[HADOOP-12614] Add a generic .isOffline() method to filesystems to probe availability

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Won't Fix
• Affects Version/s: 2.7.1
• Component/s: fs
• Assignee: Unassigned
• Reporter: Steve Loughran
• Created: 04/Dec/15 15:33
• Updated: 21/Jan/21 13:43
• Resolved: 21/Jan/21 13:43

Description

Looking at some of the spark `HistoryServer` code, they do reflection games to check whether HDFS is in safe mode or not, games which vary with version and could be at risk of failing with the client/server split (fortunately, it's all client-side). Nor do the checks apply to other filesystems, which could have their own online/offline state.

I propose adding the new methods FileSystem.isOffline(), FileContext.isOffline(), to return true if an FS knows that it is offline. For HDFS: Safe mode. For other filesystems? Maybe network state, disk being r/W, etc. Their choice. The default would be false: an FS is not offline,

obvously, !isOffline() doesn't guarantee the FS is fully functional; that's why I propose isOffline(); less dangerous than the opposite values of isLive() or isAvailable(), which may be making promises which cannot hold