that, like many process managers it has a ProcessState enumeration. We take note of the beginning state for the process, NotStarted. Next, we want to find code that does one of the following:
   • A new instance of the process is created (where are processes created or persisted?)
   • The initial state is changed to a different state (how does state change?)
2. Locate the first place in the source code where either or both of the above occur. In this case, it's the Handle method in the RegistrationProcessManagerRouter class. Important: this does not necessarily mean that the process is a command handler! Process managers are responsible for creating and retrieving aggregate roots (AR) from storage for the purpose of routing messages to the AR, so while they have methods similar in name and signature to an ICommandHandler implementation, they do not implement a command's logic.
3. Take note of the message type that is received as a parameter to the method where the state change occurs, since we now need to figure out where that message originated.
   • We also note that a new command, MakeSeatReservation, is being issued by the RegistrationProcessManager.
   • As mentioned above, this command isn't actually published by the process issuing it; rather, publication occurs when the process is saved to disk.
   • These heuristics will need to be repeated to some degree or another on any commands issued as side-effects of a process handling a command.
4. Do a find references on the OrderPlaced symbol to locate the (or a) top-most (external facing) component that publishes a message of that type via the Send method on the ICommandBus interface.
   • Since internally issued commands are indirectly published (by a repository) on save, it may be safe to assume that any non-infrastructure logic that directly calls the Send method is an external point of entry.

While there is certainly more to these heuristics than noted here, what is there is likely sufficient to demonstrate the point that even discussing the interactions is a rather lengthy, cumbersome process. That makes it easily prone to misinterpretation. You can come to understand the various command/event messaging interactions in this manner, but it is not very efficient.

When infrastructure requirements get mixed into the equation, the issue of information saturation becomes even more apparent. Being the competent, capable, developers that we all are (right?), we can start looking for ways to optimize these steps and improve the signal-to-noise ratio of relevant information.

To summarize, we have two problems:

• The number of items we are forced to keep in our heads is too great to allow efficient comprehension.
• Discussion and documentation for messaging interactions is verbose, error-prone, and complicated.

Fortunately, it is quite possible to kill two birds with a single stone, with MIL (messaging intermediate language).

MIL began as a series of LINQPad scripts and snippets that I created to help juggle all these facts while answering questions. Initially, all that these scripts accomplished was to reflect through one or more project assemblies and output the various types of messages and handlers. In discussions with members of the team it became apparent that others were experiencing the same types of problems I had. After a few chats and brainstorming sessions with members of the patterns & practices team, we came up with the idea of introducing a small domain-specific language (DSL) that would encapsulate the interactions being discussed. The tentatively named SawMIL toolbox, located at http://jelster.github.com/CqrsMessagingTools/ provides utilities, scripts, and examples that enable you to use MIL as part of your development and analysis process managers.

In MIL, messaging components and interactions are represented in a specific manner: commands, since they are requests for the system to perform some action, are denoted by ?, as in DoSomething?. Events represent something definite that happened in the system, and hence gain a ! suffix, as in SomethingHappened!.

Another important element of MIL is message publication and reception. Messages received from a messaging source (such as Azure Service Bus, nServiceBus, and so forth) are always preceded by the -> symbol, while messages that are being sent have the symbol following it. To keep the examples simple for now, the optional nil element, (a period, . ) is used to indicate explicitly a no-op (in other words, nothing is receiving the message). The following snippet shows an example of the nil element syntax:

SendCustomerInvoice? -> .
CustomerInvoiceSent! -> .

Once a command or event has been published, something needs to do something with it. Commands have one and only one handler, while events can have multiple handlers. MIL represents this relationship between message and handler by placing the name of the handler on the other side of the messaging operation, as shown in the following snippet:

SendCustomerInvoice? -> CustomerInvoiceHandler
CustomerInvoiceSent! ->
    -> CustomerNotificationHandler
    -> AccountsAgeingViewModelGenerator

Notice how the command handler is on the same line as the command, while the event is separated from its handlers? That's because in CQRS, there is a 1:1 correlation between commands and command handlers. Putting them together helps reinforce that concept, while keeping events separate from event handlers helps reinforce the idea that a given event can have 0...N handlers.

Aggregate Roots are prefixed with the @ sign, a convention that should be familiar to anyone who has ever used twitter. Aggregate roots never handle commands, but occasionally may handle events. Aggregate roots are most frequently event sources, raising events in response to business operations invoked on the aggregate. Something that should be made clear about these events, however, is that in most systems there are other elements that decide upon and actually perform the publication of domain events. This is an interesting case where business and technical requirements blur boundaries, with the requirements being met by infrastructure logic rather than application or business logic. An example of this lies in the journey code: in order to ensure consistency between event sources and event subscribers, the implementation of the repository that persists the aggregate root is the element responsible for actually publishing the events to a bus. The following snippet shows an example of the AggregateRoot syntax:

SendCustomerInvoice? -> CustomerInvoiceHandler
@Invoice::CustomerInvoiceSent! -> .

In the above example, a new language element called the scope context operator appears alongside the @AggregateRoot. Denoted by double colons (::) the scope context element may or may not have whitespace between its two characters, and is used to identify relationships between two objects. Above, the AR '@Invoice' is generating the CustomerSent! event in response to logic invoked by the CustomerInvoiceHandler event handler. The next example demonstrates use of the scope element on an AR, which generates multiple events in response to a single command:

SendCustomerInvoice? -> CustomerInvoiceHandler
@Invoice:
    :CustomerInvoiceSent! -> .
    :InvoiceAged! -> .

Scope context is also used to signify intra-element routing that does not involve infrastructure messaging apparatus:

SendCustomerInvoice? -> CustomerInvoiceHandler
@Invoice::CustomerInvoiceSent! ->
    -> InvoiceAgeingProcessRouter::InvoiceAgeingProcess

The last element that I'll introduce is the State Change element. State changes are one of the best ways to track what is happening within a system, and thus MIL treats them as first-class citizens. These statements must appear on their own line of text, and are prefixed with the '*' character. It's the only time in MIL that there is any mention or appearance of assignment because it's just that important! The following snippet shows an example of the State Change element:

SendCustomerInvoice? -> CustomerInvoiceHandler
@Invoice::CustomerInvoiceSent! ->
    -> InvoiceAgegingProcessRouter::InvoiceAgeingProcess
        *InvoiceAgeingProcess.ProcessState = Unpaid

Summary

We've just walked through the basic steps used when describing messaging interactions in a loosely coupled application. Although the interactions described are only a subset of possible interactions, MIL is evolving into a way to compactly describe the interactions of a message-based system. Different nouns and verbs (elements and actions) are represented by distinct, mnemonically significant symbols. This provides a cross-substrate (squishy human brains < - > silicon CPU) means of communicating meaningful information about systems as a whole. Although the language describes some types of messaging interactions very well, it is very much a work in progress with many elements of the language and tooling in need of development or improvement. This presents some great opportunities for people looking to contribute to OSS, so if you've been on the fence about contributing or are wondering about OSS participation, there's no time like the present to head over to http://jelster.github.com/CqrsMessagingTools/, fork the repos, and get started!

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