Document what constitutes a breaking change

[rigelrozanski]

Maybe to be in the contributing guidelines? or at least linked from the contributing guidelines, and add somewhere else

"I’ve also been thinking about this a bunch.. not sure if a change would cause one to run transactions differently it would also be breaking? couldn’t replay blocks the same way/ in a bad scenario would prevent consensus. Let’s explicitly document what a breaking change is"

"is our definition of "non-breaking" only related to state modifications?"

CC @cwgoes @ebuchman

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[ValarDragon]

I think anything that would cause a tx, or abci header, to modify state differently is a breaking change, in addition to just plain changing the structure of stores.

[rigelrozanski]

Interesting I feel like there are different levels of how breaking a change can be - like some changes might be breaking in that they introduce a new transaction type however it doesn't interfere with how historical transactions have been run and therefor it wouldn't break a testnet if you were to just start running software with this new change (you might just halt consensus if 50% of folks upgraded)

[cwgoes]

I think "inclusive state machine breaking" is a reasonable definition for now - a non-breaking change must successfully process all transactions that would have been successfully processed before the change. It can additionally successfully process transactions that would not have been successfully processed before.

[alexanderbez]

The definition @cwgoes pointed out makes intuitive sense. I think it'll still be beneficial if we could also point out/tag non-breaking changes that do break client facing APIs.

Also, since discussion has started on this, I'll move it to the "In Progress" lane.

[cwgoes]

This should be defined in #790.

[tac0turtle]

@alessio do you want to include this in your PR about releases? it goes hand in hand

[alessio]

Oh wow, is it #throwbackFriday today 😄? Thanks @marbar3778, I'll incorporate this in my PR