aweisberg commented on code in PR #4043:
URL: https://github.com/apache/cassandra/pull/4043#discussion_r2027775778


##########
src/java/org/apache/cassandra/service/paxos/v1/PrepareCallback.java:
##########
@@ -79,8 +86,28 @@ public synchronized void onResponse(Message<PrepareResponse> 
message)
         latch.decrement();
     }
 
-    public Iterable<InetAddressAndPort> replicasMissingMostRecentCommit()
+    public Iterable<InetAddressAndPort> 
replicasMissingMostRecentCommit(TableMetadata metadata)
     {
+        /**
+         * this check is only needed for mixed mode operation with 4.0 and can 
be removed once upgrade support dropped
+         * see the comment in {@link 
org.apache.cassandra.distributed.upgrade.MixedModePaxosTTLTest} for a full 
explanation.
+         */
+        if (DatabaseDescriptor.paxosStatePurging() == 
Config.PaxosStatePurging.legacy)

Review Comment:
   I wonder how you safely transition to a new Paxos state purging 
configuration? It's not atomic to change it across the entire cluster.



##########
src/java/org/apache/cassandra/service/paxos/v1/PrepareCallback.java:
##########
@@ -79,8 +86,28 @@ public synchronized void onResponse(Message<PrepareResponse> 
message)
         latch.decrement();
     }
 
-    public Iterable<InetAddressAndPort> replicasMissingMostRecentCommit()
+    public Iterable<InetAddressAndPort> 
replicasMissingMostRecentCommit(TableMetadata metadata)
     {
+        /**
+         * this check is only needed for mixed mode operation with 4.0 and can 
be removed once upgrade support dropped
+         * see the comment in {@link 
org.apache.cassandra.distributed.upgrade.MixedModePaxosTTLTest} for a full 
explanation.
+         */
+        if (DatabaseDescriptor.paxosStatePurging() == 
Config.PaxosStatePurging.legacy)
+        {
+            // In general, we need every replicas that have answered to the 
prepare (a quorum) to agree on the MRC (see
+            // coment in StorageProxy.beginAndRepairPaxos(), but basically we 
need to make sure at least a quorum of nodes
+            // have learn a commit before commit a new one otherwise that 
previous commit is not guaranteed to have reach a
+            // quorum and further commit may proceed on incomplete 
information).
+            // However, if that commit is too hold, it may have been expired 
from some of the replicas paxos table (we don't

Review Comment:
   s/hold/old, s/replicas/replica's



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