I would describe it as two fold. From an architectural (or design) point of view, it is a clean slate design. We come at it from first principles asking questions like ‘What is the problem we are trying to solve using caching in memory storage?’ and ‘What are the requirements?’ If we pose this problem as a new question today, how are we going to design an architecture that works for the scale and works for the datacenter that we have or that is available on the market? So from a design point of view, it is a clean slate.

However, new code is usually bad code (or if not bad, it’s ugly code). There is significant risk in using new coding in a critical piece of the architecture. In terms of implementation, we are not so strict. We actually prefer using existing code if it fits into the design well. So let’s say you have an existing networking library that works really well, there is no need to write your own library to handle connections. You can just copy the code that handles connections and drop that into your design. You can do that, because it serves exactly the same purpose. With implementation, we are much more practical and utilitarian in examining solutions that we have, including Memcached and Twemcached (which is our fork Redis). We will take a look at whoever has a good library out there that does something similar. gRPC is another one. There are actually a lot of code that does more or less the same thing, so we would just look at all the codebases we can find. If there is something that we can use, we will use it, and we will give credit to open source project.

In reality, what happened is we started by copying code and writing new code about one to one. We were re-using about 50% of our code from existing open source projects and I lost track of the ratio over time because then we started re-factoring and polishing the code. But we started with 50% re-use. The new code ratio has gone up slightly since then, because, as we add more features and as we re-write, we tend to bring more code of our own into the project.