Design documentation at small companies

One component of the engineering culture at Twitter (where I used to work) that I’m trying to instill at my new job is the importance of writing design documents prior to implementing complicated systems. In this essay, I will argue in favor of premeditated software design at small companies and propose what I call “precautionary migration planning” as a design doc section that caters specifically to the tradeoffs required by startups.

Traveling by map

A design document is an outline of a proposed design for a software system in writing and figures. The level of detail and formality can vary, but the purpose is to force an engineer to think about and document what a system should do and how it should be built before effort is spent on implementation.

Many large companies enforce design docs for all new projects, going so far as to prescribe document templates and design review meetings. While such a formal approach makes sense when projects require coordinated effort across multiple teams and scores of people, it would be an inappropriate amount of overhead for an engineering team at a startup.

But the baby shouldn’t be thrown out with the bathwater. Writing down and examining your thoughts prior to acting on them is a good way to avoid mistakes and prevent unwarranted technical debt. As such, even at a startup, going through a semi-formal design exercise injects a healthy amount of peer-review into the process and can increase the reliability of the systems you end up with. Not to mention the added benefit of having a good understanding of a project’s scope and thorough high-level documentation prior to writing a single line of code. Ideally, when you bring new folks onto the team, you can simply link them to a set of design docs and save yourself an hour of whiteboarding.

A straightforward analogy helps illustrate when a design doc is appropriate for a new undertaking. A design doc is like a set of directions and a map. The complexity of a journey determines whether or not directions are required. For instance, you can walk up the road to the grocery store without thinking, so you obviously don’t need a map. Similarly, if all you need to do is add a simple feature or fix a simple bug, then a rigorous design process is probably unnecessary.

However, for trips venturing into unfamiliar territory or requiring multiple vehicles, coordinating travel with a set of directions is a must. Likewise, if a system at the core of the company’s business has many moving parts and will affect the lives of numerous people over its lifetime, then a design doc will probably prove to be worthwhile.

External dependencies

After writing the first couple design documents at Whisker Labs, I’ve noticed a key difference between what I’m writing now and those I wrote at Twitter. Critically, the former tend to rely on the availability of services maintained by unfamilar people at other companies rather than acquaintances down the hall. For instance, by making use of Amazon Web Services instead of technologies stewarded in-house, our services’ uptime is reliant on the diligence of anonymous Amazon personnel. As the swashbuckling systems cliché goes, you own your availability, but you aren’t in control of all of the factors from which it derives.

Strategies exist for managing the impact of intermittent outages of third-party services. RPC interactions can be augmented with features like retries and failure accrual, or can simply return partial results as a means to limit the damage caused by temporary downtime. But at a higher level, years of experience with as-a-service offerings have shown that there is typically a threshold scale beyond which any given hosted service ceases to be economical. What we’ve observed is that almost all companies who bootstrap their software atop whatever-as-a-service solutions eventually move away from them on account of cost, reliability, and/or functionality. In the long term, everybody ends up running their own Graphite and Kafka clusters and the luckiest of us get our own datacenters.

Not to mention the trend of services simply disappearing out from under you, on account of the originating company being acquired or otherwise going out of business.

But for a scrappy, bandwidth-constrained startup team, paying someone to do the heavy lifting of distributed systems operation is a no-brainer. So what does a responsible software engineer do in such cases when business and productivity concerns demand the usage of hosted services regardless of their long-term feasibility?

Precautionary migration planning

The easy (and industry-standard) answer is to throw up your hands and say “we’ll cross that bridge when we get there.” The pricing and long-term viability of external services is entirely out of your control, so why worry about hypothetical futures that you can’t influence? People still live in Seattle and Portland even though the mega-quake is coming, right?

This is a fine answer if you’ve made the conscious decision that your #1 priority as an engineering organization is speed of execution. Depending on your product or service’s reliability requirements, the pace of your market, and your bottom line, it very well may be preferable to put your time to more immediately productive use than planning for eventualities.

On the other hand, deciding which failure modes are worth planning for is part of what makes engineering interesting. The best you can do to minimize the risk imposed by external dependencies is to come up with a feasible (but brief) plan for migrating away from them. Consider it a precautionary principle for SaaS.

This is why I’m starting to bake such a section into the design docs that I’m writing. They follow the same principles of situational awareness and premeditated action that motivates having runbooks for services, but are more akin to a heart transplant than a simple runbook item. The sections will:

  • List the system’s external dependencies whose long-term feasibility is deemed at risk (i.e. “<PaaS> will be too expensive by the time we hit <milestone>”)
  • List potential replacements for the risky dependency and give a high-level plan for migrating

The result of this exercise is a better understanding of a system’s risk profile and the paths by which the system is likely to evolve over time.

Countering the logical conclusion

In response to my initial thoughts on this strategy on Twitter, an esteemed former colleague pointed out its logical conclusion, in which the list of “hosted services” is exhaustive. In literal terms, a program’s “external dependencies” include the operating system and proprietary hardware on which it runs, all the way down to the utility company that supplies the energy powering the computer. In this light, precautionary migration planning is absurd, given that the engineering effort involved in reinventing every wheel between your program and electrons in circuits is well beyond most companies’ capabilities.

However, I don’t think that this argument refutes the usefulness of such planning. When done pragmatically, focusing on a reasonable subset of a system’s dependencies, a team gains the ability to act quickly when migrations are deemed necessary.

One way to differentiate external dependencies is by whether or not they are truly fundamental to a service’s operation. If the power goes out, a program (or at least a stricken instance thereof) is unrecoverable regardless of any migration plan. Thus such planning is only relevant for partial failure modes, such as the loss of a hosted database or the end-of-LTS for a specific operating system version.


Even small ships carry maps. I’ve made a case for the use of design documents at startups, but a key takeaway is that their use varies from organization to organization. For some businesses, time spent planning for hypothetical futures is not time well spent. For others, it’s a valuable hedge against undesirable outcomes.

Experience has shown that once an engineering organization reachs a certain size, a reasonably-rigorous design process is well worth having in place. A startup team’s habits tend to ossify into company culture, which is motivation to start thinking about a team’s design process early. Even if you decide against design documentation in the early stage of your company, going through the mental exercise of considering its implications will increase your team’s operational awareness.

Thanks to Marcel Molina and Gary Tsang for reading and providing feedback on drafts of this essay.