+ private boolean isOutOfSync(final AppendEntries appendEntries) {
+
+ final long lastIndex = lastIndex();
+ if (lastIndex == -1 && appendEntries.getPrevLogIndex() != -1) {
+
+ // The follower's log is out of sync because the leader does have an entry at prevLogIndex and this
+ // follower has no entries in it's log.
+
+ log.info("{}: The followers log is empty and the senders prevLogIndex is {}", logName(),
+ appendEntries.getPrevLogIndex());
+ return true;
+ }
+
+ if (lastIndex > -1) {
+ if (isLogEntryPresent(appendEntries.getPrevLogIndex())) {
+ final long prevLogTerm = getLogEntryTerm(appendEntries.getPrevLogIndex());
+ if (prevLogTerm != appendEntries.getPrevLogTerm()) {
+
+ // The follower's log is out of sync because the Leader's prevLogIndex entry does exist
+ // in the follower's log but it has a different term in it
+
+ log.info("{}: The prevLogIndex {} was found in the log but the term {} is not equal to the append "
+ + "entries prevLogTerm {} - lastIndex: {}, snapshotIndex: {}", logName(),
+ appendEntries.getPrevLogIndex(), prevLogTerm, appendEntries.getPrevLogTerm(), lastIndex,
+ context.getReplicatedLog().getSnapshotIndex());
+ return true;
+ }
+ } else if (appendEntries.getPrevLogIndex() != -1) {
+
+ // The follower's log is out of sync because the Leader's prevLogIndex entry was not found in it's log
+
+ log.info("{}: The log is not empty but the prevLogIndex {} was not found in it - lastIndex: {}, "
+ + "snapshotIndex: {}", logName(), appendEntries.getPrevLogIndex(), lastIndex,
+ context.getReplicatedLog().getSnapshotIndex());
+ return true;
+ }
+ }
+
+ if (appendEntries.getPrevLogIndex() == -1 && appendEntries.getPrevLogTerm() == -1
+ && appendEntries.getReplicatedToAllIndex() != -1) {
+ if (!isLogEntryPresent(appendEntries.getReplicatedToAllIndex())) {
+ // This append entry comes from a leader who has it's log aggressively trimmed and so does not have
+ // the previous entry in it's in-memory journal
+
+ log.info("{}: Cannot append entries because the replicatedToAllIndex {} does not appear to be in the "
+ + "in-memory journal", logName(), appendEntries.getReplicatedToAllIndex());
+ return true;
+ }
+
+ final List<ReplicatedLogEntry> entries = appendEntries.getEntries();
+ if (entries.size() > 0 && !isLogEntryPresent(entries.get(0).getIndex() - 1)) {
+ log.info("{}: Cannot append entries because the calculated previousIndex {} was not found in the "
+ + "in-memory journal", logName(), entries.get(0).getIndex() - 1);
+ return true;
+ }
+ }
+
+ return false;