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.

Kotlin and Java use repeatable @Tag annotations. TypeScript uses @tag(...) or @tags(...) on the reducer class; those labels are sent with its observer registration.

For self reducers, place descriptive tags on the read model itself. They remain observer labels, not append-time event tags or delivery filters.

Adding Tags

You can tag reducers in multiple ways:

Single Tag

Apply a single tag to your reducer:

C#

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);
}

View C# snippet source on GitHub

Kotlin

import io.cratis.chronicle.events.EventContext
import io.cratis.chronicle.events.EventType
import io.cratis.chronicle.observation.Reducer
import io.cratis.chronicle.observation.Tag
import io.cratis.chronicle.readModels.ReadModel

@EventType
data class TaggingReducersOrderPlaced(val totalAmount: Double)

@ReadModel
data class TaggingReducersOrderAnalytics(val orderCount: Int = 0, val totalAmount: Double = 0.0)

@Reducer
@Tag("Analytics")
class TaggingReducersOrderAnalyticsReducer {
    fun placed(event: TaggingReducersOrderPlaced, current: TaggingReducersOrderAnalytics?, context: EventContext) =
        TaggingReducersOrderAnalytics((current?.orderCount ?: 0) + 1, (current?.totalAmount ?: 0.0) + event.totalAmount)
}

View Kotlin snippet source on GitHub

Java

import io.cratis.chronicle.events.EventContext;
import io.cratis.chronicle.events.EventType;
import io.cratis.chronicle.observation.Reducer;
import io.cratis.chronicle.observation.Tag;
import io.cratis.chronicle.readModels.ReadModel;

@EventType
record TaggingReducersOrderPlaced(double totalAmount) {}

@ReadModel
record TaggingReducersOrderAnalytics(int orderCount, double totalAmount) {
    TaggingReducersOrderAnalytics() {
        this(0, 0.0);
    }
}

@Reducer
@Tag("Analytics")
class TaggingReducersOrderAnalyticsReducer {
    TaggingReducersOrderAnalytics placed(TaggingReducersOrderPlaced event, TaggingReducersOrderAnalytics current, EventContext context) {
        int count = current == null ? 0 : current.orderCount();
        double total = current == null ? 0.0 : current.totalAmount();
        return new TaggingReducersOrderAnalytics(count + 1, total + event.totalAmount());
    }
}

View Java snippet source on GitHub

Elixir

Elixir does not support this workflow yet.

View Elixir snippet source on GitHub

TypeScript

import { eventType, reducer, tag } from '@cratis/chronicle';
import { field } from '@cratis/fundamentals';

@eventType()
class TaggingReducersOrderPlaced {
    @field(Number) readonly totalAmount: number;

    constructor(totalAmount: number) {
        this.totalAmount = totalAmount;
    }
}

class TaggingReducersOrderAnalytics {
    orderCount = 0;
    totalAmount = 0;
}

@reducer('', undefined, TaggingReducersOrderAnalytics)
@tag('Analytics')
class TaggingReducersOrderAnalyticsReducer {
    taggingReducersOrderPlaced(
        event: TaggingReducersOrderPlaced,
        current: TaggingReducersOrderAnalytics | undefined
    ): TaggingReducersOrderAnalytics {
        return {
            orderCount: (current?.orderCount ?? 0) + 1,
            totalAmount: (current?.totalAmount ?? 0) + event.totalAmount
        };
    }
}

View TypeScript snippet source on GitHub

Multiple Tags (Single Attribute)

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

C#

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

public record TaggingReducersSalesReport(decimal TotalSales);

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

View C# snippet source on GitHub

Kotlin

import io.cratis.chronicle.events.EventType
import io.cratis.chronicle.observation.Reducer
import io.cratis.chronicle.observation.Tag
import io.cratis.chronicle.readModels.ReadModel

@ReadModel
data class TaggingReducersSalesReport(val totalSales: Double = 0.0)

// @Tag takes any number of tags in a single attribute
@Reducer
@Tag("Analytics", "Reporting", "Dashboard")
class TaggingReducersSalesReportReducer {
    fun on(event: TaggingReducersSaleRecorded) = TaggingReducersSalesReport(event.totalSales)
}

@EventType
data class TaggingReducersSaleRecorded(val totalSales: Double = 0.0)

View Kotlin snippet source on GitHub

Java

import io.cratis.chronicle.events.EventType;
import io.cratis.chronicle.observation.Reducer;
import io.cratis.chronicle.observation.Tag;
import io.cratis.chronicle.readModels.ReadModel;

@ReadModel
record TaggingReducersSalesReport(double totalSales) {
    TaggingReducersSalesReport() {
        this(0.0);
    }
}

// @Tag takes any number of tags in a single attribute
@Reducer
@Tag({"Analytics", "Reporting", "Dashboard"})
class TaggingReducersSalesReportReducer {
    public TaggingReducersSalesReport on(TaggingReducersSaleRecorded event) {
        return new TaggingReducersSalesReport(event.totalSales());
    }
}

@EventType
record TaggingReducersSaleRecorded(double totalSales) {}

View Java snippet source on GitHub

Elixir

Elixir does not support this workflow yet.

View Elixir snippet source on GitHub

TypeScript

import { reducer, tag } from '@cratis/chronicle';

class TaggingReducersSalesReport {
    totalSales = 0;
}

@reducer('', undefined, TaggingReducersSalesReport)
@tag('Analytics', 'Reporting', 'Dashboard')
class TaggingReducersSalesReportReducer {}

View TypeScript snippet source on GitHub

Multiple Tags (Multiple Attributes)

Apply multiple [Tag] attributes:

C#

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

public record TaggingReducersComplianceReport(string Status);

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

View C# snippet source on GitHub

Kotlin

import io.cratis.chronicle.events.EventType
import io.cratis.chronicle.observation.Reducer
import io.cratis.chronicle.observation.Tag
import io.cratis.chronicle.readModels.ReadModel

@ReadModel
data class TaggingReducersComplianceReport(val status: String = "")

@Reducer
@Tag("Analytics")
@Tag("Compliance")
@Tag("Auditing")
class TaggingReducersComplianceReportReducer {
    fun on(event: TaggingReducersAuditEntryRecorded) = TaggingReducersComplianceReport(event.status)
}

@EventType
data class TaggingReducersAuditEntryRecorded(val status: String = "")

View Kotlin snippet source on GitHub

Java

import io.cratis.chronicle.events.EventType;
import io.cratis.chronicle.observation.Reducer;
import io.cratis.chronicle.observation.Tag;
import io.cratis.chronicle.readModels.ReadModel;

@ReadModel
record TaggingReducersComplianceReport(String status) {
    TaggingReducersComplianceReport() {
        this("");
    }
}

@Reducer
@Tag("Analytics")
@Tag("Compliance")
@Tag("Auditing")
class TaggingReducersComplianceReportReducer {
    public TaggingReducersComplianceReport on(TaggingReducersAuditEntryRecorded event) {
        return new TaggingReducersComplianceReport(event.status());
    }
}

@EventType
record TaggingReducersAuditEntryRecorded(String status) {}

View Java snippet source on GitHub

Elixir

Elixir does not support this workflow yet.

View Elixir snippet source on GitHub

TypeScript

import { reducer, tag } from '@cratis/chronicle';

class TaggingReducersComplianceReport {
    status = '';
}

@reducer('', undefined, TaggingReducersComplianceReport)
@tag('Analytics')
@tag('Compliance')
@tag('Auditing')
class TaggingReducersComplianceReportReducer {}

View TypeScript snippet source on GitHub

Mixed Approach

Combine both approaches:

C#

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

public record TaggingReducersExecutiveDashboard(int MetricCount);

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

View C# snippet source on GitHub

Kotlin

import io.cratis.chronicle.events.EventType
import io.cratis.chronicle.observation.Reducer
import io.cratis.chronicle.observation.Tag
import io.cratis.chronicle.readModels.ReadModel

@ReadModel
data class TaggingReducersExecutiveDashboard(val metricCount: Int = 0)

@Reducer
@Tag("Analytics", "Reporting")
@Tag("Executive")
class TaggingReducersExecutiveDashboardReducer {
    fun on(event: TaggingReducersKpiRecorded) = TaggingReducersExecutiveDashboard(event.metricCount)
}

@EventType
data class TaggingReducersKpiRecorded(val metricCount: Int = 0)

View Kotlin snippet source on GitHub

Java

import io.cratis.chronicle.events.EventType;
import io.cratis.chronicle.observation.Reducer;
import io.cratis.chronicle.observation.Tag;
import io.cratis.chronicle.readModels.ReadModel;

@ReadModel
record TaggingReducersExecutiveDashboard(int metricCount) {
    TaggingReducersExecutiveDashboard() {
        this(0);
    }
}

@Reducer
@Tag({"Analytics", "Reporting"})
@Tag("Executive")
class TaggingReducersExecutiveDashboardReducer {
    public TaggingReducersExecutiveDashboard on(TaggingReducersKpiRecorded event) {
        return new TaggingReducersExecutiveDashboard(event.metricCount());
    }
}

@EventType
record TaggingReducersKpiRecorded(int metricCount) {}

View Java snippet source on GitHub

Elixir

Elixir does not support this workflow yet.

View Elixir snippet source on GitHub

TypeScript

import { reducer, tag } from '@cratis/chronicle';

class TaggingReducersExecutiveDashboard {
    metricCount = 0;
}

@reducer('', undefined, TaggingReducersExecutiveDashboard)
@tag('Analytics', 'Reporting')
@tag('Executive')
class TaggingReducersExecutiveDashboardReducer {}

View TypeScript snippet source on GitHub

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:

C#

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>;

View C# snippet source on GitHub

Kotlin

import io.cratis.chronicle.events.EventType
import io.cratis.chronicle.observation.Reducer
import io.cratis.chronicle.observation.Tag
import io.cratis.chronicle.readModels.ReadModel
import java.util.UUID

@ReadModel
data class TaggingReducersCategoryExamples(val id: UUID = UUID(0, 0))

// 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")
@Reducer
class TaggingReducersCategoryExamplesReducer {
    fun on(event: TaggingReducersCategoryOrderPlaced) = TaggingReducersCategoryExamples(event.id)
}

@EventType
data class TaggingReducersCategoryOrderPlaced(val id: UUID = UUID(0, 0))

View Kotlin snippet source on GitHub

Java

import io.cratis.chronicle.events.EventType;
import io.cratis.chronicle.observation.Reducer;
import io.cratis.chronicle.observation.Tag;
import io.cratis.chronicle.readModels.ReadModel;

import java.util.UUID;

@ReadModel
record TaggingReducersCategoryExamples(UUID id) {
    TaggingReducersCategoryExamples() {
        this(new UUID(0, 0));
    }
}

// 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"})
@Reducer
class TaggingReducersCategoryExamplesReducer {
    public TaggingReducersCategoryExamples on(TaggingReducersCategoryOrderPlaced event) {
        return new TaggingReducersCategoryExamples(event.id());
    }
}

@EventType
record TaggingReducersCategoryOrderPlaced(UUID id) {}

View Java snippet source on GitHub

Elixir

Elixir does not support this workflow yet.

View Elixir snippet source on GitHub

TypeScript

import { reducer, tag } from '@cratis/chronicle';

class TaggingReducersCategoryExamples {
    id = '';
}

@reducer('', undefined, TaggingReducersCategoryExamples)
// 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')
class TaggingReducersCategoryExamplesReducer {}

View TypeScript snippet source on GitHub

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.

See Also

• Tagging - Comprehensive guide to tagging in Chronicle
• Reactors Tagging - Tagging reactors