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Discussion on: Should behavioural changes be considered breaking changes under SemVer?

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turnerj profile image
James Turner Author

Interesting - yeah, in cases like an API where you can track how people used the API and what the responses were like, it gives you the tools to be confident about how people use it and what it would take to break any rules.

From the perspective of an open source library though, you really don't have either of those as options. Really everything is on the table except maybe using reflection to access internal code (as that hardly seems fair and is against the spirit of consuming the library). Something as seemingly trivial as the error message on an exception object could change the flow of the consumer's code. I don't think that is obviously a good idea but I guess what I am getting at is the grey area of "breaking change".

It might start with something like an exception message to then you find out some code is depending on the actual stack trace of an exception (again, I think this would be a really bad idea BUT it is a valid property on an exception - in C# at least - without needing to use reflection).

Going back to your example of actually monitoring, capturing and replaying activity against an API - that is quite fascinating. I mean, it completely makes sense but at the same time, seems like quite an undertaking. Would love to know more, anything you can share (eg. Is it a custom solution you made? Something off-the-shelf? Do you need to worry about sensitive data? etc) would be great.

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phlash909 profile image
Phil Ashby

Ah the joys of maintaining Open Source libraries :)

In this case, possibly caveat utilitor (user beware) applies, and provided their tests don't fail they can use a new version of the library. They chose to use your library after all, and there is no commercial contract keeping them there or forcing their use of a specific version (unlike many SaaS things with APIs!). Serious users may want to write some test cases for you, so you both know when their contract is broken, they may even like to fix that breakage? This leverages the value of open source to provide visibility and options for all parties.

Regarding the operational sampling / replay thing - we don't do this yet (I did say toying with the idea, not shipping :)), but we've been looking at putting what amounts to a transaction recorder in the sidecars that terminate TLS and manage request routing in our stack. The problems are less technical than legal/privacy for us, being a major processor of sensitive info. We already record API call failures into an incident log for investigation, giving us another option to build requests that exhibit the same failure but with synthetic data, that we can push back up the pipeline to open development areas.

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