NEventLite

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NEventLite - An extensible lightweight library for .NET that manages the Aggregate lifecycle in an Event Sourced system. Supports Event and Snapshot storage providers like EventStore/Redis or SQL Server. Built with dependency injection in mind and seamlessly integrates with AspNetCore.

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# NEventLite [![Build status](https://ci.appveyor.com/api/projects/status/yfc041h8uvt7u3dq?svg=true)](https://ci.appveyor.com/project/dasiths/neventlite) [![NuGet](https://img.shields.io/nuget/v/NEventLite.svg)](https://www.nuget.org/packages/NEventLite) [![Downloads](https://img.shields.io/nuget/dt/NEventLite.svg)](https://www.nuget.org/packages/NEventLite/)

## An extensible lightweight library for .NET that manages the Aggregate lifecycle in an **Event Sourced** system

Supports `Event` and `Snapshot` storage providers like EventStore/Redis or SQL Server. Built with dependency injection in mind and seamlessly integrates with AspNetCore.

### Give a star :star: if you appreciate the effort

## What is Event Sourcing?

> Use an append-only store to record the full series of events that describe actions taken on data in a domain, rather than storing just the current state.

Start here [https://dasith.me/2016/12/02/event-sourcing-examined-part-1-of-3/](https://dasith.me/2016/12/02/event-sourcing-examined-part-1-of-3/)

## :goal_net: What does NEventLite solve?
NEventLite makes it easier to implement the event sourcing pattern in your .NET project. The library is opinionated and introduces some patterns to manage the life cycle of your [Aggregates](https://martinfowler.com/bliki/DDD_Aggregate.html) in an event sourced system. It will manage Creating, Loading, Mutating and Persisting Aggregates and Events.

## :warning: What doesn't it solve?

NEventLite is **not a framework** that manages your application end to end. It doesn't enforce ports and adapters pattern or any of the application level concerns. The aim is to do one thing (Manage aggregate lifecycle) and do that well. If you need to implement command and event handlers you can have a look at something like [SimpleMediator](https://github.com/dasiths/SimpleMediator) or [Brighter](https://github.com/BrighterCommand/Brighter) and NEventLite will complement them nicely.

## What about v1.0? Wasn't it advertised as a framework? 
*NEventLite V1.0 tried to solve similar problems but the scope of the project very large and it was decided to narrow down the scope. If you're still looking for reference it's hosted [here](https://github.com/dasiths/NEventLite/blob/master/legacy/v1.0).*

## :eyes: Before you start

- The library targets .NET Standard 2.0
- *Optional:* Installation of EventStore - [https://eventstore.com/](https://eventstore.com/) (You can use the in memory event and snapshot providers when developing)

## :hammer: Using It

1. Install and reference the NuGet `NEventLite`

In the NuGet Package Manager Console, type:

```ps
    Install-Package NEventLite
    ```

2. Define the events. They are simple pocos that will be serialized and stored in `EventStorage` when changes are saved. Events use `Guid` for Id by *default* but they can be changed to use any data type as Id. See `Event<TAggregate, TAggregateKey, TEventKey>` for reference.

```csharp
        public class ScheduleCreatedEvent : Event<Schedule>
        {
            public string ScheduleName { get; set; }

public ScheduleCreatedEvent(Guid scheduleId, string scheduleName) : base(Guid.NewGuid(), scheduleId)
            {
                ScheduleName = scheduleName;
            }
        }

public class TodoCreatedEvent : Event<Schedule>
        {
            public Guid TodoId { get; set; }
            public string Text { get; set; }

public TodoCreatedEvent(Guid aggregateId, long targetVersion, Guid todoId, string text) : base(Guid.NewGuid(), aggregateId, targetVersion)
            {
                TodoId = todoId;
                Text = text;
            }
        }
    ```

3. Define the Aggregate (`Schedule.cs` in the sample). Aggregates use `Guid` as Id by *default* but just like the events, they can be changed to the Id type of your choice. See `AggregateRoot<TAggregateKey>` for reference.

```csharp
        public class Schedule : AggregateRoot
        {
            public IList<Todo> Todos { get; private set; }
            public string ScheduleName { get; private set; }

// To create or mutate, call the constructor or methods.

// Constructor
            public Schedule(string scheduleName)
            {
                // Pattern: Create an event and apply it
                var newScheduleId = Guid.NewGuid();
                var @event = new ScheduleCreatedEvent(newScheduleId, scheduleName);
                ApplyEvent(@event);
            }

public Guid AddTodo(string text)
            {
                // Pattern: Create an event and apply it
                var newTodoId = Guid.NewGuid();
                var @event = new TodoCreatedEvent(Id, CurrentVersion, newTodoId, text);
                ApplyEvent(@event);
                return newTodoId;
            }

// ** Mutations happen through applying events **
            // The library identifies the internal event handler methods though a special method attribute.

[InternalEventHandler]
            public void OnScheduleCreated(ScheduleCreatedEvent @event)
            {
                ScheduleName = @event.ScheduleName;
                Todos = new List<Todo>();
            }

[InternalEventHandler]
            public void OnTodoCreated(TodoCreatedEvent @event)
            {
                var todo = new Todo(@event.TodoId, @event.Text);
                Todos.Add(todo);
            }
        }
    ```

4. Use the built in `Session` and `Repository` implementations to manage the Aggregate lifecycle. This is an example of how you would create and update a `Schedule` in your domain layer using some sort of command handler.

```csharp
    // We recommend using a DI container to inject the Session. Keep it scoped (per request in a web application)

public class CreateScheduleCommandHandler
        {
            private readonly ISession<Schedule> _session;

public CreateScheduleCommandHandler(ISession<Schedule> session)
            {
                _session = session;
            }

public async Task<Guid> HandleAsync()
            {
                var schedule = new Schedule("my new schedule");
                _session.Attach(schedule);
                await _session.SaveAsync();
                return schedule.Id;
            }
        }

public class AddTodoCommandHandler
        {
            private readonly ISession<Schedule> _session;

public AddTodoCommandHandler(ISession<Schedule> session)
            {
                _session = session;
            }

public async Task<Guid> HandleAsync(Guid scheduleId)
            {
                var schedule = await _session.GetByIdAsync(scheduleId);
                var id = schedule.AddTodo("test todo 1");
                await _session.SaveAsync();
                return id;
            }
        }
    ```

5. If you are using it in ASPNET.CORE consider using the `NEventLite.Extensions.Microsoft.DependencyInjection` NuGet package to setup your container in the dependency injection example below.

## :syringe: Dependency Injection

The library is built with DI as a first class concept. Wiring it up is easy. This is an example of how you would do it with `Microsoft.Extensions.DependencyInjection`. You can find a detailed [example in the sample console app](https://github.com/dasiths/NEventLite/blob/master/src/Samples/NEventLite.Samples.ConsoleApp/DependencyInjection.cs).

```csharp
    public static ServiceProvider GetContainer() {
        var services = new ServiceCollection();

// >> Step 1: Register the storage provider

// Wiring up the event storage provider. "https://eventstore.com/" in this example
            // Event Store specific settings etc
            services.AddScoped<IEventStoreSettings, EventStoreSettings>(
                (sp) => new EventStoreSettings(SnapshotFrequency, PageSize));
            services.AddScoped<IEventStoreStorageConnectionProvider, EventStoreStorageConnectionProvider>();
            services.AddScoped<IEventStoreStorageCore, EventStoreStorageCore>();

// The storage provider implementations
            services.AddScoped<IEventStorageProvider<Guid>, EventStoreEventStorageProvider>();
            services.AddScoped<ISnapshotStorageProvider<Guid>, EventStoreSnapshotStorageProvider>();

// >> Step 2: Register the Repository and Session

// Use the extension method "ScanAndRegisterAggregates()" in the
            // "NEventLite.Extensions.Microsoft.DependencyInjection" nuget library as shown below

services.ScanAndRegisterAggregates();

// Or if you prefer to register the manually

// Register the repository
            services.AddScoped<IRepository<Schedule, Guid, Guid>, Repository<Schedule, ScheduleSnapshot, Guid, Guid, Guid>>();
            // If you prefer to work without snapshots and use events only repository
            // services.AddScoped<IRepository<Schedule, Guid, Guid>, EventOnlyRepository<Schedule>>();

// register the session implementation for the Aggregate
            services.AddScoped<ISession<Schedule>, Session<Schedule>>();
            // or if prefer to you use the more detailed interface
            // services.AddScoped<ISession<Schedule, Guid, Guid>, Session<Schedule>>();

// >> Step 3: Register the other required dependencies

// Use the defaults
            services.AddSingleton<IClock, DefaultSystemClock>();
            services.AddSingleton<IEventPublisher, DefaultNoOpEventPublisher>();

// Or
            // use your own implementation
            // services.AddSingleton<IClock, MyClock>();
            // services.AddSingleton<IEventPublisher, MyEventPublisher>();

var container = services.BuildServiceProvider();
        return container;
    }
```

If you want to use it with a different dependency injection framework, you can look at how the assembly scanning and registration is implemented for `Microsoft.Extensions.DependencyInjection` as an example and come up with your own implementation. The file is [located here](https://github.com/dasiths/NEventLite/blob/master/src/Extensions/NEventLite.Extensions.Microsoft.DependencyInjection/Extensions.cs).

```csharp
    public static void ScanAndRegisterAggregates(this ServiceCollection services, IList<Assembly> assemblies)
    {
        foreach (var a in assemblies.GetAllAggregates()) // Use the built in GetAllAggregates() extensions method to find aggregate information
        {
            services.RegisterAggregate(a);
        }
    }

public static void RegisterAggregate(this ServiceCollection services, AggregateInformation a)
    {
        // Register full generic types
        services.AddScoped(typeof(IRepository<,,>).MakeGenericType(a.Aggregate, a.AggregateKey, a.EventKey),
            a.Snapshot != null
                ? typeof(Repository<,,,,>).MakeGenericType(a.Aggregate, a.Snapshot, a.AggregateKey,
                    a.EventKey, a.SnapshotKey)
                : typeof(EventOnlyRepository<,,>).MakeGenericType(a.Aggregate, a.AggregateKey, a.EventKey));

services.AddScoped(typeof(ISession<,,>).MakeGenericType(a.Aggregate, a.AggregateKey, a.EventKey),
            typeof(Session<,,>).MakeGenericType(a.Aggregate, a.AggregateKey, a.EventKey));

// Register the convenience GUID scoped ISession interface as well
        if (a.AggregateKey == typeof(Guid) && a.EventKey == typeof(Guid))
        {
            services.AddScoped(typeof(ISession<>).MakeGenericType(a.Aggregate),
                typeof(Session<>).MakeGenericType(a.Aggregate));
        }
    }
```

## :ledger: Storage providers

The library contains storage provider implementation for **[EventStore](https://eventstore.com/)** and we plan to include a few more in the future. We have also included an in memory event and snapshot storage provider to get you up and running faster.

It's very easy to implement your own as well. Implement `IEventStorageProvider` and `ISnapshotStorageProvider` interfaces shown below and register then in your DI. If you need help look at the `NEventLite.StorageProviders.EventStore` project in the repository.

```csharp
    // Interface for persisting and reading events
    public interface IEventStorageProvider<TEventKey>
    {
        Task<IEnumerable<IEvent<AggregateRoot<TAggregateKey, TEventKey>, TAggregateKey, TEventKey>>> GetEventsAsync<TAggregate, TAggregateKey>(TAggregateKey aggregateId, long start, long count)
            where TAggregate : AggregateRoot<TAggregateKey, TEventKey>;

Task<IEvent<AggregateRoot<TAggregateKey, TEventKey>, TAggregateKey, TEventKey>> GetLastEventAsync<TAggregate, TAggregateKey>(TAggregateKey aggregateId)
            where TAggregate : AggregateRoot<TAggregateKey, TEventKey>;

Task SaveAsync<TAggregate, TAggregateKey>(TAggregateKey aggregateId, IEnumerable<IEvent<AggregateRoot<TAggregateKey, TEventKey>, TAggregateKey, TEventKey>> events)
            where TAggregate: AggregateRoot<TAggregateKey, TEventKey>;
    }

// Interface for persisting and reading snapshots
    public interface ISnapshotStorageProvider<in TSnapshotKey>
    {
        int SnapshotFrequency { get; }

Task<TSnapshot> GetSnapshotAsync<TSnapshot, TAggregateKey>(TAggregateKey aggregateId)
            where TSnapshot : ISnapshot<TAggregateKey, TSnapshotKey>;

Task SaveSnapshotAsync<TSnapshot, TAggregateKey>(TSnapshot snapshot)
            where TSnapshot : ISnapshot<TAggregateKey, TSnapshotKey>;
    }

// Implement the interfaces
    // Then register them in your DI
    services.AddScoped<IEventStorageProvider<YourKeyType>, MyEventStorageProvider>();
    services.AddScoped<ISnapshotStorageProvider<YourKeyType>, MySnapshotStorageProvider>();
```

## Examples

There are more examples in the [Samples folder](https://github.com/dasiths/NEventLite/tree/master/src/Samples) if you need help figuring out how to put everything together.

## Notes

Please feel free to contribute and improve the code as you see fit. Please raise an issue if you find a bug or have an improvement idea. The repository is shared under the MIT license.

**Share the :heart: and let your friends and colleagues know about this cool project. Thank you.**

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