Cannot specify JoinGroupRequest protocol priority

[DanielMorsing]

Versions

Present on every version that supports JoinGroupRequest

Problem Description

The kafka protocol for JoinGroupRequest allows you to specify priority on group protocols for seamless rollout of new protocols while a consumer group is running. This is done by changing the ordering on the wire for the GroupProtocols. See here for more details

Sarama exposes this field as a map, making it impossible to specify an ordering.

[eapache]

Note that the join group request allows for multiple protocol/metadata pairs. This enables rolling upgrades without downtime. The coordinator chooses a single protocol which all members support.

This is what is in the wiki; I don't read that as implying that order is important. The coordinator should only choose a protocol once all of the group members support it, regardless of order?

[DanielMorsing]

Later in the wiki:

Once all members have upgraded, the coordinator will choose whichever protocol is listed first in the GroupProtocols array.

If you're switching from "protov1" to "protov2", you want to deploy your binaries such that they can handle both in the switchover, but once every consumer has been updated, you want to ensure that "protov2" is chosen. The protocol ordering allows for doing this.

[eapache]

Oh yes you're right I'm sorry I must have missed that the first read through.

Fixing this without breaking API compatibility will be a bit awkward but should be possible.