Declaring PII on a concept

The most robust way to declare that a value is personally identifiable is to mark the ConceptAs<T> type itself with [PII]. Doing so means the declaration lives exactly once — on the type — and every event property that uses that type is automatically encrypted, regardless of where or how many times it appears across your event model.

Client coverage

This page covers the property type path — Chronicle resolves [PII] through four paths in total (property, declaring type, property type, constructor parameter), and marking a concept’s type once is not C#-only:

Path	C#	Kotlin/Java	TypeScript	Elixir
Property	yes	yes	yes	yes
Declaring type	yes	yes	yes	no
Property type (concept)	yes	yes	yes	yes
Constructor parameter	yes	yes	no	n/a

Every client supports the pattern on this page. C#, Kotlin/Java and TypeScript each have a ConceptAs<T>; Elixir has Chronicle.Concept, which serializes as the value it wraps in the same way. Declare the marker once on the concept and every event and read model property using that type is encrypted — see Marking data as PII for the full path table and the two remaining gaps.

Tip

For background on the ConceptAs<T> pattern, see Fundamentals.Concepts.

Why concept-level declaration is preferred

When you mark a property directly with [PII], you must remember to repeat the attribute on every event or read model that carries that value. If a new event is added months later and the developer forgets the annotation, a plaintext value is written to the event log with no warning.

A concept type solves this by making the protection part of the type’s identity. You cannot use PersonName without the encryption — the two are inseparable.

C#

using Cratis.Chronicle.Compliance.GDPR;
using Cratis.Chronicle.Events;

// ❌ Property-level: requires repetition across every event
[EventType]
public record PiiConceptsComparisonEmployeeRegistered([PII] string Name, string Department);

[EventType]
public record PiiConceptsComparisonEmployeeNameChanged([PII] string NewName);  // must remember [PII] again

// ✅ Concept-level: declare once, apply everywhere automatically
[PII]
public record PiiConceptsComparisonPersonName(string Value) : ConceptAs<string>(Value)
{
    public static readonly PiiConceptsComparisonPersonName NotSet = new(string.Empty);
    public static implicit operator string(PiiConceptsComparisonPersonName name) => name.Value;
    public static implicit operator PiiConceptsComparisonPersonName(string value) => new(value);
}

[EventType]
public record PiiConceptsComparisonEmployeeRegisteredGood(PiiConceptsComparisonPersonName Name, string Department);  // Name is encrypted

[EventType]
public record PiiConceptsComparisonEmployeeNameChangedGood(PiiConceptsComparisonPersonName NewName);  // also encrypted, no extra annotation needed

View C# snippet source on GitHub

Kotlin

import io.cratis.chronicle.compliance.Pii
import io.cratis.chronicle.concepts.ConceptAs
import io.cratis.chronicle.events.EventType

// Property-level: requires repetition across every event
@EventType
data class PiiConceptsComparisonEmployeeRegistered(@Pii val name: String, val department: String)

@EventType
data class PiiConceptsComparisonEmployeeNameChanged(@Pii val newName: String) // must remember @Pii again

// Concept-level: declare once, apply everywhere automatically
@Pii
data class PiiConceptsComparisonPersonName(override val value: String) : ConceptAs<String>

// name is encrypted
@EventType
data class PiiConceptsComparisonEmployeeRegisteredGood(val name: PiiConceptsComparisonPersonName, val department: String)

// also encrypted, no extra annotation needed
@EventType
data class PiiConceptsComparisonEmployeeNameChangedGood(val newName: PiiConceptsComparisonPersonName)

View Kotlin snippet source on GitHub

Java

import io.cratis.chronicle.compliance.Pii;
import io.cratis.chronicle.concepts.ConceptAs;
import io.cratis.chronicle.events.EventType;

// Property-level: requires repetition across every event
@EventType
record PiiConceptsComparisonEmployeeRegistered(@Pii String name, String department) {
}

// must remember @Pii again
@EventType
record PiiConceptsComparisonEmployeeNameChanged(@Pii String newName) {
}

// Concept-level: declare once, apply everywhere automatically
@Pii
record PiiConceptsComparisonPersonName(String value) implements ConceptAs<String> {
    @Override
    public String getValue() {
        return value;
    }
}

// name is encrypted
@EventType
record PiiConceptsComparisonEmployeeRegisteredGood(PiiConceptsComparisonPersonName name, String department) {
}

// also encrypted, no extra annotation needed
@EventType
record PiiConceptsComparisonEmployeeNameChangedGood(PiiConceptsComparisonPersonName newName) {
}

View Java snippet source on GitHub

Elixir

# Property-level: requires repeating pii/1,2 on every event.
defmodule MyApp.Compliance.PiiWithConcepts.EmployeeRegisteredComparison do
  use Chronicle.Events.EventType, id: "pii-with-concepts-employee-registered-comparison"
  defstruct [:name, :department]

  pii(:name)
end

defmodule MyApp.Compliance.PiiWithConcepts.EmployeeNameChangedComparison do
  use Chronicle.Events.EventType, id: "pii-with-concepts-employee-name-changed-comparison"
  defstruct [:new_name]

  # Easy to forget — a plaintext value would be written with no warning.
  pii(:new_name)
end

# Concept-level: declare once, apply everywhere automatically.
defmodule MyApp.Compliance.PiiWithConcepts.PersonNameComparison do
  use Chronicle.Concept, type: :string
  pii()
end

defmodule MyApp.Compliance.PiiWithConcepts.EmployeeRegisteredComparisonGood do
  use Chronicle.Events.EventType, id: "pii-with-concepts-employee-registered-comparison-good"

  defstruct name: %MyApp.Compliance.PiiWithConcepts.PersonNameComparison{}, department: ""
end

defmodule MyApp.Compliance.PiiWithConcepts.EmployeeNameChangedComparisonGood do
  use Chronicle.Events.EventType, id: "pii-with-concepts-employee-name-changed-comparison-good"

  # Also encrypted — no extra annotation needed.
  defstruct new_name: %MyApp.Compliance.PiiWithConcepts.PersonNameComparison{}
end

View Elixir snippet source on GitHub

TypeScript

import { eventType, pii } from '@cratis/chronicle';
import { ConceptAs } from '@cratis/fundamentals';

// ❌ Property-level: requires repetition across every event
@eventType()
class PiiConceptsComparisonEmployeeRegistered {
    @pii() name = '';
    department = '';
}

@eventType()
class PiiConceptsComparisonEmployeeNameChanged {
    @pii() newName = ''; // must remember @pii() again
}

// ✅ Concept-level: declare once, apply everywhere automatically
@pii()
class PiiConceptsComparisonPersonName extends ConceptAs<string> {
    static readonly valueType = String;

    constructor(value: string) {
        super(value);
    }
}

@eventType()
class PiiConceptsComparisonEmployeeRegisteredGood {
    name: PiiConceptsComparisonPersonName = new PiiConceptsComparisonPersonName(''); // encrypted
    department = '';
}

@eventType()
class PiiConceptsComparisonEmployeeNameChangedGood {
    newName: PiiConceptsComparisonPersonName = new PiiConceptsComparisonPersonName(''); // also encrypted, no extra annotation needed
}

View TypeScript snippet source on GitHub

Defining a PII concept type

Follow the standard ConceptAs<T> pattern and add [PII] to the record declaration:

C#

using Cratis.Chronicle.Compliance.GDPR;

[PII]
public record PiiConceptsPersonName(string Value) : ConceptAs<string>(Value)
{
    public static readonly PiiConceptsPersonName NotSet = new(string.Empty);

    public static implicit operator string(PiiConceptsPersonName name) => name.Value;
    public static implicit operator PiiConceptsPersonName(string value) => new(value);
}

View C# snippet source on GitHub

Kotlin

import io.cratis.chronicle.compliance.Pii
import io.cratis.chronicle.concepts.ConceptAs

@Pii
data class PiiConceptsPersonName(override val value: String) : ConceptAs<String>

View Kotlin snippet source on GitHub

Java

import io.cratis.chronicle.compliance.Pii;
import io.cratis.chronicle.concepts.ConceptAs;

@Pii
record PiiConceptsPersonName(String value) implements ConceptAs<String> {
    @Override
    public String getValue() {
        return value;
    }
}

View Java snippet source on GitHub

Elixir

defmodule MyApp.Compliance.PiiWithConcepts.PersonName do
  use Chronicle.Concept, type: :string
  pii()
end

View Elixir snippet source on GitHub

TypeScript

import { pii } from '@cratis/chronicle';
import { ConceptAs } from '@cratis/fundamentals';

@pii()
class PiiConceptsPersonName extends ConceptAs<string> {
    static readonly valueType = String;

    constructor(value: string) {
        super(value);
    }
}

View TypeScript snippet source on GitHub

Another example, documenting why the value is sensitive with the optional details parameter:

C#

using Cratis.Chronicle.Compliance.GDPR;

[PII("National ID number — sensitive personal identifier")]
public record PiiConceptsNationalIdNumber(string Value) : ConceptAs<string>(Value)
{
    public static readonly PiiConceptsNationalIdNumber NotSet = new(string.Empty);

    public static implicit operator string(PiiConceptsNationalIdNumber id) => id.Value;
    public static implicit operator PiiConceptsNationalIdNumber(string value) => new(value);
}

View C# snippet source on GitHub

Kotlin

import io.cratis.chronicle.compliance.Pii
import io.cratis.chronicle.concepts.ConceptAs

@Pii(description = "National ID number — sensitive personal identifier")
data class PiiConceptsNationalIdNumber(override val value: String) : ConceptAs<String>

View Kotlin snippet source on GitHub

Java

import io.cratis.chronicle.compliance.Pii;
import io.cratis.chronicle.concepts.ConceptAs;

@Pii(description = "National ID number — sensitive personal identifier")
record PiiConceptsNationalIdNumber(String value) implements ConceptAs<String> {
    @Override
    public String getValue() {
        return value;
    }
}

View Java snippet source on GitHub

Elixir

defmodule MyApp.Compliance.PiiWithConcepts.NationalIdNumber do
  use Chronicle.Concept, type: :string
  pii("National ID number — sensitive personal identifier")
end

View Elixir snippet source on GitHub

TypeScript

import { pii } from '@cratis/chronicle';
import { ConceptAs } from '@cratis/fundamentals';

@pii('National ID number — sensitive personal identifier')
class PiiConceptsNationalIdNumber extends ConceptAs<string> {
    static readonly valueType = String;

    constructor(value: string) {
        super(value);
    }
}

View TypeScript snippet source on GitHub

Optional constructor parameters

In the .NET client, a concept parameter that defaults to null retains its underlying type and [PII] metadata in the precise schema. Supplying a value therefore encrypts it just as it does for a required parameter. Read models always use this shape; event registration uses it when the kernel advertises precise event type schema support.

If an older schema omitted the classification, adding it does not encrypt earlier values. See Adding PII to an existing event type before assuming that erasure protects that history.

Placement

Concept types belong in the feature folder they are primarily associated with, or at the feature root if they are shared across slices. Do not create a separate Concepts/ folder — keep concepts co-located with the code that uses them.

Features/
├── Employees/
│   ├── PersonName.cs          ← shared across Registration and Updates slices
│   ├── Registration/
│   │   ├── Registration.cs    ← uses PersonName
│   └── Updates/
│       ├── Updates.cs         ← also uses PersonName — encryption is automatic

Combining with details

Use the optional details parameter on [PII] to document the legal basis or purpose of the PII classification. This is stored in the event schema and can be surfaced by compliance tooling.

C#

using Cratis.Chronicle.Compliance.GDPR;

[PII("Collected under GDPR Art. 6(1)(b) — necessary for contract performance. Retention: contract duration + 7 years.")]
public record PiiConceptsLegalName(string Value) : ConceptAs<string>(Value)
{
    public static readonly PiiConceptsLegalName NotSet = new(string.Empty);

    public static implicit operator string(PiiConceptsLegalName name) => name.Value;
    public static implicit operator PiiConceptsLegalName(string value) => new(value);
}

View C# snippet source on GitHub

Kotlin

import io.cratis.chronicle.compliance.Pii
import io.cratis.chronicle.concepts.ConceptAs

@Pii(
    description = "Collected under GDPR Art. 6(1)(b) — necessary for contract performance. " +
        "Retention: contract duration + 7 years."
)
data class PiiConceptsLegalName(override val value: String) : ConceptAs<String>

View Kotlin snippet source on GitHub

Java

import io.cratis.chronicle.compliance.Pii;
import io.cratis.chronicle.concepts.ConceptAs;

@Pii(
    description = "Collected under GDPR Art. 6(1)(b) — necessary for contract performance. "
        + "Retention: contract duration + 7 years."
)
record PiiConceptsLegalName(String value) implements ConceptAs<String> {
    @Override
    public String getValue() {
        return value;
    }
}

View Java snippet source on GitHub

Elixir

defmodule MyApp.Compliance.PiiWithConcepts.LegalName do
  use Chronicle.Concept, type: :string

  pii(
    "Collected under GDPR Art. 6(1)(b) — necessary for contract performance. " <>
      "Retention: contract duration + 7 years."
  )
end

View Elixir snippet source on GitHub

TypeScript

import { pii } from '@cratis/chronicle';
import { ConceptAs } from '@cratis/fundamentals';

@pii('Collected under GDPR Art. 6(1)(b) — necessary for contract performance. Retention: contract duration + 7 years.')
class PiiConceptsLegalName extends ConceptAs<string> {
    static readonly valueType = String;

    constructor(value: string) {
        super(value);
    }
}

View TypeScript snippet source on GitHub

What cannot be marked PII

EventSourceId types

Any type inheriting from EventSourceId or EventSourceId<T> cannot be marked with [PII]. Chronicle throws PIINotSupportedOnEventSourceId if you attempt this.

C#

using Cratis.Chronicle.Compliance.GDPR;
using Cratis.Chronicle.Events;

// ❌ Throws PIINotSupportedOnEventSourceId
[PII]
public record PiiConceptsEmployeeId(Guid Value) : EventSourceId<Guid>(Value);

View C# snippet source on GitHub

Kotlin

import io.cratis.chronicle.compliance.Pii
import io.cratis.chronicle.concepts.EventSourceId

// Throws PiiNotSupportedOnEventSourceId
@Pii
data class PiiConceptsEmployeeId(override val value: String) : EventSourceId

View Kotlin snippet source on GitHub

Java

import io.cratis.chronicle.compliance.Pii;
import io.cratis.chronicle.concepts.EventSourceId;

// Throws PiiNotSupportedOnEventSourceId
@Pii
record PiiConceptsEmployeeId(String value) implements EventSourceId {
    @Override
    public String getValue() {
        return value;
    }
}

View Java snippet source on GitHub

Elixir

# Raises ArgumentError at compile time — any Chronicle.Concept declared with
# event_source_id: true cannot also declare pii/0,1.
#
# defmodule MyApp.Compliance.PiiWithConcepts.EmployeeId do
#   use Chronicle.Concept, type: :uuid, event_source_id: true
#   pii()
# end

View Elixir snippet source on GitHub

TypeScript

import { pii } from '@cratis/chronicle';

// TypeScript has no dedicated EventSourceId<T> type to mark PII on directly - the event
// source identifier is always the conventional 'eventSourceId' property, and marking it
// @pii() throws PIINotSupportedOnEventSourceId for the same reason C# forbids [PII] on a
// concept deriving from EventSourceId<T>.
class PiiConceptsEmployeeId {
    @pii() eventSourceId = '';
}

View TypeScript snippet source on GitHub

Use a non-sensitive surrogate key as the event source identifier and store sensitive identity values in a separate event property:

C#

using Cratis.Chronicle.Events;

// ✅ Surrogate key as event source identifier
public record PiiConceptsSurrogateEmployeeId(Guid Value) : EventSourceId<Guid>(Value)
{
    public static PiiConceptsSurrogateEmployeeId New() => new(Guid.NewGuid());
}

// ✅ Sensitive value stored in a PII-marked concept type
[EventType]
public record PiiConceptsSurrogateEmployeeRegistered(PiiConceptsNationalIdNumber NationalId, PiiConceptsPersonName Name);

View C# snippet source on GitHub

Kotlin

import io.cratis.chronicle.concepts.EventSourceId
import io.cratis.chronicle.events.EventType

// Surrogate key as event source identifier - not marked @Pii
data class PiiConceptsSurrogateEmployeeId(override val value: String) : EventSourceId

// Sensitive values stored in PII-marked concept types instead
@EventType
data class PiiConceptsSurrogateEmployeeRegistered(
    val nationalId: PiiConceptsNationalIdNumber,
    val name: PiiConceptsPersonName
)

View Kotlin snippet source on GitHub

Java

import io.cratis.chronicle.concepts.EventSourceId;
import io.cratis.chronicle.events.EventType;

// Surrogate key as event source identifier - not marked @Pii
record PiiConceptsSurrogateEmployeeId(String value) implements EventSourceId {
    @Override
    public String getValue() {
        return value;
    }
}

// Sensitive values stored in PII-marked concept types instead
@EventType
record PiiConceptsSurrogateEmployeeRegistered(
        PiiConceptsNationalIdNumber nationalId,
        PiiConceptsPersonName name) {
}

View Java snippet source on GitHub

Elixir

# Surrogate key as event source identifier — no pii declared.
defmodule MyApp.Compliance.PiiWithConcepts.SurrogateEmployeeId do
  use Chronicle.Concept, type: :uuid, event_source_id: true
end

defmodule MyApp.Compliance.PiiWithConcepts.SurrogateNationalId do
  use Chronicle.Concept, type: :string
  pii("National ID number — sensitive personal identifier")
end

defmodule MyApp.Compliance.PiiWithConcepts.SurrogatePersonName do
  use Chronicle.Concept, type: :string
  pii()
end

# Sensitive values stored in PII-marked concept fields instead.
defmodule MyApp.Compliance.PiiWithConcepts.SurrogateEmployeeRegistered do
  use Chronicle.Events.EventType, id: "pii-with-concepts-surrogate-employee-registered"

  defstruct national_id: %MyApp.Compliance.PiiWithConcepts.SurrogateNationalId{},
            name: %MyApp.Compliance.PiiWithConcepts.SurrogatePersonName{}
end

View Elixir snippet source on GitHub

TypeScript

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

// ✅ Surrogate key as event source identifier - TypeScript event source identifiers are
// plain strings, so a randomly generated Guid works well with no dedicated identity type
// required.
function createSurrogateEmployeeId(): string {
    return Guid.create().toString();
}

// ✅ Sensitive values stored in PII-marked concept properties instead
@eventType()
class PiiConceptsSurrogateEmployeeRegistered {
    @field(PiiConceptsNationalIdNumber) readonly nationalId: PiiConceptsNationalIdNumber;
    @field(PiiConceptsPersonName) readonly name: PiiConceptsPersonName;

    constructor(nationalId: PiiConceptsNationalIdNumber, name: PiiConceptsPersonName) {
        this.nationalId = nationalId;
        this.name = name;
    }
}

View TypeScript snippet source on GitHub