Event Sourcing is a design pattern that has gained increasing popularity in recent years. It is a way of building software applications that is based on the idea of capturing all changes made to the application state as a sequence of events. This approach allows developers to reconstruct the state of the application at any point in time by replaying the events in the correct order.
In this article, we will discuss how to implement the Event Sourcing pattern in C# using EFCore. We will also provide examples to illustrate the concepts covered.
What is Event Sourcing?
Event Sourcing is a pattern that involves capturing all changes made to the state of an application as a sequence of events. Each event represents a change to the state of the application and is stored in a persistent event store. The state of the application is then reconstructed by replaying the events in the correct order.
The key advantage of Event Sourcing is that it provides a complete audit trail of all changes made to the state of the application. This can be useful for debugging purposes and for compliance with regulatory requirements.
Implementing Event Sourcing with EFCore
In order to implement the Event Sourcing pattern with EFCore, we need to create an event store that will persist all the events that are generated by the application. We can use a relational database as the event store and map the events to database tables using EFCore.
Defining the Event Model
The first step is to define the event model. The event model should include all the data that is required to represent a change to the state of the application. For example, if we were implementing Event Sourcing for a banking application, the event model might include the following properties:
Event Id
Account Id
Transaction Type (e.g. deposit, withdrawal)
Amount
Timestamp
We can define the event model as a C# class, like this:
public class Event
{
public int Id { get; set; }
public int AccountId { get; set; }
public string TransactionType { get; set; }
public decimal Amount { get; set; }
public DateTime Timestamp { get; set; }
}
Defining the Event Store
The next step is to define the event store. We can use a relational database as the event store and map the events to database tables using EFCore.
To create the event store, we need to define a DbContext that includes a DbSet for the event model. We can define the DbContext like this:
public class EventStoreContext : DbContext
{
public DbSet<Event> Events { get; set; }
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EventStore;Trusted_Connection=True;");
}
}
This defines a DbContext that includes a DbSet for the Event model. We also specify the connection string for the database that we will use to store the events.
Storing Events in the Event Store
Now that we have defined the event model and the event store, we can start storing events in the event store. Whenever a change is made to the state of the application, we create a new event and add it to the event store.
We can use EFCore to add new events to the event store like this:
using (var context = new EventStoreContext())
{
var newEvent = new Event
{
AccountId = 123,
TransactionType = "deposit",
Amount = 100,
Timestamp = DateTime.Now
};
context.Events.Add(newEvent);
context.SaveChanges();
}
This creates a new event and This creates a new event and adds it to the Events DbSet in the event store context. The SaveChanges method then persists the event to the database.
Reconstructing the State of the Application
The final step in implementing Event Sourcing with EFCore is to reconstruct the state of the application by replaying the events in the correct order. We can use EFCore to retrieve the events from the event store and apply them to the application state.
We can retrieve the events from the event store like this:
using (var context = new EventStoreContext())
{
var events = context.Events.OrderBy(e => e.Timestamp).ToList();
foreach (var e in events)
{
// Apply the event to the application state
}
}
This retrieves all the events from the event store in the correct order based on their timestamps. We can then iterate over the events and apply them to the application state.
In order to apply the events to the application state, we need to define a function that takes an event as input and updates the application state accordingly. For example, if we were implementing Event Sourcing for a banking application, the function might look like this:
private void ApplyEvent(Event e)
{
var account = _accounts.SingleOrDefault(a => a.Id == e.AccountId);
if (account == null)
{
account = new Account { Id = e.AccountId };
_accounts.Add(account);
}
if (e.TransactionType == "deposit")
{
account.Balance += e.Amount;
}
else if (e.TransactionType == "withdrawal")
{
account.Balance -= e.Amount;
}
}
This function takes an event as input and updates the application state by adding or subtracting the amount from the account balance.
We can then apply the events to the application state like this:
using (var context = new EventStoreContext())
{
var events = context.Events.OrderBy(e => e.Timestamp).ToList();
foreach (var e in events)
{
ApplyEvent(e);
}
}
This iterates over the events in the correct order and applies them to the application state by calling the ApplyEvent function.
Conclusion
In this article, we have discussed how to implement the Event Sourcing pattern in C# using EFCore. We have defined the event model, created the event store, stored events in the event store, and reconstructed the state of the application by replaying the events in the correct order.
Event Sourcing is a powerful pattern that provides a complete audit trail of all changes made to the state of the application. It can be used in a variety of applications, from banking to e-commerce to healthcare. By using EFCore to implement Event Sourcing, we can take advantage of the rich set of features and performance optimizations that EFCore provides.