[ovirt-devel] Making event types backwards compatible?

[Juan Hernández]

On 05/30/2017 07:20 AM, Michal Skrivanek wrote:
>> On 29 May 2017, at 11:44, Juan Hernández <jhernand at redhat.com> wrote:
>>
>>> On 05/29/2017 11:27 AM, Michal Skrivanek wrote:
>>>
>>>> On 29 May 2017, at 10:39, Juan Hernández <jhernand at redhat.com> wrote:
>>>>
>>>> Hello,
>>>>
>>>> It has been recently requested that the API provides event types:
>>>>
>>>> [RFE] Expose event types to API
>>>> https://bugzilla.redhat.com/1453170
>>>>
>>>> Currently the API provides the event code and description, for example:
>>>>
>>>> <event href="/ovirt-engine/api/events/8021" id="8021">
>>>>   <code>19</code>
>>>>   <description>Host myhost failed to recover.</description
>>>>   ...
>>>> </event>
>>>>
>>>> There is no documentation of what is the meaning of codes, except the
>>>> source code of the engine itself. This forces some applications to add
>>>> their own code to name mapping. For example, the 'ovirt' Ruby gem used
>>>> by older versions of ManageIQ to interact with oVirt contains the following:
>>>>
>>>> https://github.com/ManageIQ/ovirt/blob/v0.17.0/lib/ovirt/event.rb#L25-L485
>>>>
>>>> We could avoid this by adding to the API a new event attribute that
>>>> indicates the type:
>>>>
>>>> <event href="/ovirt-engine/api/events/8021" id="8021">
>>>>   <code>19</code>
>>>>   <type>host_recover_failure</type>
>>>>   <description>Host myhost failed to recover.</description>
>>>>   ...
>>>> </event>
>>>>
>>>> Ideally this should be defined as an enum, so that it will be
>>>> represented as an enum in the SDKs. Alternatively it could just be an
>>>> string, and we could reuse the 'name' attribute:
>>>>
>>>> <event href="/ovirt-engine/api/events/8021" id="8021">
>>>>   <code>19</code>
>>>>   <name>host_recover_failure</name>
>>>>   <description>Host myhost failed to recover.</description>
>>>>   ...
>>>> </event>
>>>>
>>>> However, the key point to making this useful would be to keep the types
>>>> (or names) backwards compatible, so that users of the API can rely on
>>>> their values and meanings.
>>>>
>>>> So this is my question to you: can we commit to keep the names and
>>>> meanings of the backend event types backwards compatible?
>>>
>>> Do we even have to make it bw compatible?
>>> I guess it depends on the actual usage of those names…
>>> The ovirt ruby gem itself doesn’t do much with it
>>
>> We need to make keep it backwards compatible or else tell users "don't
>> rely on these values, as they may change without notice".
>>
>> The 'ovirt' gem doesn't do anything special, it just creates its own
>> code to name mapping. But the users of the 'ovirt' gem (the ManageIQ
>> oVirt provider) do rely on the name. For example:
>>
>>
>> https://github.com/ManageIQ/manageiq-providers-ovirt/blob/master/app/models/manageiq/providers/redhat/infra_manager/event_parser.rb#L80-L92
>>
>> That means that if we ever change the meaning of a code the ManageIQ
>> provider, for example, will break.
> 
> Right,then it indeed needs to stay stable.
> But how is maintaining the enum string different from the code? It is
> the same information, so if MIQ doesn't use the name directly then it
> doesn't really matter if it's a code or string.

It is true that we could commit to keep the meanings of numeric codes
backwards compatible, and document them somehow. If we commit to do that
then I'd say the issue is mostly solved.

However, if we use an enum we have the additional advantage it is more
readable (in the SDKs in particular) and that the Java compiler would
help us to detect potential backwards compatibility breaking changes. In
addition changes to the enum would need to be done and reviewed in the
specification of the API, which would give us (us == the API
maintainers) the opportunity to review/discuss/document/decide about
them explicitly.

> Perhaps deprecate the code and keep the name fixed?

Yes, that is the idea, if we commit to keep them stable, see the bug.