EventSourcing

A .NET Core event sourcing framework.

Easy to be integrated in ASP.NET Core web application, Lambda function or Azure function.

Support various of event store:

• in file system as plain text file (see File system setup instructions)
• in AWS DynamoDB (see DynamoDB setup instructions)
• in Azure CosmosDB (see CosmosDB setup instructions)
• in any relational database supported by EF Core, e.g., Microsoft SQL Server,MySQL, etc. (see EF Core setup instructions)

NuGet packages

• JKang.EventSourcing
• JKang.EventSourcing.Persistence.FileSystem
• JKang.EventSourcing.Persistence.EfCore
• JKang.EventSourcing.Persistence.DynamoDB
• JKang.EventSourcing.Persistence.CosmosDB
• JKang.EventSourcing.Persistence.S3
• JKang.EventSourcing.Persistence.Caching

Quick Start:

Let's implement a simple gift card management system with the following use cases:

• Create gift cards with initial credit
• Debit the gift card specifying amount while overpaying is not allowed

I'm adopting DDD (Domain Driven Design) approach and implement the GiftCard entity as an Rich Domain Aggregate which encapsulates/protects its internal data/state, and contains itself business logics ensuring data integrity.

Step 1 - Create aggregate events

public sealed class GiftCardCreated : AggregateCreatedEvent<Guid>
{
    public GiftCardCreated(Guid aggregateId, DateTime timestamp, decimal initialCredit)
        : base(aggregateId, timestamp)
    {
        InitialCredit = initialCredit;
    }

    public decimal InitialCredit { get; }
}

public class GiftCardDebited : AggregateEvent<Guid>
{
    public GiftCardDebited(Guid aggregateId, int aggregateVersion, DateTime timestamp, decimal amount)
        : base(aggregateId, aggregateVersion, timestamp)
    {
        Amount = amount;
    }

    public decimal Amount { get; }
}

Notes:

• It's recommended to implement aggregate event in an immutable way.
• Inheriting from AggregateEvent<TKey> or AggregateCreatedEvent<TKey> is not mandatory, but an aggreagte event must at least implement IAggregateEvent<TKey> interface.
• In order to use built-in event stores, please make sure event can be properly serialized using Json.NET.

Step 2 - Create domain aggregate

public class GiftCard : Aggregate<Guid>
{
    /// <summary>
    /// Constructor for creating an new gift card from scratch
    /// </summary>
    public GiftCard(decimal initialCredit)
        : base(new GiftCardCreated(Guid.NewGuid(), DateTime.UtcNow, initialCredit))
    { }

    /// <summary>
    /// Constructor for rehydrating gift card from historical events
    /// </summary>
    public GiftCard(Guid id, IEnumerable<IAggregateEvent<Guid>> savedEvents)
        : base(id, savedEvents)
    { }

    /// <summary>
    /// Constructor for rehydrating gift card from a snapshot + historical events after the snapshot
    /// </summary>
    public GiftCard(Guid id, IAggregateSnapshot<Guid> snapshot, IEnumerable<IAggregateEvent<Guid>> savedEvents)
        : base(id, snapshot, savedEvents)
    { }

    public decimal Balance { get; private set; }

    public void Debit(decimal amout)
        => ReceiveEvent(new GiftCardDebited(Id, GetNextVersion(), DateTime.UtcNow, amout));

    protected override void ApplyEvent(IAggregateEvent<Guid> @event)
    {
        if (@event is GiftCardCreated created)
        {
            Balance = created.InitialCredit;
        }
        else if (@event is GiftCardDebited debited)
        {
            if (debited.Amount < 0)
            {
                throw new InvalidOperationException("Negative debit amout is not allowed.");
            }

            if (Balance < debited.Amount)
            {
                throw new InvalidOperationException("Not enough credit");
            }

            Balance -= debited.Amount;
        }
    }
}

Notes:

• Please ensure that state of domain aggregate can only be changed by applying aggregate events.
• Inheriting from Aggregate<TKey> is not mandatory, but the minimum requirements for implementing a domain aggregate are:
  • Implement IAggregate<TKey> interface
  • Have a public constructor with signature MyAggregate(TKey id, IEnumerable<IAggregateEvent<TKey>> savedEvents)
  • Have a public constructor with signature MyAggregate(TKey id, IAggregateSnapshot<TKey> snapshot, IEnumerable<IAggregateEvent<TKey>> savedEvents)

Step 3 - Implement repository

By definition of Event Sourcing, persisting an aggregate insists on persisting all historical events.

public interface IGiftCardRepository
{
    Task SaveGiftCardAsync(GiftCard giftCard);
    Task<GiftCard> FindGiftCardAsync(Guid id);
}

public class GiftCardRepository : AggregateRepository<GiftCard, Guid>, 
    IGiftCardRepository
{
    public GiftCardRepository(IEventStore<GiftCard, Guid> eventStore)
        : base(eventStore)
    { }

    public Task SaveGiftCardAsync(GiftCard giftCard) =>
        SaveAggregateAsync(giftCard);

    public Task<GiftCard> FindGiftCardAsync(Guid id) =>
        FindAggregateAsync(id);
}

Step 4 - Register your repository interface and configure event store in dependency injection framework

services
    .AddScoped<IGiftCardRepository, GiftCardRepository>();

services
    .AddEventSourcing(builder =>
    {
        builder.UseTextFileEventStore<GiftCard, Guid>(x =>
            x.Folder = "C:/Temp/GiftcardEvents");
    });

Notes:

• You can choose other persistence store provided such as CosmosDB or DynamoDB etc.

Step 5 - implmement use cases

// create a new gift card with initial credit 100
var giftCard = new GiftCard(100);

// persist the gift card
await _repository.SaveGiftCardAsync(giftCard);

// rehydrate the giftcard
giftCard = await _repository.FindGiftCardAsync(giftCard.Id);

// payments
giftCard.Debit(40); // ==> balance: 60
giftCard.Debit(50); // ==> balance: 10
giftCard.Debit(20); // ==> invalid operation exception

FAQs

How to programmatically initialize event store?

See this page.

How to use snapshots to optimize performance?

See this page.

How to improve performance using caching?

Consider install the nuget package JKang.EventSourcing.Persistence.Caching and inherit the CachedAggregateRepository class. It leverages Microsoft.Extensions.Caching.Distributed.IDistributedCache to cache aggregate every time after loaded from or saved into repository.

Consider configuring a short sliding expiration (e.g., 5 sec) to reduce the chance of having cache out of date.

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