"Cannot create tracker" exception thrown from ModelFactory

Question

I am facing the following error:

ERROR Cannot create tracker. Exception: System.IndexOutOfRangeException Message: Index was outside the bounds of the array. Source: mscorlib at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add) at Sitecore.Analytics.Model.Framework.ModelFactory.GetConcreteType(Type elementType) at Sitecore.Analytics.Model.Framework.ModelFactory.CreateElement(Type elementType) at Sitecore.Analytics.Model.Framework.ModelFactory.CreateFacet(Type facetType) at Sitecore.Analytics.Model.Framework.Faceted.AddFacet(String name, Type type) at Sitecore.Analytics.Model.Framework.ModelFactory.CreateContact(ID id, IReadOnlyDictionary`2 facets) at Sitecore.Analytics.Data.ContactFactory.Create(ID id) at Sitecore.Analytics.Data.ContactRepository.CreateContact(ID id) at Sitecore.Analytics.Pipelines.EnsureSessionContext.LoadContact.Process(InitializeTrackerArgs args) at (Object , Object[] ) at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args) at Sitecore.Analytics.Pipelines.EnsureSessionContext.EnsureSessionContextPipeline.Run(InitializeTrackerArgs args) at Sitecore.Analytics.DefaultTracker.EnsureSessionContext() at Sitecore.Analytics.Pipelines.CreateTracker.GetTracker.Process(CreateTrackerArgs args) at (Object , Object[] ) at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args) at Sitecore.Analytics.Tracker.Initialize()

Per this post I verified that we have those config values present in our solution.

Still, randomly I get this error on one of the CD servers and not on all. Any idea why?

Answer 1

This is a known bug in Sitecore. Use this fix to solve the issue.

To track the status of this bug with Sitecore Support, you can use the reference number 95735.

Explanation

The ModelFactory class uses a static dictionary under the cover. It stores a mapping of model interfaces to their implementation types, as defined in the configuration (see Sitecore.Analytics.Model.config). This dictionary is purely an optimization—it helps not to read the configuration every time.

public static class ModelFactory
{
    private static readonly IDictionary<Type, Type> typeMap = new Dictionary<Type, Type>();

    // ...
}

Since this dictionary is static, it can be accessed by several threads simultaneously. The Dictionary class is not meant to be used concurrently, which is why its Insert method throws the obscure error you're seeing. This happens when two threads insert a new entry at the same time.

This should be fixed by either respecting concurrency in the ModelFactory class (e.g. apply locks), or by using a ConcurrentDictionary instead. The fix I linked at the top substitutes the dictionary with a concurrent dictionary.