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"Buy Me A Coffee"


Diffy is used in production at:

and blogged about by cloud infrastructure providers like:

Visit our engineering blog to keep up with the latest developments in Diffy.

Diffy is being actively developed and maintained by the engineering team at Sn126.

Feel free to contact us via discord, linkedin, or twitter.

What is Diffy?

Diffy finds potential bugs in your service using running instances of your new code and your old code side by side. Diffy behaves as a proxy and multicasts whatever requests it receives to each of the running instances. It then compares the responses, and reports any regressions that may surface from those comparisons. The premise for Diffy is that if two implementations of the service return “similar” responses for a sufficiently large and diverse set of requests, then the two implementations can be treated as equivalent and the newer implementation is regression-free.

How does Diffy work?

Diffy acts as a proxy that accepts requests drawn from any source that you provide and multicasts each of those requests to three different service instances:

  1. A candidate instance running your new code
  2. A primary instance running your last known-good code
  3. A secondary instance running the same known-good code as the primary instance

As Diffy receives a request, it is multicast and sent to your candidate, primary, and secondary instances. When those services send responses back, Diffy compares those responses and looks for two things:

  1. Raw differences observed between the candidate and primary instances.
  2. Non-deterministic noise observed between the primary and secondary instances. Since both of these instances are running known-good code, you should expect responses to be in agreement. If not, your service may have non-deterministic behavior, which is to be expected. Diffy Topology

Diffy measures how often primary and secondary disagree with each other vs. how often primary and candidate disagree with each other. If these measurements are roughly the same, then Diffy determines that there is nothing wrong and that the error can be ignored.


Detailed Diffy Documenation is available here.


Please reach out to for support. We look forward to hearing from you.

Code of Conduct

  1. Bug reports are welcome even if submitted anonymously via fresh github accounts.
  2. Anonymous feature and support requests will be ignored.


Copyright (C) 2019 Sn126, Inc.

This license allows reusers to copy and distribute the material in 
any medium or format in unadapted form only, for noncommercial purposes 
only, and only so long as attribution is given to the creator. 

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International Public License
for more details.

You should have received a copy of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0
International Public License along with this program. If not, see