Read Model Interception

Read model interception lets you apply cross-cutting operations to every read model instance before it is served to a client. Common uses include decryption, field masking, localization, and audit enrichment. Interceptors run automatically for all query types — controller-based, model-bound, and observable (WebSocket and SSE).

How It Works

When a query returns, the framework passes each read model instance through every registered interceptor for that type before serializing the response. For collections every item is intercepted individually. For observable queries the interception happens on every emission.

sequenceDiagram
    participant Client
    participant Framework
    participant Interceptor

    Client->>Framework: GET /api/accounts
    Framework->>Framework: Execute query
    loop For each item
        Framework->>Interceptor: Intercept(item)
        Interceptor-->>Framework: (mutated in place)
    end
    Framework-->>Client: QueryResult with intercepted data

Implementing an Interceptor

Implement IInterceptReadModel<TReadModel>. No DI registration is required — the framework discovers all implementations in your assemblies automatically via ITypes and creates instances on demand, resolving any constructor dependencies from the service provider.

public class DecryptAccountNumbers : IInterceptReadModel<AccountSummary>
{
    readonly IEncryptionService _encryption;

    public DecryptAccountNumbers(IEncryptionService encryption)
    {
        _encryption = encryption;
    }

    public Task Intercept(AccountSummary readModel)
    {
        readModel.AccountNumber = _encryption.Decrypt(readModel.AccountNumber);
        return Task.CompletedTask;
    }
}

Note: The Intercept method mutates the read model in place — it does not return a value. Arc captures and applies your changes automatically.

Multiple Interceptors

You can register any number of interceptors for the same read model type. They run in the order they are discovered.

public class MaskSensitiveFields : IInterceptReadModel<AccountSummary>
{
    public Task Intercept(AccountSummary readModel)
    {
        readModel.AccountNumber = $"****{readModel.AccountNumber[^4..]}";
        return Task.CompletedTask;
    }
}

public class EnrichWithLocale : IInterceptReadModel<AccountSummary>
{
    readonly ILocalizationService _locale;

    public EnrichWithLocale(ILocalizationService locale)
    {
        _locale = locale;
    }

    public Task Intercept(AccountSummary readModel)
    {
        readModel.FormattedBalance = _locale.FormatCurrency(readModel.Balance);
        return Task.CompletedTask;
    }
}

Both interceptors run for every AccountSummary returned by any query.

Observable Queries

Interceptors apply equally to observable (real-time) queries. Each time the observable emits new data, every item passes through the registered interceptors before the payload is sent to the client.

// No changes needed in your observable query — interception is automatic.
[HttpGet("observable")]
public ISubject<IEnumerable<AccountSummary>> GetAccountSummaries()
{
    return _collection.Observe();
}

Type Safety