Re: [dpdk-dev] [PATCH v8 2/4] doc: changes to abi policy introducing major abi versions

[Ray Kinsella <mdr@ashroe.eu>]

On 06/11/2019 09:06, Thomas Monjalon wrote:
> 06/11/2019 09:49, Ray Kinsella:
>> On 06/11/2019 00:11, Thomas Monjalon wrote:
>>> 05/11/2019 16:24, Ray Kinsella:
>>>> +#. Major ABI versions are declared every **year** and are then supported for one
>>>> +   year, typically aligned with the :ref:`LTS release <stable_lts_releases>`.
>>>
>>> As discussed earlier, a major ABI version can be declared less often
>>> than one year in the future.
>>> An ABI is supported more than one year, because of the LTS branches.
>>> That's why I propose to replace with this sentence:
>>> "
>>> Major ABI versions are declared regularly and are then supported for
>>> at least one year, typically aligned with the :ref:`LTS release <stable_lts_releases>`.
>>> "
>>
>> So look, this one was a decision of the technical board.
> 
> The techboard didn't decide to change the ABI every year.
> We decided to review the duration after the first year, with a plan to extend.
> 
>> My position is still what was agreed was, "declared every year, and supported for one year".
>> I like it, it's crystal clear what is the policy, until we change the policy.
> 
> I think it gives a wrong message.
> 
>> That said, I can make the change no problem, but I need some others to chime in to ok it. 
>> Perhaps at the head of the Techboard today?
> 
> Yes I add it to the techboard meeting.
> 
>>>> +#. The ABI version is managed at a project level in DPDK, with the ABI version
>>>> +   reflected in all library's soname.
>>>
>>> Yes, even the experimental libraries should have the same version,
>>> with the minor number incremented at each release.
>>> (just a comment to change a policy at the end of this patch)
>>
>> It's described elsewhere in the document, experimental libraries have a major 
>> ABI version of 0, to indicate they exist outside of ABI management.
>> Minor number then changes as the experimental library changes as before.
> 
> Yes, but you cannot say "reflected in all library's soname".

ACK - I will clarify

> 
>>>> +   In 2019, the DPDK community stated its intention to move to ABI stable
>>>> +   releases, over a number of release cycles. This change begins with
>>>> +   maintaining ABI stability through one year of DPDK releases starting from
>>>> +   DPDK 19.11. This policy will be reviewed in 2020, with intention of
>>>> +   lengthening the stability period.
>>>
>>> Great, the schedule is clear here.
>>>
>>>> +A major ABI version is declared every year, aligned with that year's LTS
>>>> +release, e.g. v19.11. This ABI version is then supported for one year by all
>>>> +subsequent releases within that time period, until the next LTS release, e.g.
>>>> +v20.11.
>>>
>>> Let's reword like this:
>>> "
>>> A new major ABI version can be declared when a new LTS branch is started,
>>> e.g. ABI 19 for DPDK 19.11.0.
>>> This major ABI version is then supported until the next one,
>>> e.g. ABI 20 for DPDK 20.11.0.
>>> All ABI changes must be detailed in the release notes.
>>> "
>>
>> This is more ambiguous, although what I said above stands.
> 
> What you said is wrong because of 2 reasons:
> - it is not always one year for an major ABI

Well that is a point of disagreement.

> - it is always longer because of LTS branch

Well I was pretty careful to qualify the ABI policy applies to releases over the year.
To distinguish it from LTS branch. 

> 
>> If there is general agreement with changing this part of the policy, I am ok to make 
>> the change.
> 
> Yes let's review with the techboard.
> 
>>>> +   ABI breakages due to changes such as reorganizing public
>>>> +   structure fields for aesthetic or readability purposes should be avoided.
>>>
>>> Why it should be avoided?
>>> If the ABI is broken anyway, I don't see any reason to not break it more.
>>
>> This is text from the original ABI Policy - I think the general sentiment still stands.
>> Just because you have an ABI Breakage window doesn't mean you should feel free to break
>> the ABI. The 3 ACKs required from Technical Board member to change the ABI, are another
>> reflection of this. 
>>
>> As a general rule.
>> Unnecessary changes should still be avoided, to reduce ABI churn between ABI versions.
> 
> I agree we must avoid unnecessary API changes because it requires apps to adapt.
> But if the change is only ABI, and we are in an ABI-change window,
> I don't see any issue> 
>>>> +#. ABI breaking changes (including an alternative map file) can be included with
>>>> +   deprecation notice, in wrapped way by the ``RTE_NEXT_ABI`` option, to provide
>>>> +   more details about oncoming changes. ``RTE_NEXT_ABI`` wrapper will be removed
>>>> +   at the declaration of the next major ABI version.
>>>
>>> I missed that in discussions.
>>> I thought we wanted to wait for the next major ABI.
>>> If we allow NEXT_ABI ifdef, we will have 2 DPDK versions
>>> (stable and next ABI) to test.
>>
>> This is text from the original ABI Policy - the purpose remains the same.
>> If you add an ABI breaking change in say 20.02, that clearly can't see the light of day until 20.11
>>
>> You may still opt to prepare the community for the change, by putting your code out wrapped 
>> in a NEXT_ABI and flagging it. You then get the option for people, so inclined, to build and try your code.
>>
>> I can't see it being used often, but it is another tool in the box of managing ABI change.
> 
> OK, let's keep this tool.
> 
>>>> +Libraries marked as ``experimental`` are entirely not considered part of an ABI
>>>> +version, and may change without warning at any time. Experimental libraries
>>>> +always have a major version of ``0`` to indicate they exist outside of
>>>> +ABI Versioning, with the minor version incremented with each ABI change
>>>> +to library.
>>>
>>> It means not all libraries will have the same ABI version.
>>> It is contrary of "ABI version is managed at a project level",
>>> and I don't see a real benefit of a different version number.
>>
>> There is a benefit, major version 0 is a very clear indication that 
>> the library exists outside of ABI management. 
>> A library isn't in the ABI, until it is in the ABI - an then it gets
>> added to the major version number. 
>>
>>> Anyway, some experimental functions can live inside a library
>>> with a stable ABI version number
>>
>> True, but if an entire library is experimental - let's be crystal 
>> clear about that. 
> 
> I would like to see what others think.
> 
>