Observing DbSet<>

Entity Framework Core observation support allows you to monitor changes to your entities in real-time using reactive extensions. This feature enables you to create observable queries that automatically update when data changes, either through your application or external database modifications.

Configuration

To enable observation support, you only need to register the observation services in your service collection.

Register Observation Services

Add observation services to your service collection:

services.AddEntityFrameworkCoreObservation();

This registers the necessary services for tracking entity changes and database-level notifications.

If your DbContext inherits from BaseDbContext and is registered using the Arc extension methods (AddDbContextWithConnectionString or AddReadOnlyDbContext), observation support is automatically enabled when the services are registered. No additional configuration is needed.

Manual Configuration (Advanced)

If you're not using BaseDbContext or the Arc registration methods, you can manually add observation support at registration time:

services.AddPooledDbContextFactory<MyDbContext>((serviceProvider, options) =>
{
    options.UseSqlServer(connectionString)
           .AddObservation(serviceProvider);
});

services.AddScoped(serviceProvider =>
{
    var factory = serviceProvider.GetRequiredService<IDbContextFactory<MyDbContext>>();
    return factory.CreateDbContext();
});

Important: When using pooled DbContext factories (AddPooledDbContextFactory), all configuration must be done at registration time. You cannot modify options in OnConfiguring when pooling is enabled.

Usage

Once configured, you can create observable queries using extension methods on DbSet<TEntity>.

Observe a Collection

Monitor changes to a collection of entities:

var observable = dbContext.MyEntities.Observe();
observable.Subscribe(entities => 
{
    // Handle updated collection
    Console.WriteLine($"Collection updated: {entities.Count()} items");
});

Observe with Filter

Apply filters to observe specific entities:

var observable = dbContext.Orders.Observe(order => order.Status == OrderStatus.Pending);
observable.Subscribe(pendingOrders => 
{
    // Handle updates to pending orders only
});

Observe a Single Entity

Monitor changes to a specific entity:

var observable = dbContext.Products.ObserveSingle(p => p.Sku == "ABC123");
observable.Subscribe(product => 
{
    // Handle updates to the specific product
});

Observe by Id

Monitor a single entity using its identifier:

var observable = dbContext.Customers.ObserveById<Customer, Guid>(customerId);
observable.Subscribe(customer => 
{
    // Handle updates to the customer
});

How It Works

The observation feature combines two notification mechanisms:

In-Process Changes: Changes made through your application's DbContext are tracked via EF Core interceptors
Database-Level Changes: External changes are detected using database-specific notification mechanisms:
- SQL Server: Uses SqlDependency or polling
- PostgreSQL: Uses LISTEN/NOTIFY
- SQLite: Uses polling

When any change is detected, the observable query is re-executed and subscribers are notified with the updated results.

Change Detection

The observation system detects changes when:

Entities are added, modified, or deleted through SaveChanges() or SaveChangesAsync()
External processes modify the database (via database-level notifications)
Changes match the filter criteria of your observable query

Best Practices

Use filters to limit the scope of observations and improve performance
Dispose of subscriptions when no longer needed to prevent memory leaks
Consider using ObserveSingle or ObserveById when monitoring individual entities
Be mindful of database notification limits and capabilities for your specific database provider

Example: Real-Time Dashboard

public class OrderDashboard
{
    private readonly IDisposable _subscription;

    public OrderDashboard(MyDbContext dbContext)
    {
        _subscription = dbContext.Orders
            .Observe(o => o.Status == OrderStatus.Processing)
            .Subscribe(processingOrders =>
            {
                UpdateDashboard(processingOrders);
            });
    }

    public void Dispose()
    {
        _subscription?.Dispose();
    }

    private void UpdateDashboard(IEnumerable<Order> orders)
    {
        // Update UI or metrics
    }
}

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