When the user for inter broker communication is not a super user and ACLs are configured with allow.everyone.if.no.acl.found=false, the cluster will not serve data. This is extremely confusing to debug because there is no security negotiation problem or indication of an error other than no data can make it in or out of the broker. If one knew to look in the authorizer log, it would be more clear, but that didn't make it into my workflow at least. Here's an example of a problematic debugging scenario
SASL_SSL, SSL, SASL_PLAINTEXT ports on the brokers
SASL user specified in `super.users`
SSL specified as the inter broker protocol
The only way I could figure out ACLs were an issue without gleaning it through configuration inspection was that controlled shutdown indicated that a cluster action had failed.
It would be good if we could be more transparent about the failure here.