One of the key elements of Google's software engineering culture is the use of defining software designs through design docs. These are relatively informal documents that the primary author or authors of a software system or application create before they embark on the coding project. The design doc documents the high level implementation strategy and key design decisions with emphasis on the trade-offs that were considered during those decisions.
As software engineers our job is not to produce code per se, but rather to solve problems. Unstructured text, like in the form of a design doc, may be the better tool for solving problems early in a project lifecycle, as it may be more concise and easier to comprehend, and communicates the problems and solutions at a higher level than code.
Besides the original documentation of a software design, design docs fulfill the following functions in the software development lifecycle:
Design docs are informal documents and thus don’t follow a strict guideline for their content. Rule #1 is: Write them in whatever form makes the most sense for the particular project.
Having said that, a certain structure has established itself as really useful.
This section gives the reader a very rough overview of the landscape in which the new system is being built and what is actually being built. This isn’t a requirements doc. Keep it succinct! The goal is that readers are brought up to speed but some previous knowledge can be assumed and detailed info can be linked to. This section should be entirely focused on objective background facts.
A short list of bullet points of what the goals of the system are, and, sometimes more importantly, what non-goals are. Note, that non-goals aren’t negated goals like “The system shouldn’t crash”, but rather things that could reasonably be goals, but are explicitly chosen not to be goals. A good example would be “ACID compliance”; when designing a database, you’d certainly want to know whether that is a goal or non-goal. And if it is a non-goal you might still select a solution that provides it, if it doesn’t introduce trade-offs that prevent achieving the goals.
This section should start with an overview and then go into details.
The design doc is the place to write down the trade-offs you made in designing your software. Focus on those trade-offs to produce a useful document with long-term value. That is, given the context (facts), goals and non-goals (requirements), the design doc is the place to suggest solutions and show why a particular solution best satisfies those goals.