Tagging Reducers

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

[Tag] and [Tags] label the reducer itself. They do not filter which appended events the reducer handles. For event filtering based on appended metadata, see Filter reducers and reactors by tag, Filter reducers and reactors by event source type, and Filter reducers and reactors by event stream type.

Adding Tags

You can tag reducers in multiple ways:

Single Tag

Apply a single tag to your reducer:

using Cratis.Chronicle;
using Cratis.Chronicle.Events;
using Cratis.Chronicle.Reducers;

[EventType]
public record TaggingReducersOrderPlaced(decimal TotalAmount);

public record TaggingReducersOrderAnalytics(int OrderCount, decimal TotalAmount);

[Tag("Analytics")]
public class TaggingReducersOrderAnalyticsReducer : IReducerFor<TaggingReducersOrderAnalytics>
{
    public TaggingReducersOrderAnalytics Placed(TaggingReducersOrderPlaced @event, TaggingReducersOrderAnalytics? current, EventContext context) =>
        new((current?.OrderCount ?? 0) + 1, (current?.TotalAmount ?? 0m) + @event.TotalAmount);
}

Multiple Tags (Single Attribute)

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

using Cratis.Chronicle;
using Cratis.Chronicle.Reducers;

public record TaggingReducersSalesReport(decimal TotalSales);

[Tag("Analytics", "Reporting", "Dashboard")]
public class TaggingReducersSalesReportReducer : IReducerFor<TaggingReducersSalesReport>;

Multiple Tags (Multiple Attributes)

Apply multiple [Tag] attributes:

using Cratis.Chronicle;
using Cratis.Chronicle.Reducers;

public record TaggingReducersComplianceReport(string Status);

[Tag("Analytics")]
[Tag("Compliance")]
[Tag("Auditing")]
public class TaggingReducersComplianceReportReducer : IReducerFor<TaggingReducersComplianceReport>;

Mixed Approach

Combine both approaches:

using Cratis.Chronicle;
using Cratis.Chronicle.Reducers;

public record TaggingReducersExecutiveDashboard(int MetricCount);

[Tag("Analytics", "Reporting")]
[Tag("Executive")]
public class TaggingReducersExecutiveDashboardReducer : IReducerFor<TaggingReducersExecutiveDashboard>;

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 reducers: Use tags to group reducers that serve similar purposes

Common Tag Examples

Here are some common patterns for tagging reducers:

using Cratis.Chronicle;
using Cratis.Chronicle.Reducers;

public record TaggingReducersCategoryExamples(Guid Id);

// By domain
[Tag("Sales", "Inventory", "Customer")]
// By purpose
[Tag("Analytics", "Reporting", "Dashboard", "Auditing")]
// By stakeholder
[Tag("Executive", "Operations", "Finance")]
// By data type
[Tag("Aggregates", "Summaries", "Metrics")]
public class TaggingReducersCategoryExamplesReducer : IReducerFor<TaggingReducersCategoryExamples>;

Querying by Tag

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

Filtering and searching for specific reducers
Organizing reducers in administrative interfaces
Generating documentation
Managing reducer deployments by tag

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