SendMessageController.cs

public class SendMessageController : Controller
{
    private readonly MyDataContext _dataContext;

    public SendMessageController(MyDataContext dataContext)
    {
        _dataContext = dataContext;
    }
    
    [HttpGet("/")]
    public async Task<string> Get([FromServices] IMessageSession session)
    {
        var id = Guid.NewGuid().ToString();
        
        await _dataContext.MyEntities.AddAsync(new MyEntity { Id = id, Processed = false });
        await _dataContext.SaveChangesAsync();
        
        var message = new MyMessage { EntityId = id };
        await session.SendLocal(message);
        
        return $"Message with entity ID '{id}' sent to endpoint";
    }
}

public class MyHandler : IHandleMessages<MyMessage>
{
    private static readonly ILog Log = LogManager.GetLogger<MyHandler>();
    private readonly MyDataContext _dataContext;

    public MyHandler(MyDataContext dataContext)
    {
        _dataContext = dataContext;
    }

    public async Task Handle(MyMessage message, IMessageHandlerContext context)
    {
        Log.Info("Message received at endpoint");

        var entity = await _dataContext.MyEntities
            .Where(e => e.Id == message.EntityId)
            .FirstAsync(cancellationToken: context.CancellationToken);
        
        entity.Processed = true;
    }
}