Low Code Ceremony Sagas with Jasper & Marten

You’ll need at least Jasper v2.0.0-alpha-4 if you want to recreate the saga support in this post. All the sample code for this post is in an executable sample on GitHub. Jasper does support sagas with EF Core and Sql Server or Postgresql, but Marten is where most of the effort is going just at the moment.

The Saga pattern is a way to solve the issue of logical, long-running transactions that necessarily need to span over multiple operations. In the approaches I’ve encountered throughout my career, this has generally meant persisting a “saga state” of some sort in a database that is used within a message handling framework to “know” what steps have been completed, and what’s outstanding.

Jumping right into an example, consider a very simple order management service that will have steps to:

1. Create a new order
2. Complete the order
3. Or alternatively, delete new orders if they have not been completed within 1 minute

For the moment, I’m going to ignore the underlying persistence and just focus on the Jasper message handlers to implement the order saga workflow with this simplistic saga code:

using Baseline.Dates;
using Jasper;

namespace OrderSagaSample;

public record StartOrder(string Id);

public record CompleteOrder(string Id);

public record OrderTimeout(string Id) : TimeoutMessage(1.Minutes());

public class Order : Saga
{
    public string? Id { get; set; }

    // By returning the OrderTimeout, we're triggering a "timeout"
    // condition that will process the OrderTimeout message at least
    // one minute after an order is started
    public OrderTimeout Start(StartOrder order, ILogger<Order> logger)
    {
        Id = order.Id; // defining the Saga Id.

        logger.LogInformation("Got a new order with id {Id}", order.Id);
        // creating a timeout message for the saga
        return new OrderTimeout(order.Id);
    }

    public void Handle(CompleteOrder complete, ILogger<Order> logger)
    {
        logger.LogInformation("Completing order {Id}", complete.Id);

        // That's it, we're done. This directs Jasper to delete the
        // persisted saga state after the message is done.
        MarkCompleted();
    }

    public void Handle(OrderTimeout timeout, ILogger<Order> logger)
    {
        logger.LogInformation("Applying timeout to order {Id}", timeout.Id);

        // That's it, we're done. Delete the saga state after the message is done.
        MarkCompleted();
    }
}

I’m just aiming for a quick sample rather than exhaustive documentation here, but a few notes:

• Jasper leans a bit on type and naming conventions to discover message handlers and to “know” how to call these message handlers. Some folks will definitely not like the magic, but this approach leads to substantially less code and arguably complexity compared to existing .Net tools
• Jasper supports the idea of scheduled messages, and the new TimeoutMessage base class up there is just a way to utilize that support for “saga timeout” conditions
• Jasper generally tries to adapt to your application code rather than force a lot of mandatory framework artifacts into your message handler code

Now let’s move over to the service bootstrapping and add Marten in as our persistence mechanism in the Program file:

using Jasper;
using Jasper.Persistence.Marten;
using Marten;
using Oakton;
using Oakton.Resources;
using OrderSagaSample;

var builder = WebApplication.CreateBuilder(args);

// Not 100% necessary, but enables some extra command line diagnostics
builder.Host.ApplyOaktonExtensions();

// Adding Marten
builder.Services.AddMarten(opts =>
    {
        var connectionString = builder.Configuration.GetConnectionString("Marten");
        opts.Connection(connectionString);
        opts.DatabaseSchemaName = "orders";
    })

    // Adding the Jasper integration for Marten.
    .IntegrateWithJasper();


builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

// Do all necessary database setup on startup
builder.Services.AddResourceSetupOnStartup();

// The defaults are good enough here
builder.Host.UseJasper();

var app = builder.Build();

// Just delegating to Jasper's local command bus for all
app.MapPost("/start", (StartOrder start, ICommandBus bus) => bus.InvokeAsync(start));
app.MapPost("/complete", (CompleteOrder start, ICommandBus bus) => bus.InvokeAsync(start));
app.MapGet("/all", (IQuerySession session) => session.Query<Order>().ToListAsync());
app.MapGet("/", (HttpResponse response) =>
{
    response.Headers.Add("Location", "/swagger");
    response.StatusCode = 301;
});

app.UseSwagger();
app.UseSwaggerUI();

return await app.RunOaktonCommands(args);

Off screen, I’ve started up a docker container for Postgresql to get a blank database. With that running, I’ll start the application up with the usual dotnet run command and open up the Swagger page:

You’ll get a lot of SQL in your terminal on the first run as Marten sets up the database for you, that’s perfectly normal.

I’m going to first create a new order for “Shoes” and execute the /create endpoint:

And verify that it’s persisted by checking the /all endpoint:

If I’m quick enough, I’ll post {"Id": "Shoes"} to /complete, and then verify through the /all endpoint that the “Shoes” order has been completed.

Otherwise, if I’m too slow to complete the order, the timeout message will be applied to our order and you’ll see evidence of that in the logging output like so:

And that’s it, one working saga implementation with database backed persistence through Marten. The goal of Jasper is to make this kind of server side development as low ceremony and easy to use as possible, so any feedback about what you do or don’t like in this sample would be very helpful.

Related Posts

I’ve spit out quite a bit of blogging content the past several weeks on both Marten and Jasper:

• Marten just got better for CQRS architectures about some new functionality dreamed up by Oskar Dudycz and I to smooth out some repetitive friction when using Marten with a CQRS style architecture
• (Re) Introducing Jasper as a Command Bus about the ongoing Jasper 2.0 reboot
• A Vision for Low Ceremony CQRS with Event Sourcing that combined some new functionality in Marten together with Jasper’s execution pipeline and middleware strategy to show how CQRS command handlers can be built with very minimal code ceremony
• Building a More Useful Outbox for Reliable Messaging about Jasper’s transactional outbox implementation
• Ping/Pong Jasper Style as a quick little sample of using very basic Jasper messaging between processes
• Using Rabbit MQ with Jasper. Basically the same thing as the previous post, but this time using Rabbit MQ to move messages around.
• A Vision for Stateful Resources at Development or Deployment Time