Tagging Projections

Tags provide a way to organize and tag your projections for better discoverability and management. By applying the [Tag] attribute to your projection classes, you can assign one or more tags that describe the purpose or domain of the projection.

[Tag] and [Tags] on projections label the projection definition. They do not filter which appended events the projection observes. Use projection event declarations and event sequence selection to control projection input, and use reducer or reactor filters when you need metadata-based filtering for appended events. See Filter reducers and reactors by appended event metadata.

Adding Tags

You can tag projections in multiple ways:

Single Tag

Apply a single tag to your projection:

using Cratis.Chronicle.Observation;
using Cratis.Chronicle.Projections;

[Tag("Analytics")]
public class OrderAnalyticsProjection : IProjectionFor<OrderAnalytics>
{
    public ProjectionId Identifier => "order-analytics";

    public void Define(IProjectionBuilderFor<OrderAnalytics> builder)
    {
        builder
            .From<OrderPlaced>(_ => _
                .Set(m => m.OrderId).To(e => e.OrderId))
            .From<ItemAddedToOrder>(_ => _
                .Add(m => m.TotalAmount).With(e => e.Price * e.Quantity));
    }
}

Multiple Tags (Single Attribute)

Use the params feature to specify multiple tags in a single attribute:

[Tag("Analytics", "Reporting", "Dashboard")]
public class SalesReportProjection : IProjectionFor<SalesReport>
{
    public ProjectionId Identifier => "sales-report";

    public void Define(IProjectionBuilderFor<SalesReport> builder)
    {
        // Projection definition
    }
}

Multiple Tags (Multiple Attributes)

Apply multiple [Tag] attributes:

[Tag("Analytics")]
[Tag("Compliance")]
[Tag("Auditing")]
public class ComplianceReportProjection : IProjectionFor<ComplianceReport>
{
    public ProjectionId Identifier => "compliance-report";

    public void Define(IProjectionBuilderFor<ComplianceReport> builder)
    {
        // Projection definition
    }
}

Mixed Approach

Combine both approaches:

[Tag("Analytics", "Reporting")]
[Tag("Executive")]
public class ExecutiveDashboardProjection : IProjectionFor<ExecutiveDashboard>
{
    public ProjectionId Identifier => "executive-dashboard";

    public void Define(IProjectionBuilderFor<ExecutiveDashboard> builder)
    {
        // Projection definition
    }
}

Model-Bound Projections

Tags also work with model-bound projections:

using Cratis.Chronicle.Projections;

[Tag("Inventory", "Operations")]
public record ProductInventory(
    Guid ProductId,
    string Name,
    int QuantityInStock,
    decimal UnitPrice);

Best Practices

Use meaningful names: Choose tag names that clearly describe the purpose or domain
Be consistent: Establish tag naming conventions across your organization
Don't over-tag: Apply only relevant tags; too many can reduce their usefulness
Group related projections: Use tags to group projections that serve similar purposes

Common Tag Examples

Here are some common patterns for tagging projections:

// By domain
[Tag("Sales")]
[Tag("Inventory")]
[Tag("Customer")]

// By purpose
[Tag("Analytics")]
[Tag("Reporting")]
[Tag("Dashboard")]
[Tag("Search")]

// By stakeholder
[Tag("Executive")]
[Tag("Operations")]
[Tag("Finance")]

// By consistency model
[Tag("Immediate")]
[Tag("Eventual")]

// By data type
[Tag("Aggregates")]
[Tag("Lists")]
[Tag("Details")]

Querying by Tag

Tags stored in the event store definition can be used for:

Filtering and searching for specific projections
Organizing projections in administrative interfaces
Generating documentation
Managing projection deployments by tag
Grouping projections for rebuild operations

Note: The specific querying capabilities depend on your Chronicle setup and tooling.

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