Passive Projections

Mark a projection as passive (not actively observing) using [Passive]:

C#

using Cratis.Chronicle.Events;
using Cratis.Chronicle.Keys;
using Cratis.Chronicle.Projections.ModelBound;
using Cratis.Chronicle.ReadModels;

[EventType]
public record MbPassiveSnapshotCreated(string Data);

[Passive]
public record MbPassiveHistoricalSnapshot(
    [Key]
    Guid Id,

    [SetFrom<MbPassiveSnapshotCreated>(nameof(MbPassiveSnapshotCreated.Data))]
    string Data);

Kotlin

import io.cratis.chronicle.events.EventType
import io.cratis.chronicle.projections.FromEvent
import io.cratis.chronicle.projections.SetFrom
import io.cratis.chronicle.readModels.Passive
import io.cratis.chronicle.readModels.ReadModel

@EventType
data class MbPassiveSnapshotCreated(val data: String)

@Passive
@ReadModel
@FromEvent(MbPassiveSnapshotCreated::class)
data class MbPassiveHistoricalSnapshot(
    @SetFrom("data", MbPassiveSnapshotCreated::class)
    val data: String = ""
)

Java

import io.cratis.chronicle.events.EventType;
import io.cratis.chronicle.projections.FromEvent;
import io.cratis.chronicle.projections.SetFrom;
import io.cratis.chronicle.readModels.Passive;
import io.cratis.chronicle.readModels.ReadModel;

@EventType
record MbPassiveSnapshotCreated(String data) {}

@Passive
@ReadModel
@FromEvent(eventType = MbPassiveSnapshotCreated.class)
class MbPassiveHistoricalSnapshot {
    @SetFrom(propertyPath = "data", eventType = MbPassiveSnapshotCreated.class)
    public String data = "";
}

Elixir

defmodule MyApp.Events.PassiveSnapshotCreated do
  use Chronicle.Events.EventType, id: "passive-snapshot-created-v1"

  defstruct [:data]
end

defmodule MyApp.ReadModels.PassiveHistoricalSnapshot do
  use Chronicle.ReadModels.ReadModel, passive: true

  alias MyApp.Events.PassiveSnapshotCreated

  defstruct [:id, :data]

  from PassiveSnapshotCreated,
    set: [id: :event_source_id, data: :data]
end

TypeScript

import { eventType, Guid, passive, readModel, setFrom } from '@cratis/chronicle';

@eventType()
class MbPassiveSnapshotCreated {
    data = '';
}

@readModel()
@passive
class MbPassiveHistoricalSnapshot {
    id: Guid = Guid.empty;

    @setFrom(MbPassiveSnapshotCreated, 'data')
    data = '';
}

When to use

• On-demand projections that are only rebuilt when explicitly requested
• Historical snapshots
• Resource-intensive projections that shouldn’t run continuously

Best practices

• Use for expensive projections that are only needed occasionally
• Consider the trade-off between resource usage and data freshness
• Ensure you have mechanisms to trigger rebuilds when needed