See HDFS-13150 for full design.
This JIRA is to add the in-memory cache of recent edit transactions on the JournalNode. This JIRA does not include accesses to this cache.
[Edit Tail Fast Path] Allow SbNN to tail in-progress edits from JN via RPC
[Edit Tail Fast Path Pt 2] Add ability for JournalNode to serve edits via RPC