Transaction Scoping in FubuMVC with RavenDb and StructureMap

TL;DR – We built unit of work and transactional boundary mechanics into our RavenDb integration with FubuMVC (with StructureMap supplying the object scoping) that I’m still happy with and I’d consider using the same conceptual design again in the future.

Why write this now long after giving up on FubuMVC as an OSS project? We still use FubuMVC very heavily at work, I’m committed to supporting StructureMap as long as it’s relevant, and RavenDb is certainly going strong. I’ve seen other people trying to accomplish the same kind of IoC integration and object scoping that I’m showing here on other frameworks and this might be useful to those folks. Mostly though, this post might benefit some of our internal teams and almost every enterprise software project involves some kind of transaction management.

Transaction Boundaries

If you’re working on any kind of software system that persists state, you probably need to be consciously aware of the ACID rules (Atomicity, Consistency, Isolation, Durability) to determine where your transaction boundaries should be. Roughly speaking, you want to prevent any chance of a system getting into an invalid state because some database changes succeeded while other database changes within the same logical operation fail. The canonical example is transfering money between two different bank accounts. For that use case, you’re probably making at least three different changes to the underlying database:

1. Subtract the amount of the transfer from the first account
2. Add the amount of the transfer to the second account
3. Write some sort of log record for the transfer

All three database operations above need to succeed together, or all three should be rolled back to prevent your system from getting into an invalid state. Failing to manage your transaction boundaries could result in the bank customer either losing some of their funds or the bank magically giving more money to the consumer. Following the ACID rules, we would make sure that all three operations are within the same database transaction so that they all have to succeed or fail together.

Using event sourcing and eventual consistency flips this on its head a little bit. In that case you might just capture the log record, then rebuild the “read side” views for the two accounts asynchronously to reflect the transfer. Needless to say, that can easily generate a different set of problems when there’s any kind of lag in the “eventual.”

The Unit of Work Pattern

Historically, one of the easier ways to collect and organize database operations into transactions has been the Unit of Work pattern that’s now largely built into most every persistence framework. In RavenDb, you use the IDocumentSession service as the logical unit of work. Using RavenDb, we can just collect all the documents that need to be persisted within a single transaction by calling the Delete() or Store() methods. When we’re ready to commit the entire unit of work as a transaction, we just call the SaveChanges() method to execute all our batched up persistence operations.

One of the big advantages to using the unit of work pattern is the ability to easily coordinate transaction boundaries across multiple unrelated classes, services, or functions — as long as each is working against the same unit of work object. The challenge then is to control the object lifecycle of the RavenDb IDocumentSession object in the lifecycle of a logical operation. While you can certainly use some sort of transaction controller (little ‘c’) to manage the lifecyle of an IDocumentSession object and coordinate its usage within the actual workers, we built IDocumentSession lifecycle handling directly into our FubuMVC.RavenDb library so that you can treat an HTTP POST, PUT, or DELETE in FubuMVC or the handling of a single message in the FubuTransportation service bus as a logical unit of work and simply let the framework itself do all your transaction boundary kind of bookkeeping for you.

IDocumentSession Lifecycle Management 

First off, FubuMVC utilizes the StructureMap Nested Container feature to create an isolated object scope per HTTP request — meaning that any type registered with the default lifecyle that is resolved by StructureMap within a request (or message) will be the same object instance. In the case of IDocumentSession, this scoping means that every concrete type built by StructureMap will have the same shared instance of IDocumentSession.

To make it concrete, let’s say that you have these three classes that all take in an IDocumentSession as a constructor argument:

    public class RavenUsingEndpoint
    {
        private readonly IDocumentSession _session;
        private readonly Worker1 _worker1;
        private readonly Lazy<Worker2> _worker2;

        public RavenUsingEndpoint(IDocumentSession session, Worker1 worker1, Lazy<Worker2> worker2)
        {
            _session = session;
            _worker1 = worker1;
            _worker2 = worker2;
        }

        public string post_session_is_the_same()
        {
            _session.ShouldBeTheSameAs(_worker1.Session);
            _session.ShouldBeTheSameAs(_worker2.Value.Session);

            return "All good!";
        }
    }

    public class Worker1
    {
        public IDocumentSession Session { get; set; }

        public Worker1(IDocumentSession session)
        {
            Session = session;
        }
    }

    public class Worker2 : Worker1
    {
        public Worker2(IDocumentSession session) : base(session)
        {
        }
    }

When a FubuMVC request executes that uses the RavenUsingEndpoint.post_session_is_the_same() method, FubuMVC is creating a RavenUsingEndpoint object, a Worker1 object, a Lazy<Worker2> object, and one single IDocumentSession object that is injected both into RavenUsingEndpoint and Worker1. Additionally, if the Lazy<Worker2> is evaluated, a Worker2 object will be created from the active nested container and the exact same IDocumentSession will again be injected into Worker2. With this type of scoping, we can use RavenUsingEndpoint, Worker1, and Worker2 in the same logical transaction without worrying about how we’re passing around IDocumentSession or dealing with any sort of less efficient transaction scoping tied to the thread.

TransactionalBehavior

So step 1, and I’d argue the hardest step, was making sure that the same IDocumentSession object was used by each handler in the request. Step 2 is to execute the transaction.

Utilizing FubuMVC’s Russian Doll model, we insert a new “behavior” into the request handling chain for simple transaction management:

    public class TransactionalBehavior : WrappingBehavior
    {
        private readonly ISessionBoundary _session;

        // ISessionBoundary is a FubuMVC.RavenDb service
        // that just tracks the current IDocumentSession
        public TransactionalBehavior(ISessionBoundary session)
        {
            _session = session;
        }

        protected override void invoke(Action action)
        {
            // The 'action' below is just executing
            // the handlers nested inside this behavior
            action();

            // Saves all changes *if* an IDocumentSession
            // object was created for this request
            _session.SaveChanges();
        }
    }

The sequence of events inside a FubuMVC request using the RavenDb unit of work mechanics would be to:

1. Determine which chain of behaviors should be executed for the incoming url (routing)
2. Create a new StructureMap nested container
3. Using the new nested container, build the entire “chain” of nested behaviors
4. The TransactionalBehavior starts by delegating to the inner behaviors, one of which would be…
5. A behavior that invokes the RavenUsingEndpoint.post_session_is_the_same() method
6. The TransactionBehavior object, assuming there are no exceptions, executes all the queued up persistence actions as a single transaction
7. At the end of the request, FubuMVC will dispose the nested container for the request, which will also call Dispose() on any IDisposable object created by the nested container during the request, including the IDocumentSession object.

How does the TransactionalBehavior get applied?

In the early days we got slap happy with conventions and the simple inclusion of the FubuMVC.RavenDb library in the applications bin path was enough to add the TransactionBehavior to any route that responded to the PUT, POST, or DELETE http verbs. For FubuMVC 2.0 I made this behavior “opt in” instead of being automatic, so you just had to either include a policy to specify which routes or message handling chains needed the transactional semantics:

    public class NamedEntityRegistry : FubuRegistry
    {
        public NamedEntityRegistry()
        {
            Services(x => x.ReplaceService(new RavenDbSettings { RunInMemory = true}));

            // Applies the TransactionalBehavior to all PUT, POST, DELETE routes
            Policies.Local.Add<TransactionalBehaviorPolicy>();
        }
    }

You could also use attributes for fine grained application if there was a need for that.

Explicit Transaction Boundaries

I fully realize that many people will dislike the API usage with the nested closure below, but it’s worked out well for us in my opinion — especially when it’s rarely used as the exception case rather than the normal every day usage.

All the above stuff is great as long as your within the context of an HTTP request or a FubuTransportation message that represents a single logical transaction, but that leaves plenty of circumstances where you need to explicitly control transaction boundaries. To that end, we’ve used the ITransaction service in FubuPersistence to mark a logical transaction boundary by again executing a logical transaction within the scope of a completely isolated nested container for the operation.

Let’s say you have a simple transaction worker class like this one:

    public class ExplicitWorker
    {
        private readonly IDocumentSession _session;

        public ExplicitWorker(IDocumentSession session)
        {
            _session = session;
        }

        public void DoSomeWork()
        {
            // do something that results in writes
            // to IDocumentSession
        }
    }

To execute the ExplicitWorker.DoSomeWork() method in its own isolated transaction, I would invoke the ITransaction service like this:

// Just spinning up an empty FubuMVC application
// FubuMVC.RavenDb is picked up automatically by the assembly
// being in the bin path
using (var runtime = FubuApplication.DefaultPolicies().StructureMap().Bootstrap())
{
    // Pulling a new instance of the ITransaction service
    // from the main application container
    var transaction = runtime.Factory.Get<ITransaction>();
            
    transaction.Execute<ExplicitWorker>(x => x.DoSomeWork());
}

At runtime, the sequence of actions is:

1. Create a new StructureMap nested container
2. Resolve a new instance of ExplicitWorker from that nested container
3. Execute the Action<ExplicitWorker> passed into the ITransaction against the newly created ExplicitWorker object to execute the logical unit of work
4. Assuming that the action didn’t throw an exception, submit all changes to RavenDb as a single transaction
5. Dispose the nested container

Use this strategy when you need to execute transactions outside the scope of FubuMVC HTTP requests or FubuTransportation message handling, or anytime when it’s appropriate to execute multiple transactions for a single HTTP request or FT message.