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kgrzybek
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Full Modular Monolith application with Domain-Driven Design approach.

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Modular Monolith with DDD

Full Modular Monolith .NET application with Domain-Driven Design approach.

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FrontEnd application

FrontEnd application : Modular Monolith With DDD: FrontEnd React application

Table of contents

1. Introduction

  1.1 Purpose of this Repository

  1.2 Out of Scope

  1.3 Reason

  1.4 Disclaimer

  1.5 Give a Star

  1.6 Share It

2. Domain

  2.1 Description

  2.2 Conceptual Model

  2.3 Event Storming

3. Architecture

  3.0 C4 Model

  3.1 High Level View

  3.2 Module Level View

  3.3 API and Module Communication

  3.4 Module Requests Processing via CQRS

  3.5 Domain Model Principles and Attributes

  3.6 Cross-Cutting Concerns

  3.7 Modules Integration

  3.8 Internal Processing

  3.9 Security

  3.10 Unit Tests

  3.11 Architecture Decision Log

  3.12 Architecture Unit Tests

  3.13 Integration Tests

  3.14 System Integration Testing

  3.15 Event Sourcing

  3.16 Database change management

  3.17 Continuous Integration

  3.18 Static code analysis

4. Technology

5. How to Run

6. Contribution

7. Roadmap

8. Authors

9. License

10. Inspirations and Recommendations

1. Introduction

1.1 Purpose of this Repository

This is a list of the main goals of this repository:

  • Showing how you can implement a monolith application in a modular way
  • Presentation of the full implementation of an application
    • This is not another simple application
    • This is not another proof of concept (PoC)
    • The goal is to present the implementation of an application that would be ready to run in production
  • Showing the application of best practices and object-oriented programming principles
  • Presentation of the use of design patterns. When, how and why they can be used
  • Presentation of some architectural considerations, decisions, approaches
  • Presentation of the implementation using Domain-Driven Design approach (tactical patterns)
  • Presentation of the implementation of Unit Tests for Domain Model (Testable Design in mind)
  • Presentation of the implementation of Integration Tests
  • Presentation of the implementation of Event Sourcing
  • Presentation of C4 Model
  • Presentation of diagram as text approach

1.2 Out of Scope

This is a list of subjects which are out of scope for this repository: - Business requirements gathering and analysis - System analysis - Domain exploration - Domain distillation - Domain-Driven Design strategic patterns - Architecture evaluation, quality attributes analysis - Integration, system tests - Project management - Infrastructure - Containerization - Software engineering process - Deployment process - Maintenance - Documentation

1.3 Reason

The reason for creating this repository is the lack of something similar. Most sample applications on GitHub have at least one of the following issues: - Very, very simple - few entities and use cases implemented - Not finished (for example there is no authentication, logging, etc..) - Poorly designed (in my opinion) - Poorly implemented (in my opinion) - Not well described - Assumptions and decisions are not clearly explained - Implements "Orders" domain - yes, everyone knows this domain, but something different is needed - Implemented in old technology - Not maintained

To sum up, there are some very good examples, but there are far too few of them. This repository has the task of filling this gap at some level.

1.4 Disclaimer

Software architecture should always be created to resolve specific business problems. Software architecture always supports some quality attributes and at the same time does not support others. A lot of other factors influence your software architecture - your team, opinions, preferences, experiences, technical constraints, time, budget, etc.

Always functional requirements, quality attributes, technical constraints and other factors should be considered before an architectural decision is made.

Because of the above, the architecture and implementation presented in this repository is one of the many ways to solve some problems. Take from this repository as much as you want, use it as you like but remember to always pick the best solution which is appropriate to the problem class you have.

1.5 Give a Star

My primary focus in this project is on quality. Creating a good quality product involves a lot of analysis, research and work. It takes a lot of time. If you like this project, learned something or you are using it in your applications, please give it a star :star:. This is the best motivation for me to continue this work. Thanks!

1.6 Share It

There are very few really good examples of this type of application. If you think this repository makes a difference and is worth it, please share it with your friends and on social networks. I will be extremely grateful.

2. Domain

2.1 Description

Definition:

Domain - A sphere of knowledge, influence, or activity. The subject area to which the user applies a program is the domain of the software. Domain-Driven Design Reference, Eric Evans

The Meeting Groups domain was selected for the purposes of this project based on the Meetup.com system.

Main reasons for selecting this domain:

  • It is common, a lot of people use the Meetup site to organize or attend meetings
  • There is a system for it, so everyone can check this implementation against a working site which supports this domain
  • It is not complex so it is easy to understand
  • It is not trivial - there are some business rules and logic and it is not just CRUD operations
  • You don't need much specific domain knowledge unlike other domains like financing, banking, medical
  • It is not big so it is easier to implement

Meetings

The main business entities are

Member
,
Meeting Group
and
Meeting
. A
Member
can create a
Meeting Group
, be part of a
Meeting Group
or can attend a
Meeting
.

A

Meeting Group Member
can be an
Organizer
of this group or a normal
Member
.

Only an

Organizer
of a
Meeting Group
can create a new
Meeting
.

A

Meeting
has attendees, not attendees (
Members
which declare they will not attend the
Meeting
) and
Members
on the
Waitlist
.

A

Meeting
can have an attendee limit. If the limit is reached,
Members
can only sign up to the
Waitlist
.

A

Meeting Attendee
can bring guests to the
Meeting
. The number of guests allowed is an attribute of the
Meeting
. Bringing guests can be unallowed.

A

Meeting Attendee
can have one of two roles:
Attendee
or
Host
. A
Meeting
must have at least one
Host
. The
Host
is a special role which grants permission to edit
Meeting
information or change the attendees list.

A

Member
can comment
Meetings
. A
Member
can reply to, like other
Comments
.
Organizer
manages commenting of
Meeting
by
Meeting Commenting Configuration
.
Organizer
can delete any
Comment
.

Each

Meeting Group
must have an organizer with active
Subscription
. One organizer can cover 3
Meeting Groups
by his
Subscription
.

Additionally, Meeting organizer can set an

Event Fee
. Each
Meeting Attendee
is obliged to pay the fee. All guests should be paid by
Meeting Attendee
too.

Administration

To create a new

Meeting Group
, a
Member
needs to propose the group. A
Meeting Group Proposal
is sent to
Administrators
. An
Administrator
can accept or reject a
Meeting Group Proposal
. If a
Meeting Group Proposal
is accepted, a
Meeting Group
is created.

Payments

Each

Member
who is the
Payer
can buy the
Subscription
. He needs to pay the
Subscription Payment
.
Subscription
can expire so
Subscription Renewal
is required (by
Subscription Renewal Payment
payment to keep
Subscription
active).

When the

Meeting
fee is required, the
Payer
needs to pay
Meeting Fee
(through
Meeting Fee Payment
).

Users

Each

Administrator
,
Member
and
Payer
is a
User
. To be a
User
,
User Registration
is required and confirmed.

Each

User
is assigned one or more
User Role
.

Each

User Role
has set of
Permissions
. A
Permission
defines whether
User
can invoke a particular action.

2.2 Conceptual Model

Definition:

Conceptual Model - A conceptual model is a representation of a system, made of the composition of concepts that are used to help people know, understand, or simulate a subject the model represents. Wikipedia - Conceptual model

Conceptual Model

PlantUML version:

VisualParadigm version (not maintained, only for demonstration):

Conceptual Model of commenting feature

2.3 Event Storming

While a Conceptual Model focuses on structures and relationships between them, behavior and events that occur in our domain are more important.

There are many ways to show behavior and events. One of them is a light technique called Event Storming which is becoming more popular. Below are presented 3 main business processes using this technique: user registration, meeting group creation and meeting organization.

Note: Event Storming is a light, live workshop. One of the possible outputs of this workshop is presented here. Even if you are not doing Event Storming workshops, this type of process presentation can be very valuable to you and your stakeholders.

User Registration process



Meeting Group creation


Meeting organization


Payments Download high resolution file


3. Architecture

3.0 C4 Model

C4 model is a lean graphical notation technique for modelling the architecture of software systems.

As can be found on the website of the author of this model (Simon Brown): The C4 model was created as a way to help software development teams describe and communicate software architecture, both during up-front design sessions and when retrospectively documenting an existing codebase

Model C4 defines 4 levels (views) of the system architecture: System Context, Container, Component and Code. Below are examples of each of these levels that describe the architecture of this system.

Note: The PlantUML (diagram as text) component was used to describe all C4 model levels. Additionally, for levels C1-C3, a C4-PlantUML plug-in connecting PlantUML with the C4 model was used.

3.0.1 C1 System Context

3.0.2 C2 Container

3.0.3 C3 Component (high-level)

3.0.4 C3 Component (module-level)

3.0.5 C4 Code (meeting group aggregate)

3.1 High Level View

Module descriptions:

  • API - Very thin ASP.NET MVC Core REST API application. Main responsibilities are:
    1. Accept request
    2. Authenticate and authorize request (using User Access module)
    3. Delegate work to specific module sending Command or Query
    4. Return response
  • User Access - responsible for user authentication, authorization and registration
  • Meetings - implements Meetings Bounded Context: creating meeting groups, meetings
  • Administration - implements Administration Bounded Context: implements administrative tasks like meeting group proposal verification
  • Payments - implements Payments Bounded Context: implements all functionalities associated with payments
  • In Memory Events Bus - Publish/Subscribe implementation to asynchronously integrate all modules using events (Event Driven Architecture).

Key assumptions:

  1. API contains no application logic
  2. API communicates with Modules using a small interface to send Queries and Commands
  3. Each Module has its own interface which is used by API
  4. Modules communicate each other only asynchronously using Events Bus - direct method calls are not allowed
  5. Each Module has it's own data in a separate schema - shared data is not allowed
    • Module data could be moved into separate databases if desired
  6. Modules can only have a dependency on the integration events assembly of other Module (see Module level view)
  7. Each Module has its own Composition Root, which implies that each Module has its own Inversion-of-Control container
  8. API as a host needs to initialize each module and each module has an initialization method
  9. Each Module is highly encapsulated - only required types and members are public, the rest are internal or private

3.2 Module Level View

Each Module has Clean Architecture and consists of the following submodules (assemblies):

  • Application - the application logic submodule which is responsible for requests processing: use cases, domain events, integration events, internal commands.
  • Domain - Domain Model in Domain-Driven Design terms implements the applicable Bounded Context
  • Infrastructure - infrastructural code responsible for module initialization, background processing, data access, communication with Events Bus and other external components or systems
  • IntegrationEvents - Contracts published to the Events Bus; only this assembly can be called by other modules

Note: Application, Domain and Infrastructure assemblies could be merged into one assembly. Some people like horizontal layering or more decomposition, some don't. Implementing the Domain Model or Infrastructure in separate assembly allows encapsulation using the

internal
keyword. Sometimes Bounded Context logic is not worth it because it is too simple. As always, be pragmatic and take whatever approach you like.

3.3 API and Module Communication

The API only communicates with Modules in two ways: during module initialization and request processing.

Module initialization

Each module has a static

Initialize
method which is invoked in the API
Startup
class. All configuration needed by this module should be provided as arguments to this method. All services are configured during initialization and the Composition Root is created using the Inversion-of-Control Container.
public static void Initialize(
    string connectionString,
    IExecutionContextAccessor executionContextAccessor,
    ILogger logger,
    EmailsConfiguration emailsConfiguration)
{
    var moduleLogger = logger.ForContext("Module", "Meetings");

ConfigureCompositionRoot(connectionString, executionContextAccessor, moduleLogger, emailsConfiguration);

QuartzStartup.Initialize(moduleLogger);

EventsBusStartup.Initialize(moduleLogger);

}

Request processing

Each module has the same interface signature exposed to the API. It contains 3 methods: command with result, command without result and query.

public interface IMeetingsModule
{
    Task ExecuteCommandAsync(ICommand command);

Task ExecuteCommandAsync(ICommand command);

Task<tresult> ExecuteQueryAsync<tresult>(IQuery<tresult> query);

}

Note: Some people say that processing a command should not return a result. This is an understandable approach but sometimes impractical, especially when you want to immediately return the ID of a newly created resource. Sometimes the boundary between Command and Query is blurry. One example is

AuthenticateCommand
- it returns a token but it is not a query because it has a side effect.

3.4 Module Requests Processing via CQRS

Processing of Commands and Queries is separated by applying the architectural style/pattern Command Query Responsibility Segregation (CQRS).

Commands are processed using Write Model which is implemented using DDD tactical patterns:

internal class CreateNewMeetingGroupCommandHandler : ICommandHandler
{
    private readonly IMeetingGroupRepository _meetingGroupRepository;
    private readonly IMeetingGroupProposalRepository _meetingGroupProposalRepository;

internal CreateNewMeetingGroupCommandHandler(
    IMeetingGroupRepository meetingGroupRepository,
    IMeetingGroupProposalRepository meetingGroupProposalRepository)
{
    _meetingGroupRepository = meetingGroupRepository;
    _meetingGroupProposalRepository = meetingGroupProposalRepository;
}

public async Task<unit> Handle(CreateNewMeetingGroupCommand request, CancellationToken cancellationToken)
{
    var meetingGroupProposal = await _meetingGroupProposalRepository.GetByIdAsync(request.MeetingGroupProposalId);

    var meetingGroup = meetingGroupProposal.CreateMeetingGroup();

    await _meetingGroupRepository.AddAsync(meetingGroup);

    return Unit.Value;
}

}

Queries are processed using Read Model which is implemented by executing raw SQL statements on database views:

internal class GetAllMeetingGroupsQueryHandler : IQueryHandler>
{
    private readonly ISqlConnectionFactory _sqlConnectionFactory;

internal GetAllMeetingGroupsQueryHandler(ISqlConnectionFactory sqlConnectionFactory)
{
    _sqlConnectionFactory = sqlConnectionFactory;
}

public async Task<list>&gt; Handle(GetAllMeetingGroupsQuery request, CancellationToken cancellationToken)
{
    var connection = _sqlConnectionFactory.GetOpenConnection();

    const string sql = "SELECT " +
                       "[MeetingGroup].[Id], " +
                       "[MeetingGroup].[Name], " +
                       "[MeetingGroup].[Description], " +
                       "[MeetingGroup].[LocationCountryCode], " +
                       "[MeetingGroup].[LocationCity]" +
                       "FROM [meetings].[v_MeetingGroups] AS [MeetingGroup]";
    var meetingGroups = await connection.QueryAsync<meetinggroupdto>(sql);

    return meetingGroups.AsList();
}

}

Key advantages:

Disadvantage: - Mediator pattern introduces extra indirection and is harder to reason about which class handles the request

For more information: Simple CQRS implementation with raw SQL and DDD

3.5 Domain Model Principles and Attributes

The Domain Model, which is the central and most critical part in the system, should be designed with special attention. Here are some key principles and attributes which are applied to Domain Models of each module:

  1. High level of encapsulation

    All members are

    private
    by default, then
    internal
    - only
    public
    at the very edge.
  2. High level of PI (Persistence Ignorance)

    No dependencies to infrastructure, databases, etc. All classes are POCOs.

  3. Rich in behavior

    All business logic is located in the Domain Model. No leaks to the application layer or elsewhere.

  4. Low level of Primitive Obsession

    Primitive attributes of Entites grouped together using ValueObjects.

  5. Business language

    All classes, methods and other members are named in business language used in this Bounded Context.

  6. Testable

    The Domain Model is a critical part of the system so it should be easy to test (Testable Design).

public class MeetingGroup : Entity, IAggregateRoot
{
    public MeetingGroupId Id { get; private set; }

private string _name;

private string _description;

private MeetingGroupLocation _location;

private MemberId _creatorId;

private List<meetinggroupmember> _members;

private DateTime _createDate;

private DateTime? _paymentDateTo;

internal static MeetingGroup CreateBasedOnProposal(
    MeetingGroupProposalId meetingGroupProposalId,
    string name,
    string description,
    MeetingGroupLocation location, MemberId creatorId)
{
    return new MeetingGroup(meetingGroupProposalId, name, description, location, creatorId);
}

 public Meeting CreateMeeting(
        string title,
        MeetingTerm term,
        string description,
        MeetingLocation location,
        int? attendeesLimit,
        int guestsLimit,
        Term rsvpTerm,
        MoneyValue eventFee,
        List<memberid> hostsMembersIds,
        MemberId creatorId)
    {
        this.CheckRule(new MeetingCanBeOrganizedOnlyByPayedGroupRule(_paymentDateTo));

        this.CheckRule(new MeetingHostMustBeAMeetingGroupMemberRule(creatorId, hostsMembersIds, _members));

        return new Meeting(this.Id,
            title,
            term,
            description,
            location,
            attendeesLimit,
            guestsLimit,
            rsvpTerm,
            eventFee,
            hostsMembersIds,
            creatorId);
    }

3.6 Cross-Cutting Concerns

To support Single Responsibility Principle and Don't Repeat Yourself principles, the implementation of cross-cutting concerns is done using the Decorator Pattern. Each Command processor is decorated by 3 decorators: logging, validation and unit of work.

Logging

The Logging decorator logs execution, arguments and processing of each Command. This way each log inside a processor has the log context of the processing command.

internal class LoggingCommandHandlerDecorator : ICommandHandler where T:ICommand
{
    private readonly ILogger _logger;
    private readonly IExecutionContextAccessor _executionContextAccessor;
    private readonly ICommandHandler _decorated;

public LoggingCommandHandlerDecorator(
    ILogger logger,
    IExecutionContextAccessor executionContextAccessor,
    ICommandHandler<t> decorated)
{
    _logger = logger;
    _executionContextAccessor = executionContextAccessor;
    _decorated = decorated;
}
public async Task<unit> Handle(T command, CancellationToken cancellationToken)
{
    if (command is IRecurringCommand)
    {
        return await _decorated.Handle(command, cancellationToken);
    }
    using (
        LogContext.Push(
            new RequestLogEnricher(_executionContextAccessor),
            new CommandLogEnricher(command)))
    {
        try
        {
            this._logger.Information(
                "Executing command {Command}",
                command.GetType().Name);

            var result = await _decorated.Handle(command, cancellationToken);

            this._logger.Information("Command {Command} processed successful", command.GetType().Name);

            return result;
        }
        catch (Exception exception)
        {
            this._logger.Error(exception, "Command {Command} processing failed", command.GetType().Name);
            throw;
        }
    }
}

private class CommandLogEnricher : ILogEventEnricher
{
    private readonly ICommand _command;

    public CommandLogEnricher(ICommand command)
    {
        _command = command;
    }
    public void Enrich(LogEvent logEvent, ILogEventPropertyFactory propertyFactory)
    {
        logEvent.AddOrUpdateProperty(new LogEventProperty("Context", new ScalarValue($"Command:{_command.Id.ToString()}")));
    }
}

private class RequestLogEnricher : ILogEventEnricher
{
    private readonly IExecutionContextAccessor _executionContextAccessor;
    public RequestLogEnricher(IExecutionContextAccessor executionContextAccessor)
    {
        _executionContextAccessor = executionContextAccessor;
    }
    public void Enrich(LogEvent logEvent, ILogEventPropertyFactory propertyFactory)
    {
        if (_executionContextAccessor.IsAvailable)
        {
            logEvent.AddOrUpdateProperty(new LogEventProperty("CorrelationId", new ScalarValue(_executionContextAccessor.CorrelationId)));
        }
    }
}

}

Validation

The Validation decorator performs Command data validation. It checks rules against Command arguments using the FluentValidation library.

internal class ValidationCommandHandlerDecorator : ICommandHandler where T:ICommand
{
    private readonly IList> _validators;
    private readonly ICommandHandler _decorated;

public ValidationCommandHandlerDecorator(
    IList<ivalidator>&gt; validators,
    ICommandHandler<t> decorated)
{
    this._validators = validators;
    _decorated = decorated;
}

public Task<unit> Handle(T command, CancellationToken cancellationToken)
{
    var errors = _validators
        .Select(v =&gt; v.Validate(command))
        .SelectMany(result =&gt; result.Errors)
        .Where(error =&gt; error != null)
        .ToList();

    if (errors.Any())
    {
        var errorBuilder = new StringBuilder();

        errorBuilder.AppendLine("Invalid command, reason: ");

        foreach (var error in errors)
        {
            errorBuilder.AppendLine(error.ErrorMessage);
        }

        throw new InvalidCommandException(errorBuilder.ToString(), null);
    }

    return _decorated.Handle(command, cancellationToken);
}

}

Unit Of Work

All Command processing has side effects. To avoid calling commit on every handler,

UnitOfWorkCommandHandlerDecorator
is used. It additionally marks
InternalCommand
as processed (if it is Internal Command) and dispatches all Domain Events (as part of Unit Of Work).
public class UnitOfWorkCommandHandlerDecorator : ICommandHandler where T:ICommand
{
    private readonly ICommandHandler _decorated;
    private readonly IUnitOfWork _unitOfWork;
    private readonly MeetingsContext _meetingContext;

public UnitOfWorkCommandHandlerDecorator(
    ICommandHandler<t> decorated,
    IUnitOfWork unitOfWork,
    MeetingsContext meetingContext)
{
    _decorated = decorated;
    _unitOfWork = unitOfWork;
    _meetingContext = meetingContext;
}

public async Task<unit> Handle(T command, CancellationToken cancellationToken)
{
    await this._decorated.Handle(command, cancellationToken);

    if (command is InternalCommandBase)
    {
        var internalCommand =
            await _meetingContext.InternalCommands.FirstOrDefaultAsync(x =&gt; x.Id == command.Id,
                cancellationToken: cancellationToken);

        if (internalCommand != null)
        {
            internalCommand.ProcessedDate = DateTime.UtcNow;
        }
    }

    await this._unitOfWork.CommitAsync(cancellationToken);

    return Unit.Value;
}

}

3.7 Modules Integration

Integration between modules is strictly asynchronous using Integration Events and the In Memory Event Bus as broker. In this way coupling between modules is minimal and exists only on the structure of Integration Events.

Modules don't share data so it is not possible nor desirable to create a transaction which spans more than one module. To ensure maximum reliability, the Outbox / Inbox pattern is used. This pattern provides accordingly "At-Least-Once delivery" and "At-Least-Once processing".

The Outbox and Inbox is implemented using two SQL tables and a background worker for each module. The background worker is implemented using the Quartz.NET library.

Saving to Outbox:

Processing Outbox:

3.8 Internal Processing

The main principle of this system is that you can change its state only by calling a specific Command.

Commands can be called not only by the API, but by the processing module itself. The main use case which implements this mechanism is data processing in eventual consistency mode when we want to process something in a different process and transaction. This applies, for example, to Inbox processing because we want to do something (calling a Command) based on an Integration Event from the Inbox.

This idea is taken from Alberto's Brandolini's Event Storming picture called "The picture that explains “almost” everything" which shows that every side effect (domain event) is created by invoking a Command on Aggregate. See EventStorming cheat sheet article for more details.

Implementation of internal processing is very similar to implementation of the Outbox and Inbox. One SQL table and one background worker for processing. Each internally processing Command must inherit from

InternalCommandBase
class:
internal abstract class InternalCommandBase : ICommand
{
    public Guid Id { get; }

protected InternalCommandBase(Guid id)
{
    this.Id = id;
}

}

This is important because the

UnitOfWorkCommandHandlerDecorator
must mark an internal Command as processed during committing:
public async Task Handle(T command, CancellationToken cancellationToken)
{
    await this._decorated.Handle(command, cancellationToken);

if (command is InternalCommandBase)
{
    var internalCommand =
        await _meetingContext.InternalCommands.FirstOrDefaultAsync(x =&gt; x.Id == command.Id,
            cancellationToken: cancellationToken);

    if (internalCommand != null)
    {
        internalCommand.ProcessedDate = DateTime.UtcNow;
    }
}

await this._unitOfWork.CommitAsync(cancellationToken);

return Unit.Value;

}

3.9 Security

Authentication

Authentication is implemented using JWT Token and Bearer scheme using IdentityServer. For now, only one authentication method is implemented: forms style authentication (username and password) via the OAuth2 Resource Owner Password Grant Type. It requires implementation of the

IResourceOwnerPasswordValidator
interface:
public class ResourceOwnerPasswordValidator : IResourceOwnerPasswordValidator
{
    private readonly IUserAccessModule _userAccessModule;

public ResourceOwnerPasswordValidator(IUserAccessModule userAccessModule)
{
    _userAccessModule = userAccessModule;
}

public async Task ValidateAsync(ResourceOwnerPasswordValidationContext context)
{
    var authenticationResult = await _userAccessModule.ExecuteCommandAsync(new AuthenticateCommand(context.UserName, context.Password));
    if (!authenticationResult.IsAuthenticated)
    {
        context.Result = new GrantValidationResult(
            TokenRequestErrors.InvalidGrant,
            authenticationResult.AuthenticationError);
        return;
    }
    context.Result = new GrantValidationResult(
        authenticationResult.User.Id.ToString(),
        "forms",
        authenticationResult.User.Claims);
}

}

Authorization

Authorization is achieved by implementing RBAC (Role Based Access Control) using Permissions. Permissions are more granular and a much better way to secure your application than Roles alone. Each User has a set of Roles and each Role contains one or more Permission. The User's set of Permissions is extracted from all Roles the User belongs to. Permissions are always checked on

Controller
level - never Roles:
[HttpPost]
[Route("")]
[HasPermission(MeetingsPermissions.ProposeMeetingGroup)]
public async Task ProposeMeetingGroup(ProposeMeetingGroupRequest request)
{
    await _meetingsModule.ExecuteCommandAsync(
        new ProposeMeetingGroupCommand(
            request.Name,
            request.Description,
            request.LocationCity,
            request.LocationCountryCode));

return Ok();

}

3.10 Unit Tests

Definition:

A unit test is an automated piece of code that invokes the unit of work being tested, and then checks some assumptions about a single end result of that unit. A unit test is almost always written using a unit testing framework. It can be written easily and runs quickly. It’s trustworthy, readable, and maintainable. It’s consistent in its results as long as production code hasn’t changed. Art of Unit Testing 2nd Edition Roy Osherove

Attributes of good unit test

  • Automated
  • Maitainable
  • Runs very fast (in ms)
  • Consistent, Deterministic (always the same result)
  • Isolated from other tests
  • Readable
  • Can be executed by anyone
  • Testing public API, not internal behavior (overspecification)
  • Looks like production code
  • Treated as production code

Implementation

Unit tests should mainly test business logic (domain model):

Each unit test has 3 standard sections: Arrange, Act and Assert:

1. Arrange

The Arrange section is responsible for preparing the Aggregate for testing the public method that we want to test. This public method is often called (from the unit tests perspective) the SUT (system under test).

Creating an Aggregate ready for testing involves calling one or more other public constructors/methods on the Domain Model. At first it may seem that we are testing too many things at the same time, but this is not true. We need to be one hundred percent sure that the Aggregate is in a state exactly as it will be in production. This can only be ensured when we:

  • Use only public API of Domain Model
  • Don't use InternalsVisibleToAttribute class
    • This exposes the Domain Model to the Unit Tests library, removing encapsulation so our tests and production code are treated differently and it is a very bad thing
  • Don't use ConditionalAttribute classes - it reduces readability and increases complexity
  • Don't create any special constructors/factory methods for tests (even with conditional compilation symbols)
    • Special constructor/factory method only for unit tests causes duplication of business logic in the test itself and focuses on state - this kind of approach causes the test to be very sensitive to changes and hard to maintain
  • Don't remove encapsulation from Domain Model (for example: change keywords from
    internal
    /
    private
    to
    public
    )
  • Don't make methods
    protected
    to inherit from tested class and in this way provide access to internal methods/properties

Isolation of external dependencies

There are 2 main concepts - stubs and mocks:

A stub is a controllable replacement for an existing dependency (or collaborator) in the system. By using a stub, you can test your code without dealing with the dependency directly.

A mock object is a fake object in the system that decides whether the unit test has passed or failed. It does so by verifying whether the object under test called the fake object as expected. There’s usually no more than one mock per test. Art of Unit Testing 2nd Edition Roy Osherove

Good advice: use stubs if you need to, but try to avoid mocks. Mocking causes us to test too many internal things and leads to overspecification.

2. Act

This section is very easy - we execute exactly one public method on aggregate (SUT).

3. Assert

In this section we check expectations. There are only 2 possible outcomes:

  • Method completed and Domain Event(s) published
  • Business rule was broken

Simple example:

[Test]
public void NewUserRegistration_WithUniqueLogin_IsSuccessful()
{
    // Arrange
    var usersCounter = Substitute.For();

// Act
var userRegistration =
    UserRegistration.RegisterNewUser(
        "login", "password", "[email protected]",
        "firstName", "lastName", usersCounter);

// Assert
var newUserRegisteredDomainEvent = AssertPublishedDomainEvent<newuserregistereddomainevent>(userRegistration);
Assert.That(newUserRegisteredDomainEvent.UserRegistrationId, Is.EqualTo(userRegistration.Id));

}

[Test] public void NewUserRegistration_WithoutUniqueLogin_BreaksUserLoginMustBeUniqueRule() { // Arrange var usersCounter = Substitute.For(); usersCounter.CountUsersWithLogin("login").Returns(x => 1);

// Assert
AssertBrokenRule<userloginmustbeuniquerule>(() =&gt;
{
    // Act
    UserRegistration.RegisterNewUser(
        "login", "password", "[email protected]",
        "firstName", "lastName", usersCounter);
});

}

Advanced example:

[Test]
public void AddAttendee_WhenMemberIsAlreadyAttendeeOfMeeting_IsNotPossible()
{
    // Arrange
    var creatorId = new MemberId(Guid.NewGuid());
    var meetingTestData = CreateMeetingTestData(new MeetingTestDataOptions
    {
        CreatorId = creatorId
    });
    var newMemberId = new MemberId(Guid.NewGuid());
    meetingTestData.MeetingGroup.JoinToGroupMember(newMemberId);
    meetingTestData.Meeting.AddAttendee(meetingTestData.MeetingGroup, newMemberId, 0);

// Assert
AssertBrokenRule<membercannotbeanattendeeofmeetingmorethanoncerule>(() =&gt;
{
    // Act
    meetingTestData.Meeting.AddAttendee(meetingTestData.MeetingGroup, newMemberId, 0);
});

}

CreateMeetingTestData
method is an implementation of SUT Factory described by Mark Seemann which allows keeping common creation logic in one place:
protected MeetingTestData CreateMeetingTestData(MeetingTestDataOptions options)
{
    var proposalMemberId = options.CreatorId ?? new MemberId(Guid.NewGuid());
    var meetingProposal = MeetingGroupProposal.ProposeNew(
        "name", "description",
        new MeetingGroupLocation("Warsaw", "PL"), proposalMemberId);

meetingProposal.Accept();

var meetingGroup = meetingProposal.CreateMeetingGroup();

meetingGroup.UpdatePaymentInfo(DateTime.Now.AddDays(1));

var meetingTerm = options.MeetingTerm ??
                  new MeetingTerm(DateTime.UtcNow.AddDays(1), DateTime.UtcNow.AddDays(2));

var rsvpTerm = options.RvspTerm ?? Term.NoTerm;
var meeting = meetingGroup.CreateMeeting("title",
    meetingTerm,
    "description",
    new MeetingLocation("Name", "Address", "PostalCode", "City"),
    options.AttendeesLimit,
    options.GuestsLimit,
    rsvpTerm,
    MoneyValue.Zero,
    new List<memberid>(),
    proposalMemberId);

DomainEventsTestHelper.ClearAllDomainEvents(meetingGroup);

return new MeetingTestData(meetingGroup, meeting);

}

3.11 Architecture Decision Log

All Architectural Decisions (AD) are documented in the Architecture Decision Log (ADL).

More information about documenting architecture-related decisions in this way : https://github.com/joelparkerhenderson/architecturedecisionrecord

3.12 Architecture Unit Tests

In some cases it is not possible to enforce the application architecture, design or established conventions using compiler (compile-time). For this reason, code implementations can diverge from the original design and architecture. We want to minimize this behavior, not only by code review.

To do this, unit tests of system architecture, design, major conventions and assumptions have been written. In .NET there is special library for this task: NetArchTest. This library has been written based on the very popular JAVA architecture unit tests library - ArchUnit.

Using this kind of tests we can test proper layering of our application, dependencies, encapsulation, immutability, DDD correct implementation, naming, conventions and so on - everything what we need to test. Example:

More information about architecture unit tests here: https://blogs.oracle.com/javamagazine/unit-test-your-architecture-with-archunit

3.13 Integration Tests

Definition

"Integration Test" term is blurred. It can mean test between classes, modules, services, even systems - see this article (by Martin Fowler).

For this reason, the definition of integration test in this project is as follows: - it verifies how system works in integration with "out-of-process" dependencies - database, messaging system, file system or external API - it tests particular use case - it can be slow (as opposed to Unit Test)

Approach

  • Do not mock dependencies over which you have full control (like database). Full control dependency means you can always revert all changes (remove side-effects) and no one can notice it. They are not visible to others. See next point, please.
  • Use "production", normal, real database version. Some use e.g. in memory repository, some use light databases instead "production" version. This is still mocking. Testing makes sense if we have full confidence in testing. You can't trust the test if you know that the infrastructure in the production environment will vary. Be always as close to production environment as possible.
  • Mock dependencies over which you don't have control. No control dependency means you can't remove side-effects after interaction with this dependency (external API, messaging system, SMTP server etc.). They can be visible to others.

Implementation

Integration test should test exactly one use case. One use case is represented by one Command/Query processing so CommandHandler/QueryHandler in Application layer is perfect starting point for running the Integration Test:

For each test, the following preparation steps must be performed:

  1. Clear database
  2. Prepare mocks
  3. Initialize testing module
[SetUp]
public async Task BeforeEachTest()
{
    const string connectionStringEnvironmentVariable =
        "ASPNETCORE_MyMeetings_IntegrationTests_ConnectionString";
    ConnectionString = Environment.GetEnvironmentVariable(connectionStringEnvironmentVariable, EnvironmentVariableTarget.Machine);
    if (ConnectionString == null)
    {
        throw new ApplicationException(
            $"Define connection string to integration tests database using environment variable: {connectionStringEnvironmentVariable}");
    }

using (var sqlConnection = new SqlConnection(ConnectionString))
{
    await ClearDatabase(sqlConnection);
}

Logger = Substitute.For<ilogger>();
EmailSender = Substitute.For<iemailsender>();
EventsBus = new EventsBusMock();
ExecutionContext = new ExecutionContextMock(Guid.NewGuid());

PaymentsStartup.Initialize(
    ConnectionString,
    ExecutionContext,
    Logger,
    EventsBus,
    false);

PaymentsModule = new PaymentsModule();

}

After preparation, test is performed on clear database. Usually, it is the execution of some (or many) Commands and: a) running a Query or/and b) verifying mocks to check the result.

[TestFixture]
public class MeetingPaymentTests : TestBase
{
    [Test]
    public async Task CreateMeetingPayment_Test()
    {
        PayerId payerId = new PayerId(Guid.NewGuid());
        MeetingId meetingId = new MeetingId(Guid.NewGuid());
        decimal value = 100;
        string currency = "EUR";
        await PaymentsModule.ExecuteCommandAsync(new CreateMeetingPaymentCommand(Guid.NewGuid(),
            payerId, meetingId, value, currency));

    var payment = await PaymentsModule.ExecuteQueryAsync(new GetMeetingPaymentQuery(meetingId.Value, payerId.Value));

    Assert.That(payment.PayerId, Is.EqualTo(payerId.Value));
    Assert.That(payment.MeetingId, Is.EqualTo(meetingId.Value));
    Assert.That(payment.FeeValue, Is.EqualTo(value));
    Assert.That(payment.FeeCurrency, Is.EqualTo(currency));
}

}

Each Command/Query processing is a separate execution (with different object graph resolution, context, database connection etc.) thanks to Composition Root of each module. This behavior is important and desirable.

3.14 System Integration Testing

Definition

System Integration Testing (SIT) is performed to verify the interactions between the modules of a software system. It involves the overall testing of a complete system of many subsystem components or elements.

Implementation

Implementation of system integration tests is based on approach of integration testing of modules in isolation (invoking commands and queries) described in the previous section.

The problem is that in this case we are dealing with asynchronous communication. Due to asynchrony, our test must wait for the result at certain times.

To correctly implement such tests, the Sampling technique and implementation described in the Growing Object-Oriented Software, Guided by Tests book was used:

An asynchronous test must wait for success and use timeouts to detect failure. This implies that every tested activity must have an observable effect: a test must affect the system so that its observable state becomes different. This sounds obvious but it drives how we think about writing asynchronous tests. If an activity has no observable effect, there is nothing the test can wait for, and therefore no way for the test to synchronize with the system it is testing. There are two ways a test can observe the system: by sampling its observable state or by listening for events that it sends out.

Test below: 1. Creates Meeting Group Proposal in Meetings module 2. Waits until Meeting Group Proposal to verification will be available in Administration module with 10 seconds timeout 3. Accepts Meeting Group Proposal in Administration module 4. Waits until Meeting Group is created in Meetings module with 15 seconds timeout

public class CreateMeetingGroupTests : TestBase
{
    [Test]
    public async Task CreateMeetingGroupScenario_WhenProposalIsAccepted()
    {
        var meetingGroupId = await MeetingsModule.ExecuteCommandAsync(
            new ProposeMeetingGroupCommand("Name",
            "Description",
            "Location",
            "PL"));

    AssertEventually(
        new GetMeetingGroupProposalFromAdministrationProbe(meetingGroupId, AdministrationModule), 
        10000);

    await AdministrationModule.ExecuteCommandAsync(new AcceptMeetingGroupProposalCommand(meetingGroupId));

    AssertEventually(
        new GetCreatedMeetingGroupFromMeetingsProbe(meetingGroupId, MeetingsModule),
        15000);
}

private class GetCreatedMeetingGroupFromMeetingsProbe : IProbe
{
    private readonly Guid _expectedMeetingGroupId;

    private readonly IMeetingsModule _meetingsModule;

    private List<meetinggroupdto> _allMeetingGroups;

    public GetCreatedMeetingGroupFromMeetingsProbe(
        Guid expectedMeetingGroupId, 
        IMeetingsModule meetingsModule)
    {
        _expectedMeetingGroupId = expectedMeetingGroupId;
        _meetingsModule = meetingsModule;
    }

    public bool IsSatisfied()
    {
        return _allMeetingGroups != null &amp;&amp; 
               _allMeetingGroups.Any(x =&gt; x.Id == _expectedMeetingGroupId);
    }

    public async Task SampleAsync()
    {
        _allMeetingGroups = await _meetingsModule.ExecuteQueryAsync(new GetAllMeetingGroupsQuery());
    }

    public string DescribeFailureTo() 
        =&gt; $"Meeting group with ID: {_expectedMeetingGroupId} is not created";
}

private class GetMeetingGroupProposalFromAdministrationProbe : IProbe
{
    private readonly Guid _expectedMeetingGroupProposalId;

    private MeetingGroupProposalDto _meetingGroupProposal;

    private readonly IAdministrationModule _administrationModule;

    public GetMeetingGroupProposalFromAdministrationProbe(Guid expectedMeetingGroupProposalId, IAdministrationModule administrationModule)
    {
        _expectedMeetingGroupProposalId = expectedMeetingGroupProposalId;
        _administrationModule = administrationModule;
    }

    public bool IsSatisfied()
    {
        if (_meetingGroupProposal == null)
        {
            return false;
        }

        if (_meetingGroupProposal.Id == _expectedMeetingGroupProposalId &amp;&amp;
            _meetingGroupProposal.StatusCode == MeetingGroupProposalStatus.ToVerify.Value)
        {
            return true;
        }

        return false;
    }

    public async Task SampleAsync()
    {
        try
        {
            _meetingGroupProposal =
                await _administrationModule.ExecuteQueryAsync(
                    new GetMeetingGroupProposalQuery(_expectedMeetingGroupProposalId));
        }
        catch
        {
            // ignored
        }
    }

    public string DescribeFailureTo()
        =&gt; $"Meeting group proposal with ID: {_expectedMeetingGroupProposalId} to verification not created";
}

}

Poller class implementation (based on example in the book):

public class Poller
{
    private readonly int _timeoutMillis;

private readonly int _pollDelayMillis;

public Poller(int timeoutMillis)
{
    _timeoutMillis = timeoutMillis;
    _pollDelayMillis = 1000;
}

public void Check(IProbe probe)
{
    var timeout = new Timeout(_timeoutMillis);
    while (!probe.IsSatisfied())
    {
        if (timeout.HasTimedOut())
        {
            throw new AssertErrorException(DescribeFailureOf(probe));
        }

        Thread.Sleep(_pollDelayMillis);
        probe.SampleAsync();
    }
}

private static string DescribeFailureOf(IProbe probe)
{
    return probe.DescribeFailureTo();
}

}

3.15 Event Sourcing

Theory

During the implementation of the Payment module, Event Sourcing was used. Event Sourcing is a way of preserving the state of our system by recording a sequence of events. No less, no more.

It is important here to really restore the state of our application from events. If we collect events only for auditing purposes, it is an Audit Log/Trail - not the Event Sourcing.

The main elements of Event Sourcing are as follows: - Events Stream - Objects that are restored based on events. There are 2 types of such objects depending on the purpose: -- Objects responsible for the change of state. In Domain-Driven Design they will be Aggregates. -- Projections: read models prepared for a specific purpose - Subscriptions : a way to receive information about new events - Snapshots: from time to time, objects saved in the traditional way for performance purposes. Mainly used if there are many events to restore the object from the entire event history. (Note: there is currently no snapshot implementation in the project)

Tool

In order not to reinvent the wheel, the SQL Stream Store library was used. As the documentation says:

SQL Stream Store is a .NET library to assist with developing applications that use event sourcing or wish to use stream based patterns over a relational database and existing operational infrastructure.

Like every library, it has its limitations and assumptions (I recommend the linked documentation chapter "Things you need to know before adopting"). For me, the most important 2 points from this chapter are: 1. "Subscriptions (and thus projections) are *eventually consistent** and always will be."* This means that there will always be an inconsistency time from saving the event to the stream and processing the event by the projector(s). 2. "No support for ambient System.Transaction scopes enforcing the concept of the stream as the consistency and transactional boundary." This means that if we save the event to a events stream and want to save something in the same transaction, we must use TransactionScope. If we cannot use TransactionScope for some reason, we must accept the Eventual Consistency also in this case.

Other popular tools:

  • EventStore "An industrial-strength database solution built from the ground up for event sourcing."
  • Marten ".NET Transactional Document DB and Event Store on PostgreSQL"

Implementation

There are 2 main "flows" to handle: - Command handling: change of state - adding new events to stream (writing) - Projection of events to create read models

Command Handling

The whole process looks like this:

  1. We create / update an aggregate by creating an event
  2. We add changes to the Aggregate Store. This is the class responsible for writing / loading our aggregates. We are not saving changes yet.
  3. As part of Unit Of Work a) Aggregate Store adds events to the stream b) messages are added to the Outbox

Command Handler:

public class BuySubscriptionCommandHandler : ICommandHandler
{
    private readonly IAggregateStore _aggregateStore;

private readonly IPayerContext _payerContext;

private readonly ISqlConnectionFactory _sqlConnectionFactory;

public BuySubscriptionCommandHandler(
    IAggregateStore aggregateStore, 
    IPayerContext payerContext, 
    ISqlConnectionFactory sqlConnectionFactory)
{
    _aggregateStore = aggregateStore;
    _payerContext = payerContext;
    _sqlConnectionFactory = sqlConnectionFactory;
}

public async Task<guid> Handle(BuySubscriptionCommand command, CancellationToken cancellationToken)
{
    var priceList = await PriceListProvider.GetPriceList(_sqlConnectionFactory.GetOpenConnection());

    var subscriptionPayment = SubscriptionPayment.Buy(
        _payerContext.PayerId,
        SubscriptionPeriod.Of(command.SubscriptionTypeCode),
        command.CountryCode,
        MoneyValue.Of(command.Value, command.Currency),
        priceList);

    _aggregateStore.AppendChanges(subscriptionPayment);

    return subscriptionPayment.Id;
}

}

SubscriptionPayment
Aggregate:
public class SubscriptionPayment : AggregateRoot
{
    private PayerId _payerId;

private SubscriptionPeriod _subscriptionPeriod;

private string _countryCode;

private SubscriptionPaymentStatus _subscriptionPaymentStatus;

private MoneyValue _value;

protected override void Apply(IDomainEvent @event)
{
    this.When((dynamic)@event);
}

public static SubscriptionPayment Buy(
    PayerId payerId,
    SubscriptionPeriod period,
    string countryCode,
    MoneyValue priceOffer,
    PriceList priceList)
{
    var priceInPriceList = priceList.GetPrice(countryCode, period, PriceListItemCategory.New);
    CheckRule(new PriceOfferMustMatchPriceInPriceListRule(priceOffer, priceInPriceList));

    var subscriptionPayment = new SubscriptionPayment();

    var subscriptionPaymentCreated = new SubscriptionPaymentCreatedDomainEvent(
        Guid.NewGuid(),
        payerId.Value,
        period.Code,
        countryCode,
        SubscriptionPaymentStatus.WaitingForPayment.Code,
        priceOffer.Value,
        priceOffer.Currency);

    subscriptionPayment.Apply(subscriptionPaymentCreated);
    subscriptionPayment.AddDomainEvent(subscriptionPaymentCreated);

    return subscriptionPayment;
}

private void When(SubscriptionPaymentCreatedDomainEvent @event)
{
    this.Id = @event.SubscriptionPaymentId;
    _payerId = new PayerId(@event.PayerId);
    _subscriptionPeriod = SubscriptionPeriod.Of(@event.SubscriptionPeriodCode);
    _countryCode = @event.CountryCode;
    _subscriptionPaymentStatus = SubscriptionPaymentStatus.Of(@event.Status);
    _value = MoneyValue.Of(@event.Value, @event.Currency);
}

AggregateRoot
base class:
public abstract class AggregateRoot
{
    public Guid Id { get; protected set; }

public int Version { get; private set; }

private readonly List<idomainevent> _domainEvents;

protected AggregateRoot()
{
    _domainEvents = new List<idomainevent>();

    Version = -1;
}

protected void AddDomainEvent(IDomainEvent @event)
{
    _domainEvents.Add(@event);
}

public IReadOnlyCollection<idomainevent> GetDomainEvents() =&gt; _domainEvents.AsReadOnly();

public void Load(IEnumerable<idomainevent> history)
{
    foreach (var e in history)
    {
        Apply(e);
        Version++;
    }
}

protected abstract void Apply(IDomainEvent @event);

protected static void CheckRule(IBusinessRule rule)
{
    if (rule.IsBroken())
    {
        throw new BusinessRuleValidationException(rule);
    }
}

}

Aggregate Store implementation with SQL Stream Store library usage:

public class SqlStreamAggregateStore : IAggregateStore
{
    private readonly IStreamStore _streamStore;

private readonly List<idomainevent> _appendedChanges;

private readonly List<aggregatetosave> _aggregatesToSave;

public SqlStreamAggregateStore(
    ISqlConnectionFactory sqlConnectionFactory)
{
    _appendedChanges = new List<idomainevent>();

    _streamStore =
        new MsSqlStreamStore(
            new MsSqlStreamStoreSettings(sqlConnectionFactory.GetConnectionString())
                {
                    Schema = DatabaseSchema.Name
            });

    _aggregatesToSave = new List<aggregatetosave>();
}

public async Task Save()
{
    foreach (var aggregateToSave in _aggregatesToSave)
    {
        await _streamStore.AppendToStream(
            GetStreamId(aggregateToSave.Aggregate),
            aggregateToSave.Aggregate.Version,
            aggregateToSave.Messages.ToArray());
    }

    _aggregatesToSave.Clear();
}

public async Task<t> Load<t>(AggregateId<t> aggregateId) where T : AggregateRoot
{
    var streamId = GetStreamId(aggregateId);

    IList<idomainevent> domainEvents = new List<idomainevent>();
    ReadStreamPage readStreamPage;
    do
    {
        readStreamPage = await _streamStore.ReadStreamForwards(streamId, StreamVersion.Start, maxCount: 100);
        var messages = readStreamPage.Messages;
        foreach (var streamMessage in messages)
        {
            Type type = DomainEventTypeMappings.Dictionary[streamMessage.Type];
            var jsonData = await streamMessage.GetJsonData();
            var domainEvent = JsonConvert.DeserializeObject(jsonData, type) as IDomainEvent;

            domainEvents.Add(domainEvent);
        }
    } while (!readStreamPage.IsEnd);

    var aggregate = (T)Activator.CreateInstance(typeof(T), true);

    aggregate.Load(domainEvents);

    return aggregate;
}

Events Projection

The whole process looks like this:

  1. Special class
    Subscriptions Manager
    subscribes to Events Store (using SQL Store Stream library)
  2. Events Store raises
    StreamMessageRecievedEvent
  3. Subscriptions Manager
    invokes all projectors
  4. If projector know how to handle given event, it updates particular read model. In current implementation it updates special table in SQL database.

SubscriptionsManager
class implementation:
public class SubscriptionsManager
{
    private readonly IStreamStore _streamStore;

public SubscriptionsManager(
    IStreamStore streamStore)
{
    _streamStore = streamStore;
}

public void Start()
{
    long? actualPosition;

    using (var scope = PaymentsCompositionRoot.BeginLifetimeScope())
    {
        var checkpointStore = scope.Resolve<icheckpointstore>();

        actualPosition = checkpointStore.GetCheckpoint(SubscriptionCode.All);
    }

    _streamStore.SubscribeToAll(actualPosition, StreamMessageReceived);
}

public void Stop()
{
    _streamStore.Dispose();
}

private static async Task StreamMessageReceived(
    IAllStreamSubscription subscription, StreamMessage streamMessage, CancellationToken cancellationToken)
{
    var type = DomainEventTypeMappings.Dictionary[streamMessage.Type];
    var jsonData = await streamMessage.GetJsonData(cancellationToken);
    var domainEvent = JsonConvert.DeserializeObject(jsonData, type) as IDomainEvent;

    using var scope = PaymentsCompositionRoot.BeginLifetimeScope();

    var projectors = scope.Resolve<ilist>&gt;();

    var tasks = projectors
        .Select(async projector =&gt;
        {
            await projector.Project(domainEvent);
        });

    await Task.WhenAll(tasks);

    var checkpointStore = scope.Resolve<icheckpointstore>();
    await checkpointStore.StoreCheckpoint(SubscriptionCode.All, streamMessage.Position);
}

}

Example projector:

internal class SubscriptionDetailsProjector : ProjectorBase, IProjector
{
    private readonly IDbConnection _connection;

public SubscriptionDetailsProjector(ISqlConnectionFactory sqlConnectionFactory)
{
    _connection = sqlConnectionFactory.GetOpenConnection();
}

public async Task Project(IDomainEvent @event)
{
    await When((dynamic) @event);
}

private async Task When(SubscriptionRenewedDomainEvent subscriptionRenewed)
{
    var period = SubscriptionPeriod.GetName(subscriptionRenewed.SubscriptionPeriodCode);

    await _connection.ExecuteScalarAsync("UPDATE payments.SubscriptionDetails " +
                                            "SET " +
                                                "[Status] = @Status, " +
                                                "[ExpirationDate] = @ExpirationDate, " +
                                                "[Period] = @Period " +
                                            "WHERE [Id] = @SubscriptionId",
        new
        {
            subscriptionRenewed.SubscriptionId,
            subscriptionRenewed.Status,
            subscriptionRenewed.ExpirationDate,
            period
        });
}

private async Task When(SubscriptionExpiredDomainEvent subscriptionExpired)
{
    await _connection.ExecuteScalarAsync("UPDATE payments.SubscriptionDetails " +
                                         "SET " +
                                         "[Status] = @Status " +
                                         "WHERE [Id] = @SubscriptionId",
        new
        {
            subscriptionExpired.SubscriptionId,
            subscriptionExpired.Status
        });
}

private async Task When(SubscriptionCreatedDomainEvent subscriptionCreated)
{
    var period = SubscriptionPeriod.GetName(subscriptionCreated.SubscriptionPeriodCode);

    await _connection.ExecuteScalarAsync("INSERT INTO payments.SubscriptionDetails " +
                                   "([Id], [Period], [Status], [CountryCode], [ExpirationDate]) " +
                                   "VALUES (@SubscriptionId, @Period, @Status, @CountryCode, @ExpirationDate)",
        new
        {
            subscriptionCreated.SubscriptionId,
            period,
            subscriptionCreated.Status,
            subscriptionCreated.CountryCode,
            subscriptionCreated.ExpirationDate
        });
}

}

Sample view of Event Store

Sample Event Store view after execution of SubscriptionLifecycleTests Integration Test which includes following steps: 1. Creating Price List 2. Buying Subscription 3. Renewing Subscription 4. Expiring Subscription

looks like this (SQL Stream Store table - payments.Messages):

3.16 Database Change Management

Database change management is accomplished by migrations/transitions versioning. Additionally, the current state of the database structure is also versioned.

Migrations are applied using a simple DatabaseMigrator console application that uses the DbUp library. The current state of the database structure is kept in the SSDT Database Project.

The database update is performed by running the following command:

dotnet DatabaseMigrator.dll "connection_string" "scripts_directory_path"

The entire solution is described in detail in the following articles: 1. Database change management (theory) 2. Using database project and DbUp for database management (implementation)

3.17 Continuous Integration

Definition

As defined on Martin Fowler's website:

Continuous Integration is a software development practice where members of a team integrate their work frequently, usually each person integrates at least daily - leading to multiple integrations per day. Each integration is verified by an automated build (including test) to detect integration errors as quickly as possible.

Implementation

Pipeline description

CI was implemented using GitHub Actions. For this purpose, one workflow, which triggers on Pull Request to master branch or Push to master branch was created. It contains 2 jobs: - build test, execute Unit Tests and Architecture Tests - execute Integration Tests

Steps description
a) Checkout repository - clean checkout of git repository
b) Setup .NET Core - install .NET Core SDK
c) Install dependencies - resolve NuGet packages
d) Build - build solution
e) Run Unit Tests - run automated Unit Tests (see section 3.10)
f) Run Architecture Tests - run automated Architecture Tests (see section 3.12)
g) Initialize containers - setup Docker container for MS SQL Server
h) Wait for SQL Server initialization - after container initialization MS SQL Server is not ready, initialization of server itself takes some time so 30 seconds timeout before execution of next step is needed
i) Create Database - create and initialize database
j) Migrate Database - execute database upgrade using DatabaseMigrator application (see 3.16 section)
k) Run Integration Tests - perform Integration and System Integration Testing (see section 3.13 and 3.14)

Workflow definition

Workflow definition: buildPipeline.yml

Example workflow execution

Example workflow output:

3.18 Static code analysis

In order to standardize the appearance of the code and increase its readability, the StyleCopAnalyzers library was used. This library implements StyleCop rules using the .NET Compiler Platform and is responsible for the static code analysis.

Using this library is trivial - it is just added as a NuGet package to all projects. There are many ways to configure rules, but currently the best way to do this is to edit the .editorconfig file. More information can be found at the link above.

Note! Static code analysis works best when the following points are met:

  1. Each developer has an IDE that respects the rules and helps to follow them
  2. The rules are checked during the project build process as part of Continuous Integration
  3. The rules are set to help your system grow. Static analysis is not a value in itself. Some rules may not make complete sense and should be turned off. Other rules may have higher priority. It all depends on the project, company standards and people involved in the project. Be pragmatic.

4. Technology

List of technologies, frameworks and libraries used for implementation:

5. How to Run

Install .NET Core 3.1

  • Download and install .NET Core 3.1 SDK

Create database

dotnet DatabaseMigrator.dll "connection_string" "scripts_directory_path"

"connection_string" - connection string to your database
"scriptsdirectorypath" - path to migration scripts

Seed database

  • Execute SeedDatabase.sql script
  • 2 test users will be created - check the script for usernames and passwords

Configure connection string

Set a database connection string called

MeetingsConnectionString
in the root of the API project's appsettings.json or use Secrets

Example config setting in appsettings.json for a database called

ModularMonolith
:
json
{
    "MeetingsConnectionString": "Server=(localdb)\\mssqllocaldb;Database=ModularMonolith;Trusted_Connection=True;"
}

Configure startup in IDE

  • Set the Startup Item in your IDE to the API Project, not IIS Express

Authenticate

  • Once it is running you'll need a token to make API calls. This is done via OAuth2 Resource Owner Password Grant Type. By default IdentityServer is configured with the following:
  • client_id = ro.client
  • client_secret = secret
    (this is literally the value - not a statement that this value is secret!)
  • scope = myMeetingsAPI openid profile
  • grant_type = password

Include the credentials of a test user created in the SeedDatabase.sql script - for example: -

username = [email protected]
-
password = testMemberPass

Example HTTP Request for an Access Token: ```http POST /connect/token HTTP/1.1 Host: localhost:5000

granttype=password &[email protected] &password=testMemberPass &clientid=ro.client &client_secret=secret ```

This will fetch an access token for this user to make authorized API requests using the HTTP request header

Authorization: Bearer 

If you use a tool such as Postman to test your API, the token can be fetched and stored within the tool itself and appended to all API calls. Check your tool documentation for instructions.

Run using Docker Compose

You can run whole application using docker compose from root folder:

shell
docker-compose up

It will create following services:
- MS SQL Server Database - Database Migrator - Application

6. Contribution

This project is still under analysis and development. I assume its maintenance for a long time and I would appreciate your contribution to it. Please let me know by creating an Issue or Pull Request.

7. Roadmap

List of features/tasks/approaches to add:

| Name | Status | Release date | | ------------------------ | -------- | -------- | | Domain Model Unit Tests |Completed | 2019-09-10 | | Architecture Decision Log update | Completed | 2019-11-09 | | Integration automated tests | Completed | 2020-02-24 | | Migration to .NET Core 3.1 |Completed | 2020-03-04 | | System Integration Testing | Completed | 2020-03-28 | | More advanced Payments module | Completed | 2020-07-11 | | Event Sourcing implementation | Completed | 2020-07-11 | | Database Change Management | Completed | 2020-08-23 | | Continuous Integration | Completed | 2020-09-01 | | StyleCop Static Code Analysis | Completed | 2020-09-05 | | FrontEnd SPA application | Completed | 2020-11-08 | | Docker support | Completed | 2020-11-26 | | PlantUML Conceptual Model | Completed | 2021-03-22 | | C4 Model | Completed | 2021-03-29 | | Meeting comments featurel | Completed | 2021-03-30 |

NOTE: Please don't hesitate to suggest something else or a change to the existing code. All proposals will be considered.

8. Authors

Kamil Grzybek

Blog: https://kamilgrzybek.com

Twitter: https://twitter.com/kamgrzybek

LinkedIn: https://www.linkedin.com/in/kamilgrzybek/

GitHub: https://github.com/kgrzybek

8.1 Main contributors

9. License

The project is under MIT license.

10. Inspirations and Recommendations

Modular Monolith

Domain-Driven Design

Application Architecture

Software Architecture

System Architecture

Design

Craftsmanship

Testing

UML

Event Storming

Event Sourcing

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