2 files changed, 26 insertions, 23 deletions

diff --git a/src/mongo/db/repl/initial_syncer.cpp b/src/mongo/db/repl/initial_syncer.cpp
index 4b1c882b1eb..1e13256dcaf 100644
--- a/src/mongo/db/repl/initial_syncer.cpp
+++ b/src/mongo/db/repl/initial_syncer.cpp
@@ -1096,25 +1096,22 @@ void InitialSyncer::_rollbackCheckerCheckForRollbackCallback(
 
     // Set UUIDs for all non-replicated collections on secondaries. See comment in
     // ReplicationCoordinatorExternalStateImpl::initializeReplSetStorage() for the explanation of
-    // why we do this and why it is not necessary for sharded clusters.
-    if (serverGlobalParams.clusterRole != ClusterRole::ShardServer) {
-        const NamespaceString nss("admin", "system.version");
-        auto opCtx = makeOpCtx();
-        auto statusWithUUID = _storage->getCollectionUUID(opCtx.get(), nss);
-        if (!statusWithUUID.isOK()) {
-            // If the admin database does not exist, we intentionally fail initial sync. As part of
-            // SERVER-29448, we disallow dropping the admin database, so failing here is fine.
-            onCompletionGuard->setResultAndCancelRemainingWork_inlock(lock,
-                                                                      statusWithUUID.getStatus());
+    // why we do this.
+    const NamespaceString nss("admin", "system.version");
+    auto opCtx = makeOpCtx();
+    auto statusWithUUID = _storage->getCollectionUUID(opCtx.get(), nss);
+    if (!statusWithUUID.isOK()) {
+        // If the admin database does not exist, we intentionally fail initial sync. As part of
+        // SERVER-29448, we disallow dropping the admin database, so failing here is fine.
+        onCompletionGuard->setResultAndCancelRemainingWork_inlock(lock, statusWithUUID.getStatus());
+        return;
+    }
+    if (statusWithUUID.getValue()) {
+        auto schemaStatus = _storage->upgradeUUIDSchemaVersionNonReplicated(opCtx.get());
+        if (!schemaStatus.isOK()) {
+            onCompletionGuard->setResultAndCancelRemainingWork_inlock(lock, schemaStatus);
             return;
         }
-        if (statusWithUUID.getValue()) {
-            auto schemaStatus = _storage->upgradeUUIDSchemaVersionNonReplicated(opCtx.get());
-            if (!schemaStatus.isOK()) {
-                onCompletionGuard->setResultAndCancelRemainingWork_inlock(lock, schemaStatus);
-                return;
-            }
-        }
     }
 
     // Success!
diff --git a/src/mongo/db/repl/replication_coordinator_external_state_impl.cpp b/src/mongo/db/repl/replication_coordinator_external_state_impl.cpp
index fb675304cfe..f2a2ff633ad 100644
--- a/src/mongo/db/repl/replication_coordinator_external_state_impl.cpp
+++ b/src/mongo/db/repl/replication_coordinator_external_state_impl.cpp
@@ -404,12 +404,18 @@ Status ReplicationCoordinatorExternalStateImpl::initializeReplSetStorage(Operati
         // Set UUIDs for all non-replicated collections. This is necessary for independent replica
         // sets and config server replica sets started with no data files because collections in
         // local are created prior to the featureCompatibilityVersion being set to 3.6, so the
-        // collections are not created with UUIDs. This is not an issue for shard servers because
-        // the config server sends a setFeatureCompatibilityVersion command with the
-        // featureCompatibilityVersion equal to the cluster's featureCompatibilityVersion during
-        // addShard, which will add UUIDs to all collections that do not already have them. Here,
-        // we add UUIDs to the non-replicated collections on the primary. We add them on the
-        // secondaries during InitialSync.
+        // collections are not created with UUIDs. We exclude ShardServers when adding UUIDs to
+        // non-replicated collections on the primary because ShardServers are started up by default
+        // with featureCompatibilityVersion 3.4, so we don't want to assign UUIDs to them until the
+        // cluster's featureCompatibilityVersion is explicitly set to 3.6 by the config server. The
+        // below UUID addition for non-replicated collections only occurs on the primary; UUIDs are
+        // added to non-replicated collections on secondaries during InitialSync. When the config
+        // server sets the featureCompatibilityVersion to 3.6, the shard primary will add UUIDs to
+        // all the collections that need them. One special case here is if a shard is already in
+        // featureCompatibilityVersion 3.6 and a new node is started up with --shardsvr and added to
+        // that shard, the new node will still start up with featureCompatibilityVersion 3.4 and
+        // need to have UUIDs added to each collection. These UUIDs are added during InitialSync,
+        // because the new node is a secondary.
         if (serverGlobalParams.clusterRole != ClusterRole::ShardServer &&
             FeatureCompatibilityVersion::isCleanStartUp()) {
             auto schemaStatus = updateUUIDSchemaVersionNonReplicated(opCtx, true);