[GitHub] [hbase] gjacoby126 commented on a change in pull request #390: HBASE-22623 - Add RegionObserver coprocessor hook for preWALAppend

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gjacoby126 commented on a change in pull request #390: HBASE-22623 - Add RegionObserver coprocessor hook for preWALAppend
URL: https://github.com/apache/hbase/pull/390#discussion_r305577407
 
 

 ##########
 File path: hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/RegionObserver.java
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 @@ -1104,4 +1105,18 @@ default DeleteTracker postInstantiateDeleteTracker(
       throws IOException {
     return delTracker;
   }
+
+  /**
+   * Called just before the WAL Entry is appended to the WAL. Implementing this hook allows
+   * coprocessors to add extended attributes to the WALKey that then get persisted to the
+   * WAL, and are available to replication endpoints to use in processing WAL Entries.
+   * @param ctx
+   * @param key
+   * @return
+   * @throws IOException
+   */
+  default WALKeyImpl preWALAppend(ObserverContext<RegionCoprocessorEnvironment> ctx, WALKeyImpl key)
 
 Review comment:
   @apurtell - the header comments for WALEdit state that the class is meant to be read-only for coprocs. There are coprocessor hooks that take a WALEdit parameter, but unless I missed one, they all seem to be _after_ the WAL has been appended so that mutating the edit has no function. 
   
   I don't understand the reason for the restriction -- we trust coprocessor implementers to do sane things already everywhere else -- but since the code had an explicit policy I tried to honor it. 

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