Edge cases GetEventStore API

gistfile1.md

Some examples of edge cases that have to be watched out for in GetEventStore, starting with the read API. Here's the F#-API type for ReadStreamEventsForwardAsync:

  /// <see cref="EventStore.ClientAPI.StreamEventsSlice" />
  type StreamEventsSlice =
    | NotFound of StreamId
    | Deleted of StreamId
    | Success of StreamSlice
  /// Convert a <see cref="EventStore.ClientAPI.StreamEventsSlice" /> to a
  /// ConnectionApi.EventsSlice.
  and StreamSlice =
    | StreamSlice of Slice
    | EndOfStream of Slice
  and Slice =
    { Events          : ResolvedEvent list
    ; Stream          : StreamId
    ; FromEventNumber : uint32
    ; ReadDirection   : ReadDirection
    ; NextEventNumber : uint32
    ; LastEventNumber : uint32 }
  and StreamId = string
  and StreamId = string

Here's the corresponding decompiled API source:

  public class StreamEventsSlice
  {
    public readonly SliceReadStatus Status;
    public readonly string Stream;
    public readonly int FromEventNumber;
    public readonly ReadDirection ReadDirection;
    public readonly EventStore.ClientAPI.ResolvedEvent[] Events;
    public readonly int NextEventNumber;
    public readonly int LastEventNumber;
    public readonly bool IsEndOfStream;

    internal StreamEventsSlice(SliceReadStatus status, string stream, int fromEventNumber, ReadDirection readDirection, ClientMessage.ResolvedIndexedEvent[] events, int nextEventNumber, int lastEventNumber, bool isEndOfStream)
    {
      Ensure.NotNullOrEmpty(stream, "stream");
      this.Status = status;
      this.Stream = stream;
      this.FromEventNumber = fromEventNumber;
      this.ReadDirection = readDirection;
      if (events == null || events.Length == 0)
      {
        this.Events = Empty.ResolvedEvents;
      }
      else
      {
        this.Events = new EventStore.ClientAPI.ResolvedEvent[events.Length];
        for (int index = 0; index < this.Events.Length; ++index)
          this.Events[index] = new EventStore.ClientAPI.ResolvedEvent(events[index]);
      }
      this.NextEventNumber = nextEventNumber;
      this.LastEventNumber = lastEventNumber;
      this.IsEndOfStream = isEndOfStream;
    }
  }

Leaving aside the pain of having public APIs with internal c'tors (forcing this darling* to come to life), it is clear there are some implicit invariants:

• We might get IsEndOfStream = true and then all other properties are pretty much possible to ignore. This has to be abducted.
• The stream can both be not found (Status = StreamReadStatus.NotFound) and,
• previously deleted (Status = StreamReadStatus.Deleted) or it can be
• Found, having data.

Besides that, the call to ReadStreamEventsForwardAsync has two System.Int32 parameters, which always have to be zero or a positive number. Why not use System.UInt32? Also, passing a zero count makes no sense, so that should really be always a positive number.

It becomes even more interesting when passing expectedVersion and to tell the API -2 (int) for no optimistic concurrency checking, -1 for NoStream, 0 for empty stream, having runtime exceptions in all other negative cases, and on top of that, having a possibility of getting WrongExpectedVersionException as an exception on the asynchronous call, if the optimistic check doesn't pass.

footnote:

  // returns the constructor for the instance and a setter function, as a tuple, respectively
  let reflPkgFor<'a> () : (unit -> 'a) * ('a -> string -> obj -> unit) =
    let setterFor name = typeof<'a>.GetField(name, System.Reflection.BindingFlags.Instance ||| System.Reflection.BindingFlags.NonPublic)
    let memSetterFor = memoize setterFor
    let emptyCtor = fun () -> System.Runtime.Serialization.FormatterServices.GetUninitializedObject(typeof<'a>) :?> 'a
    let setter (instance : 'a) prop (value : obj) = memSetterFor prop |> (fun fi -> fi.SetValue(instance, value))
    emptyCtor, setter

@anakryko

Thanks for your input. I noticed the same thing about EndOfStream (updated gist to my current code) - but it goes to show that the returned data doesn't show me how to use it in C#.

The comments that you added about LastEventNumber are interesting and definitely something hadn't heard of before. Thanks for telling me about it.