szetszwo commented on code in PR #1362:
URL: https://github.com/apache/ratis/pull/1362#discussion_r2870030074
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ratis-server/src/test/java/org/apache/ratis/LinearizableReadTests.java:
##########
@@ -169,8 +172,17 @@ static <C extends MiniRaftCluster> void
runTestFollowerReadOnlyParallel(C cluste
writeReplies.add(new Reply(count,
leaderClient.async().send(WAIT_AND_INCREMENT)));
Thread.sleep(100);
- assertReplyExact(count, f0Client.io().sendReadOnly(QUERY, f0));
- f1Replies.add(new Reply(count, f1Client.async().sendReadOnly(QUERY,
f1)));
+ if (readIndexType == Type.REPLIED_INDEX) {
+ // With REPLIED_INDEX the read index only advances after the leader
has applied the
+ // transaction and the reply batch is flushed. WAIT_AND_INCREMENT
takes 500 ms in
+ // the state machine but we only waited 100 ms, so its reply has not
been generated
+ // yet and the follower read may only see the preceding sync
INCREMENT (count - 1).
+ assertReplyAtLeast(count - 1, f0Client.io().sendReadOnly(QUERY, f0));
+ f1Replies.add(new Reply(count - 1,
f1Client.async().sendReadOnly(QUERY, f1)));
+ } else {
+ assertReplyExact(count, f0Client.io().sendReadOnly(QUERY, f0));
+ f1Replies.add(new Reply(count, f1Client.async().sendReadOnly(QUERY,
f1)));
+ }
Review Comment:
Sorry that the repliedIndex idea may not work:
We have the invariant
- repliedIndex <= appliedIndex <= committedIndex
The repliedIndex idea is: When repliedIndex < appliedIndex, it is safe for a
follower to return any states >= repliedIndex since appliedIndex is not yet
visible to any clients -- The data is not yet stale in this sense.
However, after a follower/leader has returned a read call, its appliedIndex
A becomes visible to that client. The subsequence reads must return a state >=
A. Below is an example of the problem:
1. Leader: repliedIndex = 10 < appliedIndex = 20
2. Follower 1: appliedIndex = 18
3. Follower 2: appliedIndex = 14
4. Client first reads from Follower 1.
5. The same client reads from Follower 2 (or the Leader) <---- stale read.
Let's see if there are some ways to make it work.
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