For a cluster of substantial size, INodeAttributeProvider initialization. may take a long time and it blocks NameNode startup. We saw a large cluster where part of file system ACLs is managed by Apache Sentry, and NameNode took a few minutes for the Sentry HDFS NameNode Plugin to initialize. I suppose the same issue can arise for Apache Ranger and other INodeAttributeProviders implementation.
It would be nice to add an extra row in NameNode Web UI startup progress, in addition to "Loading fsimage", "Loading edits", "Saving checkpoint" and "Safe mode", to give a better visibility what NameNode is doing.
In addition, there might also be a need to add a similar row into Web UI for loading NameNode plugins. So filing this jira to invite more discussion.