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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

Section titled “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

Section titled “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

Section titled “Basic Example”
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

Section titled “Topics”

Reading Your Reducer-Based Read Models

Section titled “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: