1 files changed, 48 insertions, 0 deletions
diff --git a/src/mongo/db/repl/topology_coordinator.h b/src/mongo/db/repl/topology_coordinator.h
index 419a74e61a1..8c515945213 100644
--- a/src/mongo/db/repl/topology_coordinator.h
+++ b/src/mongo/db/repl/topology_coordinator.h
@@ -916,6 +916,54 @@ private:
                                         const OpTime& lastOpTimeFetched,
                                         ReadPreference readPreference);
 
+    // Does preliminary checkes to see if a new sync source should be chosen
+    // * Do we have a valid configuration -- if so, we do not change sync source.
+    // * Are we in initial sync -- if so, we do not change sync source.
+    // * Do we have a new forced sync source -- if so, we do change sync source.
+    // Returns decision and current sync source candidate if decision is kMaybe.
+    // (kMaybe indicates to continue with further checks).
+    enum class ChangeSyncSourceDecision { kNo, kYes, kMaybe };
+    std::pair<ChangeSyncSourceDecision, int> _shouldChangeSyncSourceInitialChecks(
+        const HostAndPort& currentSource) const;
+
+    // Returns true if we should choose a new sync source because chaining is disabled
+    // and there is a new primary.
+    bool _shouldChangeSyncSourceDueToNewPrimary(const HostAndPort& currentSource,
+                                                int currentSourceIndex) const;
+
+    // Change sync source if they are not ahead of us, and don't have a sync source.
+    // Note 'syncSourceIndex' is the index of our sync source's sync source. The 'currentSource'
+    // is our sync source.
+    bool _shouldChangeSyncSourceDueToSourceNotAhead(const HostAndPort& currentSource,
+                                                    int syncSourceIndex,
+                                                    bool syncSourceIsPrimary,
+                                                    const OpTime& currentSourceOpTime,
+                                                    const OpTime& lastOpTimeFetched) const;
+
+    // Change sync source if our sync source is also syncing from us when we are in primary
+    // catchup mode, forming a sync source selection cycle, and the sync source is not ahead
+    // of us.
+    // Note 'syncSourceHost' and 'syncSourceIndex' are the host and index of ourb sync source's
+    // sync source. The 'currentSource' is our sync source.
+    bool _shouldChangeSyncSourceToBreakCycle(const HostAndPort& currentSource,
+                                             const std::string& syncSourceHost,
+                                             int syncSourceIndex,
+                                             const OpTime& currentSourceOpTime,
+                                             const OpTime& lastOpTimeFetched) const;
+
+    // Returns true if we should choose a new sync source due to our current sync source being
+    // greater than maxSyncSourceLagSeconds and a better source being available.
+    bool _shouldChangeSyncSourceDueToLag(const HostAndPort& currentSource,
+                                         const OpTime& currentSourceOpTime,
+                                         const OpTime& lastOpTimeFetched,
+                                         Date_t now) const;
+
+    // Returns true if we should choose a new sync source because our current sync source does
+    // not match our strict criteria for sync source candidates, but another member does.
+    bool _shouldChangeSyncSourceDueToBetterEligibleSource(const HostAndPort& currentSource,
+                                                          int currentSourceIndex,
+                                                          const OpTime& lastOpTimeFetched,
+                                                          Date_t now) const;
     /*
      * Clear this node's sync source.
      */