Performance as Acceptance Criteria

Posted on September 19, 2019

Every software engineer has heard the most sensational part of Knuth’s famous quote about optimizations, but sometimes we do need to take a step back and look at the full context:

Programmers waste enormous amounts of time thinking about, or worrying about, the speed of noncritical parts of their programs, and these attempts at efficiency actually have a strong negative impact when debugging and maintenance are considered. We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil. Yet we should not pass up our opportunities in that critical 3%.

Time and again I’ve seen the most sensationalist part taken too seriously by engineers, myself included at times, who then simply ignore performance of systems they design and build. To be clear, the various microoptimizations we make while coding typically do not matter. What we should consider as engineers is the black box performance of our individual components and the distributed systems we build with these components.

Performance is More than Speed

Before getting started, performance needs clarification. Many engineers consider performance to simply be the speed of their code. That’s only one measurable component that could specify performance. Performance could also be the memory footprint of an application, like when it needs to run on client devices. It could also be the accuracy of user-facing results. Consider performance as a stand in for specific, definable metrics.

Component Level Performance

A component here is a single service or artifact that is somewhat standalone. These are applications or services that today probably already have integration tests running against them on every build, or is possibly a core widget embedded across multiple client facing domains that should feel snappy to the user.

Components may or may not have real time requirements to operate successfully, but most do have some unwritten performance characteristics that must be met. This could be that the page load time must be under 3 seconds, or that an ETL pipeline must support enough throughput to not cause a request backlog, or that some data must be deleted within a certain number of days to remain compliant. These are all very real performance concerns that typically are understood, but not always spelled out, and cause very real problems for an organization when performance degrades.

Performance is a Requirement

When designing new components, ask your stakeholders and designers about performance expectations. Shoot for as specific an answer as you can get - whatever response you receive will need to be made into a specific measurable metric. Whatever metric(s) you and your stakeholder agree on should be included in the acceptance criteria of the overall deliverable. This allows engineers to remain cognizant of performance goals and ensures that the client has reasonable expectations.

Performance tests should be written into the component just like unit and integration tests typically are today. gatling and jmeter are both great tools for measuring web application performance, and are typically easy to include in builds. There are various other tools for measuring client side performance or messaging throughput. Codify your performance requirements and use whatever tooling fits your needs to ensure that you don’t have performance degradation - there’s nothing worse than finding out that what looked great in dev simply won’t work at all with production workloads. Be as realistic as possible when writing performance tests and fail your build when you don’t hit your criteria.

System Level Components

Large scale systems are even more likely to have unwritten performance requirements. Multiple asynchronous events may need to complete within a deadline, or a system’s accuracy may be beholden to scheduled batch jobs. Performance characterics of these large scales systems are often even more important to pin down and codify with stake holders since it will likely specify what architectures are suitable for the design. Architecture overhauls are expensive, time consuming, and a drain on the team, but are not uncommon when the delivered product is simply unable to meet unwritten performance demands.

At this level of performance analysis consider the entire system as a black box. Identify the inputs and outputs of the system then write additional performance tests for this holistic view (for that matter, consider writing integration tests from the same view). It’s insufficient to simply sum up component level performance tests to determine if a system is performant enough - infrastructure will add measurable overhead and it’s easy to not sum component metrics correctly in the face of a complex system. The better solution is to run these tests whenever an invidual component is redeployed to your dev/test/stage environment and catch issues early.


  • Get performance requirements while collecting other project requirements
  • Include performance tests in your artifacts (and fail the build with them)
  • Write performance tests for your systems (and raise an alarm when these fail).