[GitHub] [ratis] szetszwo commented on a diff in pull request #688: RATIS-1626. Preserve recent logs when purge logs

[GitBox <gi...@apache.org>]

szetszwo commented on code in PR #688:
URL: https://github.com/apache/ratis/pull/688#discussion_r926948970


##########
ratis-server-api/src/main/java/org/apache/ratis/server/RaftServerConfigKeys.java:
##########
@@ -285,6 +285,15 @@ static void setPurgeUptoSnapshotIndex(RaftProperties properties, boolean shouldP
       setBoolean(properties::setBoolean, PURGE_UPTO_SNAPSHOT_INDEX_KEY, shouldPurgeUptoSnapshotIndex);
     }
 
+    String PURGE_PRESERVE_LOG_NUM_KEY = PREFIX + "purge.preserve.log.num";

Review Comment:
   "preserve" is a good word choice.  Let's call it ".purge. preservation" so that it is similar to ".purge.gap".
   
   BTW, it needs a "." in the beginning.  



##########
ratis-server/src/main/java/org/apache/ratis/server/impl/StateMachineUpdater.java:
##########
@@ -297,7 +299,8 @@ private void takeSnapshot() {
             CommitInfoProto::getCommitIndex);
         purgeIndex = LongStream.concat(LongStream.of(i), commitIndexStream).min().orElse(i);
       }
-      raftLog.purge(purgeIndex);
+      final long purgeIndexWithPreserve = Math.max(0L, purgeIndex - purgePreserveLogNum);
+      raftLog.purge(purgeIndexWithPreserve);

Review Comment:
   Let's change it in RaftLogBase.purge(..) so that the new code will be next to the purgeGap code.  We may add a `purgePreservation` field and use it as below
   ```
   +++ b/ratis-server/src/main/java/org/apache/ratis/server/raftlog/RaftLogBase.java
   @@ -301,6 +301,10 @@ public abstract class RaftLogBase implements RaftLog {
    
      @Override
      public final CompletableFuture<Long> purge(long suggestedIndex) {
   +    if (purgePreservation > 0) {
   +      final long currentIndex = getNextIndex() - 1;
   +      suggestedIndex = Math.min(suggestedIndex, currentIndex - purgePreservation);
   +    }
        final long lastPurge = purgeIndex.get();
        if (suggestedIndex - lastPurge < purgeGap) {
          return CompletableFuture.completedFuture(lastPurge);
   ```



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