ReadStores.md

Read model stores

In order to create query handlers that perform and enable them search across multiple fields, read models or projections are used.

Read models are a flattened views of a subset or all aggregate domain events created specifically for efficient queries.

Here's a simple example of how a read model for doing searches for usernames could look. The read model handles the UserCreated domain event event to get the username and user ID.

public class UserReadModel : IReadModel,
  IAmReadModelFor<UserAggregate, UserId, UserCreated>
{
  public string UserId { get; set; }
  public string Username { get; set; }

  public void Apply(
    IReadModelContext context,
    IDomainEvent<UserAggregate, UserId, UserCreated> domainEvent)
  {
    UserId = domainEvent.AggregateIdentity.Value;
    Username = domainEvent.AggregateEvent.Username.Value;
  }
}

Read model locators

Typically the ID of read models are the aggregate identity, but sometimes this isn't the case. Here are some examples.

• Items from a collection on the aggregate root
• Deterministic ID created from event data
• Entity within the aggregate

To create read models in these cases, use the EventFlow concept of read model locators, which is basically a mapping from a domain event to a read model ID.

As an example, consider if we could add several nicknames to a user. We might have a domain event called UserNicknameAdded similar to this.

public class UserNicknameAdded : AggregateEvent<UserAggregate, UserId>
{
  public Nickname Nickname { get; set; }
}

We could then create a read model locator that would return the ID for each nickname we add via the event like this.

public class UserNicknameReadModelLocator : IReadModelLocator
{
  public IEnumerable<string> GetReadModelIds(IDomainEvent domainEvent)
  {
    var userNicknameAdded = domainEvent as
      IDomainEvent<UserAggregate, UserId, UserNicknameAdded>;
    if (userNicknameAdded == null)
    {
      yield break;
    }

    yield return userNicknameAdded.Nickname.Id;
  }
}

And then use a read model similar to this that represent each nickname.

public class UserNicknameReadModel : IReadModel,
  IAmReadModelFor<UserAggregate, UserId, UserNicknameAdded>
{
  public string UserId { get; set; }
  public string Nickname { get; set; }

  public void Apply(
    IReadModelContext context,
    IDomainEvent<UserAggregate, UserId, UserCreated> domainEvent)
  {
    UserId = domainEvent.AggregateIdentity.Value;
    Nickname = domainEvent.AggregateEvent.Nickname.Value;
  }
}

We could then use this nickname read model to query all the nicknames for a given user by search for read models that have a specific UserId.

Read store implementations

EventFlow has built-in support for several different read model stores.

In-memory

The in-memory read store is easy to use and easy to configure. All read models are stored in-memory, so if EventFlow is restarted all read models are lost.

To configure the in-memory read model store, simply call UseInMemoryReadStoreFor<> or UseInMemoryReadStoreFor<,> with your read model as the generic argument.

var resolver = EventFlowOptions.New
  ...
  .UseInMemoryReadStoreFor<UserReadModel>()
  .UseInMemoryReadStoreFor<UserNicknameReadModel,UserNicknameReadModelLocator>()
  ...
  .CreateResolver();

Microsoft SQL Server

To configure the MSSQL read model store, simply call UseMssqlReadModel<> or UseMssqlReadModel<,> with your read model as the generic argument.

var resolver = EventFlowOptions.New
  ...
  .UseMssqlReadModel<UserReadModel>()
  .UseMssqlReadModel<UserNicknameReadModel,UserNicknameReadModelLocator>()
  ...
  .CreateResolver();

By convention, EventFlow uses the table named ReadModel-[CLASS NAME] as the table to store the read models rows in. If you need to change this, use the Table from the System.ComponentModel.DataAnnotations.Schema namespace. So in the above example, the read model UserReadModel would be stored in a table called ReadModel-UserReadModel unless stated otherwise.

To allow EventFlow to find the read models stored, a single column is required to have the MsSqlReadModelIdentityColumn attribute. This will be used to store the read model ID.

You should also create a int column that has the MsSqlReadModelVersionColumn attribute to tell EventFlow which column is used to store the read model version in.

Elasticsearch

To configure the Elasticsearch read model store, simply call UseElasticsearchReadModel<> or UseElasticsearchReadModel<,> with your read model as the generic argument.

var resolver = EventFlowOptions.New
  ...
  .ConfigureElasticsearch(new Uri("http://localhost:9200/"))
  ...
  .UseElasticsearchReadModel<UserReadModel>()
  .UseElasticsearchReadModel<UserNicknameReadModel,UserNicknameReadModelLocator>()
  ...
  .CreateResolver();

Overloads of ConfigureElasticsearch(...) is available for alternative Elasticsearch configurations.

Make sure to create any mapping the read model requires in Elasticsearch before using the read model in EventFlow.

If EventFlow is requested to purge a specific read model, it does it by deleting the index. Thus make sure to create one separate index per read model.

If you want to control the index a specific read model is stored in, create create an implementation of IReadModelDescriptionProvider and register it in the EventFlow IoC.