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https://issues.apache.org/jira/browse/KAFKA-7481?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16670137#comment-16670137
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Ismael Juma commented on KAFKA-7481:
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Yeah, in my original comment, I was not counting the "binary upgraded" state. 
My claim is that having 4 states: "binary upgraded", "binary upgraded, new 
protocol version, old persistent metadata, old message format", "binary 
upgraded, new protocol version, new persistent metadata, old message format", 
"new protocol, new persistent metadata, new message format" is not useful 
enough.

Another example, in 2.0.0 we only allow prefixed ACLs if 
inter.broker.protocol.version is 2.0. Downgrading in this case is quite 
dangerous since the older broker won't know what to do with prefixed ACLs.

> Consider options for safer upgrade of offset commit value schema
> ----------------------------------------------------------------
>
>                 Key: KAFKA-7481
>                 URL: https://issues.apache.org/jira/browse/KAFKA-7481
>             Project: Kafka
>          Issue Type: Bug
>            Reporter: Jason Gustafson
>            Priority: Blocker
>             Fix For: 2.1.0
>
>
> KIP-211 and KIP-320 add new versions of the offset commit value schema. The 
> use of the new schema version is controlled by the 
> `inter.broker.protocol.version` configuration.  Once the new inter-broker 
> version is in use, it is not possible to downgrade since the older brokers 
> will not be able to parse the new schema. 
> The options at the moment are the following:
> 1. Do nothing. Users can try the new version and keep 
> `inter.broker.protocol.version` locked to the old release. Downgrade will 
> still be possible, but users will not be able to test new capabilities which 
> depend on inter-broker protocol changes.
> 2. Instead of using `inter.broker.protocol.version`, we could use 
> `message.format.version`. This would basically extend the use of this config 
> to apply to all persistent formats. The advantage is that it allows users to 
> upgrade the broker and begin using the new inter-broker protocol while still 
> allowing downgrade. But features which depend on the persistent format could 
> not be tested.
> Any other options?



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