Unable to insert objects of equal priority into ConcurrentSkipListSet

Question

I am having an issue with a simplistic thread-safe implementation of the observer pattern using a ConcurrentSkipListSet to handle keeping track of observer priorities during insertion. The majority of observers will not have any special priority attributed to them, and following this Comparable#compareTo method will show as equal priority when compared (where priority is a value in an enum of five priorities ranging from highest to lowest):

public int compareTo(BaseLink<?> link) {
    return this.priority.compareTo(link.getPriority());
}

When I add observers of equal priorities to the ConcurrentSkipListSet , it seems like some of the added objects are simply lost during the insertion process. Changing the priorities of any of the observers I have created while testing this results in those observers being added to the set without issue, though I assume that given enough observers of the same priority the issue will arise again.

I am unsure about what is causing this issue, and of what I should do to help resolve it. Is there anything I can do to resolve this issue? Alternatively if this is an inherent problem with the ConcurrentSkipListSet, are there any other thread-safe data structures that can give me reasonably performant insertion and sorting times for unique objects?

Answer 1

I presume that you are instantiating like this ConcurrentSkipListSet(myComparator) where myComparator implements compareTo as you have shown us.

A ConcurrentSkipListSet is a Set. When you instantiate one using a Comparator, it will use it to:

1. order the set elements
2. determine when a new element is already in the set.

In your code, your Comparator.compareTo(...) is saying that every BaseLink with a given priority is the same. That's what is causing your problem.

Solution:

public int compareTo(BaseLink<?> link) {
    int res = this.priority.compareTo(link.getPriority());
    if (res == 0) {
        // tie-breaker for different links with the same priority
        res = // compare using some other key / identifier
    }
    return res;
}

If your BaseLink objects have a natural key or identifier, you could use that as the tie-breaker for objects with the same priority. Even the object's identity hashcode would do ... if you are not worried about "fairness" or "reproducibility" in the ordering of the set.