Sunday, November 10, 2013

An event store with optimistic concurrency

Like I mentioned last week - after only five posts on the subject - there still are a great deal of event sourcing nuances left to be discovered.

My current event store implementation only supports a single user. Due to an aggressive file lock, concurrently accessing an aggregate will throw an exception. Can we allow multiple users to write to and read from an event stream? Also, what can we do about users making changes to the same aggregate; can we somehow detect conflicts and avoid changes to be committed?

Multi-user

In the current version, concurrently appending to or reading from an aggregate's event stream will throw since the file will already be locked.
Parallel.For(0, 1000, (i) =>
{    
    _eventStore.CreateOrAppend(aggregateId, new EventStream(new List<IEvent>() 
    { 
        new ConcurrencyTestEvent() 
    }));
    _eventStore.GetStream(aggregateId);    
});
The exception looks like this: "System.IO.IOException: The process cannot access the file 'C:\EventStore\92f42a08-8583-4dcf-98a5-440b06f34719.txt' because it is being used by another process."

To prevent concurrent file access, we can lock code accessing the aggregate's event stream. Instead of using a global lock, we maintain a dictionary of lock objects; one lock object per aggregate.
lock (Lock.For(aggregateId))
{
    using (var stream = new FileStream(
        path, FileMode.Append, FileAccess.Write, FileShare.Read))
    {
        // Access the aggregate's event stream
    }
}

public class Lock
{
    private static ConcurrentDictionary<Guid, object> _locks = 
        new ConcurrentDictionary<Guid, object>();

    public static object For(Guid aggregateId)
    {
        var aggregateLock = _locks.GetOrAdd(aggregateId, new object());

        return aggregateLock;
    }
}     
Optimistic concurrency

Before committing changes, we want to verify that no other changes have been committed in the meanwhile. These changes could have influenced the behaviour of our aggregate significantly. Appending the last changes without considering what might have happened in the meanwhile might corrupt our aggregate's state.

One way to verify this is by using a number (or a timestamp - clocks, bah) to keep track of an aggregate's version. It's up to the client to tell us which version he expects when appending to a stream. To accommodate for this, we need to change the contract of our event store.
public interface IEventStore
{
    void Create(Guid aggregateId, EventStream eventStream);

    void Append(Guid aggregateId, EventStream eventStream, int expectedVersion);

    ReadEventStream GetStream(Guid aggregateId);
}
Clients now need to pass in the expected version when appending to a stream. The result of reading a stream will include the current version.

In the event store, we now store an index with every event.


If we append to an event stream, we will get the current version by reading the highest index - storing this in aggregate meta data would be faster for reading. If the current version doesn't match the expected version, we throw an exception.
var currentVersion = GetCurrentVersion(path);

if (currentVersion != expectedVersion)
    throw new OptimisticConcurrencyException(expectedVersion, currentVersion);

using (var stream = new FileStream(
    path, FileMode.Append, FileAccess.Write, FileShare.Read))
{
    using (var streamWriter = new StreamWriter(stream))
    {
        foreach (var @event in eventStream)
        {
            currentVersion++;

            streamWriter.WriteLine(new Record(
                aggregateId, @event, currentVersion).Serialized());
        }
    }
}
A test for that looks something like this.
try
{
    GivenEventStore();
    GivenAggregateId();
    GivenEventStreamCreated();
    WhenAppendingTwoEventStreamsWithTheSameExpectedVersion();
}
catch (OptimisticConcurrencyException ocex) 
{
    _expectedConcurrencyException = ocex;
}

[TestMethod]
public void ThenTheConcurrencyExceptionHasANiceMessage()
{
    var expected = "Version found: 3, expected: 1";
    var actual = _expectedConcurrencyException.Message

    Assert.AreEqual(expected, actual);
}
Reading the event stream doesn't change much; we now also read the current version, and return it with the event stream. 
var lines = File.ReadAllLines(path);

if (lines.Any())
{
    var records = lines.Select(x => Record.Deserialize(x, _assembly));
    var currentVersion = records.Max(x => x.Version);
    var events = records.Select(x => x.Event).ToList();

    return new ReadEventStream(events, currentVersion);
}

return null; 
And that's one way to implement optimistic concurrency. The biggest bottleneck in this approach is how we read the current version; having to read all the events to find the current version isn't very efficient.

Transactional behaviour is also missing. I've been thinking about adding a COMMIT flag after appending a set of events, and using that to resolve corruption on reads, or is this fundamentally flawed? 

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