---
title: Reducers
---

Reducers provide a way to build read models by reducing a sequence of events into a single state. Unlike projections, which focus on transforming individual events into read models, reducers process collections of events together to compute derived state.

## Key Concepts

A reducer is a specialized observer that:

- **Processes multiple events together** - Events are grouped by event source and passed to the reducer as a collection
- **Computes derived state** - The reducer method receives the current state and returns the new state after processing the events
- **Maintains temporal consistency** - All events for a given event source are processed in order
- **Supports snapshots** - The computed state can be retrieved at any point in the event stream

## When to Use Reducers

Reducers are ideal when you need to:

- **Aggregate data across multiple events** - Calculate sums, averages, or other metrics from a series of events
- **Build temporal models** - Track how state changes over time
- **Implement complex business logic** - Process events together to derive insights that span multiple events
- **Create snapshots** - Capture the state of a read model at specific points in time

## Basic Example

```csharp
public record AccountBalance(decimal Balance, DateTimeOffset LastUpdated);

public class AccountBalanceReducer : IReducerFor<AccountBalance>
{
    public AccountBalance OnDepositMade(DepositMade @event, AccountBalance? current, EventContext context)
    {
        var currentBalance = current?.Balance ?? 0m;
        return new AccountBalance(currentBalance + @event.Amount, context.Occurred);
    }

    public AccountBalance OnWithdrawalMade(WithdrawalMade @event, AccountBalance? current, EventContext context)
    {
        var currentBalance = current?.Balance ?? 0m;
        return new AccountBalance(currentBalance - @event.Amount, context.Occurred);
    }
}
```

## Topics

- [Getting Started](/chronicle/reducers/getting-started/) - Learn how to create your first reducer
- [Choose a read-model style](/chronicle/projections/choosing-a-read-model-style/) - Compare reducers with model-bound and declarative projections
- [Subscribe to External Event Stores](/chronicle/reducers/external-event-store-subscriptions/) - Configure outbox-to-inbox subscriptions for reducers
- [Passive Reducers](/chronicle/reducers/passive-reducers/) - Control when reducers actively observe events
- [Event Processing](/chronicle/reducers/event-processing/) - Understand how reducers process events
- [Event Sequence](/chronicle/reducers/event-sequence/) - Specify which event sequence a reducer observes
- [Filtering by appended event metadata](/chronicle/events/filtering/) - Limit reducers to specific tags, event source types, or event stream types
- [Tagging Reducers](/chronicle/reducers/tagging-reducers/) - How to use tags with reducers

## Reading Your Reducer-Based Read Models

Once you've defined a reducer, you can retrieve and observe the resulting read models using the `IReadModels` API:

- [Getting a Single Instance](/chronicle/read-models/getting-single-instance/) - Retrieve a specific instance by key with strong consistency
- [Getting a Collection of Instances](/chronicle/read-models/getting-collection-instances/) - Retrieve all instances for reporting and analysis
- [Getting Snapshots](/chronicle/read-models/getting-snapshots/) - Retrieve historical state snapshots grouped by correlation ID
- [Watching Read Models](/chronicle/read-models/watching-read-models/) - Observe real-time changes as events are applied