Event Store

Event Souring Library that allows storing Events into a Database. Comes with an implementation of a Azure CosmosDb EventStore.

The CosmosDb Implementation uses the following Microsoft libraries:

• Microsoft.Azure.Cosmos
• Microsoft.Extensions.Configuration.Abstractions
• Microsoft.Extensions.Configuration.Binder
• Microsoft.Extensions.Logging.Abstractions
• Microsoft.Extensions.Options

and comes with included compatability to the dependency injection of the .NET Core (Web)HostBuilder.

A Sample can be found in the samples directory.

Version

v3.x

V3 supports mainly .NET 5.0 and registration of events and event aggregators through reflection

v4

• V4 only supports .NET 6.0

It introduces the concept of Code Generated registration of events and aggregators by decorating them with the EventName attribute.

It also decouples the auto generated event type descriptor (was basically a description used to revert to a type using Type.GetType()) from the concrete implementation. This allows to version and migrate events by just adding a different descriptor.

A sample on how to use the Event Descriptors is found under Samples. The Extension Method AddCodeGeneratedEvents() is automatically generated during compilation if the package Papst.EventStore.CodeGeneration is added to the Project.

Migrate v3 events by adding a EventName attribute and add the typename as a name: [Fullename of the type],[assembly name of the type]. For the MyEventSourcingEvent in the Code Generation Sample it would look like this:

SampleCodeGeneratedEvents.MyEventSourcingEvent,SampleCodeGeneratedEvents

Breaking Change

V4 removes support for authenticating with shared keys against the cosmos DB. The implementation is still there, but changed and marked as obsolete.

Installing the Library

• Papst.EventStore.Abstractions
• Papst.EventStore.CosmosDb
• Papst.EventStore.CodeGeneration

Configuring the Code Generator

• Events must be attributed with the EventName attribute.
• EventAggregators, implementing the interface IEventAggregator or implementing the abstract class EventAggregatorBase are automatically added to the Dependency Injection.

Configuring Write and Read Event Identifier

It is possible to have multiple EventName attributes on just one event. With the attribute IsWriteName it is possible to define which Identifier is used when writing the event.

[EventName(Name = "MyEventV1", IsWriteName = false)]
[EventName(Name = "MyEventV2")]
public class MyEventsourcingEvent 
{

}

Reading Events named MyEventV1 or MyEventV2 will deserialize them into a MyEventsourcingEvent. Writing MyEventsourcingEvent to the Event Stream, will serialize them and name them MyEventV2.

Note: IsWriteName is true by default!

Configuring the Cosmos Db EventStore

The CosmosEventStore class uses the Options pattern to retrieve its configuration. Therefore a ConfigurationSection can be added to the AddCosmosEventStore extension method for the IServiceCollection. The C# representation of the Configuration Sections looks like this:

/// <summary>
/// Configuration for the Cosmos Database Connection
/// </summary>
public class CosmosEventStoreOptions
{
    /// <summary>
    /// Endpoint URL
    /// </summary>
    public string Endpoint { get; set; }

    /// <summary>
    /// Whether to try to create Database and collection after creation of the Client
    /// </summary>
    public bool InitializeOnStartup { get; set; }

    /// <summary>
    /// Name of the Collection
    /// </summary>
    public string Collection { get; set; }

    /// <summary>
    /// Name of the Database
    /// </summary>
    public string Database { get; set; }

    /// <summary>
    /// Whether to allow a Event Timestamp to be set from the outside or 
    /// to override the time before inserting the document
    /// </summary>
    public bool AllowTimeOverride { get; set; }

    /// <summary>
    /// Credential which is used to Authenticate agains the Database
    /// </summary>
    public TokenCredential? Credential { get; internal set; }
}