// Approach for emitting complete events to AWS SNS from the monolithic codebase

// Could be called from BO layer, or directly from calling applications where they bypass DAL to avoid refactor churn

// EventProducer classes will build messages and obtain the data required to produce - if the data cannot be passed by the calling code

// May need to not be async and could be better suited as a static class for ease of implementation

​

// class designed to publish events specific to contacts domain

public class ContactDomainEventProducer : IDomainEventProducer

{

    public ContactDomainEventProducer()

    {

        // initialize AWS SDK here - dependancy injection preferred

    }

​

    public async Task<bool> PublishCreatedEventAsync(BigCustomer customer, int contactId, BigContact? contact, BigContactExtra? contactExtra)

    {

        // if the calling code cannot provide all of the data we need to publish our message

        // fetch it from the database

​

        if (contact == null)

        {

            contact = BigContact.GetById(customer, contactId);

        }

​

        if (contactExtra == null)

        {

            contactExtra = BigContactExtra.GetById(customer, contactId);

        }

​

        // construct the domain event payload

        var payload = new ContactCreatedPayloadV1()

        {

            Id = contactId,

            Name = contact.ContactName,

            Town = contact.ContactTown

        };

​

        // create a domain event envelope from the payload

        var domainEvent = DomainEvent.Create<ContactCreatedPayloadV1>(MessageType.ContactCreated, payload, customer.CustomerId, "1.0");

​

        // publish the event

        return await this.PublishEventAsync(domainEvent);

    }

​

    private async Task<bool> PublishEventAsync(DomainEvent payload)

    {

        // emit to SNS using AWS SDK Here

        return true;

    }

}

​

// envelope class for event payload - includes standard message metadata

public class DomainEvent

{

    public MessageType Type { get; }

    public dynamic Message { get; }

    public int CustomerId { get; }

    public string Originator { get; }

    public string Version { get; } = "1.0";

    public Guid EventId { get; }

    public DateTime Timestamp { get; }

​

    private DomainEvent(MessageType type, dynamic message, int customerId, string originator)

    {

        this.Type = Type;

        this.Message = message;

        this.CustomerId = customerId;

        this.Originator = originator;

        this.Version = "1.0";

        this.EventId = Guid.NewGuid();

        this.Timestamp = DateTime.UtcNow;

    }

​

    // Create a domain event from a provided payload

    public static DomainEvent Create<T>(MessageType type, T message, int customerId, string originator)

    {

        return new DomainEvent(type, message, customerId, originator);

    }

}

​

// Known types of events that may be published

public enum MessageType

{

    ContactCreated,

    ContactModified,

    ContactStopped

}

​

// payload class aligned to a type of event - possibly autogenerated from AsyncAPI specificaiton

public class ContactCreatedPayloadV1

{

    public int Id { get; set; }

    public string Name { get; set; }

    public string Town { get; set; }

}