Essence

The essence of an entity is a number, sort of a handle, to its exact contents. The only requirement is that if two representations of an entity are different then their essence values must be different.

I came up with concept quite a while ago, probably 6 years ago, while I was working with very large hierarchical IC designs. We were trying to figure out how to run tools more quickly on large designs. I began to think of entire sub-hierarchies as entities. I realized that the essence value for a whole hierarchy could be calculated by combining the essence values of its sub-hierarchies.

This was around the same time that git was developed for Linux kernel development. I watched the development of git from day one. I was using git as an integral part of my development within a month or two. I got inspiration from it and realized that its underlying concepts could be used in other ways. Essentially, git uses a sha1 as an essence value for files (blobs) and directories (trees). It does it hierarchically by including the essence values of files and sub-directories in calculating the essence value of a parent directory. This was very much like what I needed.

Recently, I’ve been trying to learn more about http. I’ve read RFC 2616 (Hypertext Transfer Protocol – HTTP/1.1) in its entirety and realized that it has a concept almost exactly like my essence concept. They call it an entity tag. More specifically a strong entity tag. I was pretty excited to see this in there. So much focus is given to cache validation using timestamps, ages and expiration that I didn’t even realize that this was in there until I read it. HTTP became more than just a necessary evil to me. I think I will enjoy it.

Obviously, I didn’t come up with the essence concept but I see much more potential in this concept than has been tapped in the few applications that I’ve come across. I hope to be responsible for tapping this potential in the near future.