Add comments to HA code and loadBalance code

bindings/flow/fdb_flow.actor.cpp

@@ -94,6 +94,7 @@ void fdb_flow_test() {
 	g_network->run();
 }
 
+// FDB obj used by bindings
 namespace FDB {
 	class DatabaseImpl : public Database, NonCopyable {
 	public:

fdbclient/StorageServerInterface.h

@@ -137,7 +137,7 @@ struct StorageInfo : NonCopyable, public ReferenceCounted<StorageInfo> {
 };
 
 struct ServerCacheInfo {
-	std::vector<Tag> tags;
+	std::vector<Tag> tags; // all tags in both primary and remote DC for the key-range
 	std::vector<Reference<StorageInfo>> src_info;
 	std::vector<Reference<StorageInfo>> dest_info;
 

fdbrpc/LoadBalance.actor.h

@@ -169,7 +169,9 @@ void addLaggingRequest(Future<Optional<Reply>> reply, Promise<Void> requestFinis
 
 // Keep trying to get a reply from any of servers until success or cancellation; tries to take into account
 //   failMon's information for load balancing and avoiding failed servers
-// If ALL the servers are failed and the list of servers is not fresh, throws an exception to let the caller refresh the list of servers
+// If ALL the servers are failed and the list of servers is not fresh, throws an exception to let the caller refresh the
+// list of servers When model is set, load balance among alternatives in the same DC, aiming to balance request queue
+// length on these interfaces. If too many interfaces in the same DC are bad, try remote interfaces.
 ACTOR template <class Interface, class Request, class Multi>
 Future< REPLY_TYPE(Request) > loadBalance(
 	Reference<MultiInterface<Multi>> alternatives,
@@ -206,7 +208,12 @@ Future< REPLY_TYPE(Request) > loadBalance(
 		int badServers = 0;
 
 		for(int i=0; i<alternatives->size(); i++) {
+			// countBest(): the number of alternatives in the same locality (i.e., DC by default) as alternatives[0].
+			// if the if-statement is correct, it won't try to send requests to the remote ones.
 			if(badServers < std::min(i, FLOW_KNOBS->LOAD_BALANCE_MAX_BAD_OPTIONS + 1) && i == alternatives->countBest()) {
+				// If there are not enough local servers that are bad,
+				// we won't even try to send requests to remote servers.
+				// An interface is bad if its endpoint fails or if reply from the interface has a high penalty.
 				break;
 			}
 
@@ -217,6 +224,7 @@ Future< REPLY_TYPE(Request) > loadBalance(
 					double thisMetric = qd.smoothOutstanding.smoothTotal();
 					double thisTime = qd.latency;
 					if(FLOW_KNOBS->LOAD_BALANCE_PENALTY_IS_BAD && qd.penalty > 1.001) {
+						// penalty is sent from server.
 						++badServers;
 					}
 

fdbrpc/MultiInterface.h

@@ -43,7 +43,7 @@ std::string describe( KVPair<K,V> const& p ) { return format("%d ", p.k) + descr
 template <class T>
 struct ReferencedInterface : public ReferenceCounted<ReferencedInterface<T>> {
 	T interf;
-	int8_t distance;
+	int8_t distance; // choose one enum value in LBDistance type
 	std::string toString() const {
 		return interf.toString();
 	}
@@ -222,7 +222,8 @@ public:
 	}
 private:
 	vector<Reference<ReferencedInterface<T>>> alternatives;
-	int16_t bestCount;
+	int16_t bestCount; // The number of interfaces in the same location as alternatives[0]. The same location means
+	                   // DC by default and machine if more than one alternatives are on the same machine).
 };
 
 template <class Ar, class T> void load(Ar& ar, Reference<MultiInterface<T>>&) { ASSERT(false); }	//< required for Future<T>

fdbserver/ClusterController.actor.cpp

@@ -945,6 +945,7 @@ public:
 		}
 	}
 
+	// Check if txn system is recruited successfully in each region
 	void checkRegions(const std::vector<RegionInfo>& regions) {
 		if(desiredDcIds.get().present() && desiredDcIds.get().get().size() == 2 && desiredDcIds.get().get()[0].get() == regions[0].dcId && desiredDcIds.get().get()[1].get() == regions[1].dcId) {
 			return;

fdbserver/MasterProxyServer.actor.cpp

@@ -420,7 +420,7 @@ struct ProxyCommitData {
 	uint64_t commitVersionRequestNumber;
 	uint64_t mostRecentProcessedRequestNumber;
 	KeyRangeMap<Deque<std::pair<Version,int>>> keyResolvers;
-	KeyRangeMap<ServerCacheInfo> keyInfo;
+	KeyRangeMap<ServerCacheInfo> keyInfo; // keyrange -> all storage servers in all DCs for the keyrange
 	KeyRangeMap<bool> cacheInfo;
 	std::map<Key, applyMutationsData> uid_applyMutationsData;
 	bool firstProxy;

fdbserver/TagPartitionedLogSystem.actor.cpp

@@ -1038,7 +1038,12 @@ struct TagPartitionedLogSystem : ILogSystem, ReferenceCounted<TagPartitionedLogS
 					bestSet = bestSatelliteSet;
 				}
 
-				TraceEvent("TLogPeekLogRouterOldSets", dbgid).detail("Tag", tag.toString()).detail("Begin", begin).detail("OldEpoch", old.epochEnd).detail("RecoveredAt", recoveredAt.present() ? recoveredAt.get() : -1).detail("FirstOld", firstOld);
+				TraceEvent("TLogPeekLogRouterOldSets", dbgid)
+				    .detail("Tag", tag.toString())
+				    .detail("Begin", begin)
+				    .detail("OldEpoch", old.epochEnd)
+				    .detail("RecoveredAt", recoveredAt.present() ? recoveredAt.get() : -1)
+				    .detail("FirstOld", firstOld);
 				//FIXME: do this merge on one of the logs in the other data center to avoid sending multiple copies across the WAN
 				return Reference<ILogSystem::SetPeekCursor>( new ILogSystem::SetPeekCursor( localSets, bestSet, localSets[bestSet]->bestLocationFor( tag ), tag, begin, firstOld && recoveredAt.present() ? recoveredAt.get() + 1 : old.epochEnd, true ) );
 			}

fdbserver/WorkerInterface.actor.h

@@ -210,8 +210,11 @@ struct RecruitFromConfigurationReply {
 	std::vector<WorkerInterface> proxies;
 	std::vector<WorkerInterface> resolvers;
 	std::vector<WorkerInterface> storageServers;
-	std::vector<WorkerInterface> oldLogRouters;
-	Optional<Key> dcId;
+	std::vector<WorkerInterface> oldLogRouters; // why need oldLogRouters?
+	Optional<Key> dcId; // dcId is where master is recruited. It prefers to be in configuration.primaryDcId, but
+	                    // it can be recruited from configuration.secondaryDc: The dcId will be the secondaryDcId and
+	                    // this generation's primaryDC in memory is different from configuration.primaryDcId.
+	                    // when is dcId set?
 	bool satelliteFallback;
 
 	RecruitFromConfigurationReply() : satelliteFallback(false) {}

fdbserver/storageserver.actor.cpp

@@ -691,6 +691,7 @@ public:
 		return counters.bytesInput.getValue() - counters.bytesDurable.getValue();
 	}
 
+	// penalty used by loadBalance() to balance requests among SSes. We prefer SS with less write queue size.
 	double getPenalty() {
 		return std::max(std::max(1.0, (queueSize() - (SERVER_KNOBS->TARGET_BYTES_PER_STORAGE_SERVER -
 													  2.0 * SERVER_KNOBS->SPRING_BYTES_STORAGE_SERVER)) /