Java Optimistic Locking

Optimistic Locking

Motivation

Whatever your particular way of modelling your software, in order to be able to enforce invariants in your aggregates, you'd need to coordinate writes to it. In simple monoliths, you do that by synchronizing write access to the aggregate. When Software-Systems become distributed (or at least replicated), this coordination obviously needs to be externalized.

Pessimistic Locking

While pessimistic locking makes sure every change is strictly serializable, it has obvious drawbacks in terms of throughput and complexity (timeouts) as well as the danger of deadlock, when the scope of the lock expands to more than one aggregate. This is why we chose to implement a means of optimistic locking in Factcast.

Optimistic Locking

In general, the idea of optimistic locking is to make a change and before publishing it, make sure there was no potentially contradicting change in between. If there was, the process can safely be retried, as there was nothing published yet.

Transferred to Factcast, this means to express a body of code that:

  1. creates an understanding of the published state of the aggregates in question
  2. invokes its business logic according to that state
  3. creates the effects: either fails (if business logic decides to do so), or publishes new Fact(s)
  4. rechecks, if the state recorded in 1. is still unchanged and then
  5. either publishes the prepared Facts or retries by going back to 1.

Usage

a simple example

This code checks if an account with id newAccountId already exists, and if not - creates it by publishing the Fact accordingly.

        factcast.lock("myBankNamespace")
                .on(newAccountId)
                .attempt(() -> {
                    // check and maybe abort
                    if (repo.findById(newAccountId) !=null)
                        return Attempt.abort("Already exists.");
                    else
                      return Attempt.publish(
                        Fact.builder()
                        .ns("myBankNamespace")
                        .type("AccountCreated")
                        .aggId(newAccountId)
                        .build("{...}")
                      );            
                });

You may probably guess what happens, remembering the above steps. Let's dive into details with a more complex scenario.

a complete example

The unavoidable imaginary example, of two BankAccounts and a money transfer between them:

        factcast.lock("myBankNamespace")
                .on(sourceAccountId,targetAccountId)
                .optimistic()            // this is optional, defaults to optimistic, currently the only mode supported
                .retry(100)              // this is optional, default is 10 times
                .interval(5)             // this is optional, default is no wait interval between attempts (equals to 0)
                .attempt(() -> {
                    
                    // fetch the latest state
                    Account source = repo.findById(sourceAccountId);
                    Account target = repo.findById(targetAccountId);
                    
                    // run businesslogic on it
                    if (source.amount() < amountToTransfer)
                        return Attempt.abort("Insufficient funds.");
                    
                    if (target.isClosed())
                        return Attempt.abort("Target account is closed");
                    
                    // everything looks fine, create the Fact to be published
                    Fact toPublish = Fact.builder()
                        .ns("myBankNamespace")
                        .type("transfer")
                        .aggId(sourceAccountId)
                        .aggId(targetAccountId)
                        .build("{...}");            
                    
                    // register for publishing
                    return Attempt.publish(toPublish).andThen(()->{
                        
                        // this is only executed at max once, and only if publishing succeeded
                        log.info("Money was transferred.");
                        
                    });
                    
                });

Explanation

First, you tell factcast to record a state according to all events that have either sourceAccountId or targetAccountId in their list of aggIds and are on namespace myBankNamespace. While the namespace is not strictly necessary, it is encouraged to use it - but it depends on your decision on how to use namespaces and group Facts within them.

The number of retries is set to 100 here (default is 10, which for many systems is an acceptable default). In essence this means, that the attempt will be executed at max 100 times, before factcast gives up and throws an OptimisticRetriesExceededException which extends ConcurrentModificationException.

If interval is not set, it defaults to 0 with the effect, that the code passed into attempt is continuously retried without any pause until it either aborts, succeeds, or the max number of retries was hit (see above). Setting it to 5 means, that before retrying, a 5 msec wait happens.

Everything starts with passing a lambda to the attempt method. The lambda is of type

@FunctionalInterface
public interface Attempt {
    IntermediatePublishResult call() throws AttemptAbortedException;
    //...
}

so that it has to return an instance of IntermediatePublishResult. The only way to create such an instance are static methods on the same interface (abort & publish) in order to make it obvious. This lambda now is called according to the logic above.

Inside the lambda, you'd want to check the current state using the very latest facts from factcast (repo.findById(…)) and then check your business constraints on it (if (source.amount() < amountToTransfer)…). If the constraints do not hold, you may choose to abort the Attempt and thus abort the process. In this case, the attempt will not be retried.

On the other hand, if you choose to publish new facts using Attempt.publish(…), the state will be checked and the Fact(s) will be published if there was not change in between (otherwise a retry will be issued, see above).

Optionally, you can pass a runnable using .andThen and schedule it for execution once, if and only if the publishing succeeded. Or in other words, this runnable is executed just once or never (in case of abort or OptimisticRetriesExceededException).