Font Awesome Free 5.13.0 by @fontawesome - License - (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License)
Skip to main content

Testing after production

Why testing in production (rather than before deploying) could make sense in some cases

Basic idea

  • Tests will never be perfect, we can't catch everything! Impossible to reduce chance of failure to zero
  • It might be impractical and not worth the effort to test certain things before putting them in production

Mean time between failures versus mean time to repair

  • Mean time between failures (MTBF): indication of how often issues make it to production
  • Mean time to repair (MTTR): indication of how long it takes you to detect and fix such issues

Tradeoff MTBF versus MTTR:

  • Sometimes, it's more efficient to spend effort on getting better at detecting and fixing issues in production than on adding more automated tests
  • Best tradeoff depends on your organization
  • Do not completely abandon one in favor of the other
    • It's probably not a good idea to just throw stuff into production without any level of testing
    • Even with great tests, you need to be prepared for a bug popping up in production

Separating deployment from release

Basic idea: after deploying something, don't immediately direct full production load to it

Useful techniques:

  • Smoke tests:
    • Tests designed to check that deployment was successful and software runs properly in current environment
    • Should ideally be run automatically on deploy
  • Blue/green deployment:
    • Run old and new next to each other
    • New can get smoke tested while old still handles production load, then we can switch
    • After switching to new, we can quickly switch back if necessary
  • Canary releasing:
    • Keep old and new next to each other for longer time
    • Only direct a fraction of production load to new, increase as confidence increases
  • Branch By Abstraction and application strangulation (see Branch By Abstraction and application strangulation):
    • Techniques to gradually migrate to new code or even new system
    • Possible to direct production traffic to existing code/system but also send a copy of it to new code/system to check for differences in behavior

Monitoring and logging

  • Monitor CPU, memory, ...
  • Monitor application itself: response time, number of errors returned to client, number of submitted forms, ...
  • Collect logs about what the system is doing
    • It's especially important to log any errors that happen
  • Set up dashboards so people can quickly get an idea of the system's state
  • Set up alerts based on resource use, response time, error rates, 500 responses, ...
    • Alert early enough so team can act before things really get bad
    • Watch your signal-to-noise ratio, so people don't start ignoring alerts

Synthetic monitoring

(also called semantic monitoring)

Basic idea: monitor health of system as whole by running end-to-end scenarios against it


  • Define important scenarios or user journeys
  • Write tests for them (often make sense to start from end-to-end tests)
  • Periodically run these against production
    • Depending on importance of scenario or journey, failure can trigger on-call alert


  • Often way better at finding out if something's wrong than lower-level metrics
    • Still, you'll likely need lower-level metrics to help you find the exact location of the issue
  • Make sure it doesn't affect actual production customers!