ConcurrentHashSet | A ConcurrentHashSet implementation

Been having some problems with a web based .Net(C#) application. I'm using the LazyCache library to cache frequent JSON responses (some in & around 80+KB) for users belonging to the same company across user sessions.

One of the things we need to do is to keep track of the cache keys for a particular company so when any user in the company makes mutating changes to items being cached we need to clear the cache for those items for that particular company's users to force the cache to be repopulated immediately upon the receiving the next request.

We choose LazyCache library as we wanted to do this in memory without needing to use an external cache source such as Redis etc as we don't have heavy usage.

One of the problems we have using this approach is we need to keep track of all the cache keys belonging to a particular customer anytime we cache. So when any mutating change is made by company user's to the relevant resource we need to expire all the cache keys belonging to that company.

To achieve this we have a global cache which all web controllers have access to.

I'm having a method which calls a WCF service multiple times in parallel. To prevent an overload on the target system, I want to use PLinq's ability to limit the number of parallel executions. Now I wonder how I could rewrite my method in an efficient way.

Here's my current implementation:

Let's consider the following following code:

I'm currently using rx-java 2 and have a use case where multiple Observables need to be consumed by single Camel Route subscriber. Using this solution as a reference, I have a partly working solution. RxJava - Merged Observable that accepts more Observables at any time?

I'm planning to use a PublishProcessor that will be subscribed to one camel reactive stream subscriber and then maintain a ConcurrentHashSet> where I can dynamically add new Observable.
I'm currently stuck on how can I add/manage Flowable instances with PublishProcessor? I'm really new to rx java, so any help is appreciated! This is what I have so far :

Recently I switched from HashSet to a collection someone else posted called 'ConcurrentHashSet', I decided to opt out of locking my own HashSet as I was using it a lot and just seemed a safer bet to use a pre-made thread-safe class, but I've hit an issue.

When I used HashSet (the default HashSet class) I was getting my HashSet values using the First and FirstOrDefault methods, the issue is I can no longer use these methods and I'm not sure why or how to re-implement them, is it even possible? It may be really simple, I'm unsure.

I was hoping someone would know and could point me in the right direction. Heres the class that I picked up off another stack overflow answer, although I'm not sure if this is the original.