This post is in continuation of the design for an object cache layer as described here. A specific use case now explains the aging of objects in the cache. While the object storage saved the objects for durability, the cache made it available for workloads to save on the cost of going deep into object storage.  The cache also translated the user workloads into an append-only workload for the object storage. This simplifies versioning and replication which can now be done in object storage at little or no cost. A Garbage Collection System demonstrates this aging.  While the cache can implement any one of the caching policies for retention of object, it can also be delegated to the user workload where the workload specifies which objects have aged. Then the cache merely schedules the storage of these objects into the object storage. Such a policy is best demonstrated in a user workload that implements a garbage collection system. And once it works well in the user workload, the logic can then be moved into the cache layer.
In this post and the next few, we bring up the .Net garbage collection as an example of such an aging policy. The .Net garbage collection is a generational markdown compactor. The compactor uses the notion of a generation to identify objects by their usage. The most used objects belong to a younger generation and those that are less used, belong to the older generation. The generation gives us an indication of age. The age of the object on the other hand, is the time from last use. We will describe the sweep and the collection shortly but we proceed with the notion that there is a metric that lets us identify the age of the object which the cache then rolls to object storage.

#codingexercise

Check if a number has less set than unset bits. 

boolean hasLessSetThanUnsetBits(int n) 

{ 

    int set = 0; 

    int unset = 0; 

    while (n) 

    { 

        if (n & 0x1) { 

             set++; 

        } else {  

             unset++; 

        } 

        n = n >> 1; 

    } 

    return (set < unset); 

}