SERVER-33562 Move FeatureCompatibilityVersion::kCollection/kDatabase into namespace_string.h

and clean up some file dependencies

17 files changed, 123 insertions, 136 deletions

diff --git a/src/mongo/db/commands/SConscript b/src/mongo/db/commands/SConscript
index 6d492b765a2..d6aeba17592 100644
--- a/src/mongo/db/commands/SConscript
+++ b/src/mongo/db/commands/SConscript
@@ -60,6 +60,7 @@ env.Library(
     ],
     LIBDEPS=[
         '$BUILD_DIR/mongo/base',
+        '$BUILD_DIR/mongo/db/namespace_string',
     ],
 )
 
diff --git a/src/mongo/db/commands/feature_compatibility_version.cpp b/src/mongo/db/commands/feature_compatibility_version.cpp
index e9b98c38c63..b617228c4b7 100644
--- a/src/mongo/db/commands/feature_compatibility_version.cpp
+++ b/src/mongo/db/commands/feature_compatibility_version.cpp
@@ -36,6 +36,7 @@
 #include "mongo/db/commands/feature_compatibility_version_documentation.h"
 #include "mongo/db/commands/feature_compatibility_version_parser.h"
 #include "mongo/db/dbdirectclient.h"
+#include "mongo/db/namespace_string.h"
 #include "mongo/db/operation_context.h"
 #include "mongo/db/repl/optime.h"
 #include "mongo/db/repl/storage_interface.h"
@@ -53,9 +54,6 @@ namespace mongo {
 
 using repl::UnreplicatedWritesBlock;
 
-constexpr StringData FeatureCompatibilityVersion::kCollection;
-constexpr StringData FeatureCompatibilityVersion::kDatabase;
-
 Lock::ResourceMutex FeatureCompatibilityVersion::fcvLock("featureCompatibilityVersionLock");
 
 void FeatureCompatibilityVersion::setTargetUpgrade(OperationContext* opCtx) {
@@ -101,7 +99,7 @@ void FeatureCompatibilityVersion::setIfCleanStartup(OperationContext* opCtx,
     const bool storeUpgradeVersion = serverGlobalParams.clusterRole != ClusterRole::ShardServer;
 
     UnreplicatedWritesBlock unreplicatedWritesBlock(opCtx);
-    NamespaceString nss(FeatureCompatibilityVersion::kCollection);
+    NamespaceString nss(NamespaceString::kServerConfigurationNamespace);
 
     {
         CollectionOptions options;
@@ -109,7 +107,7 @@ void FeatureCompatibilityVersion::setIfCleanStartup(OperationContext* opCtx,
         uassertStatusOK(storageInterface->createCollection(opCtx, nss, options));
     }
 
-    // We then insert the featureCompatibilityVersion document into the "admin.system.version"
+    // We then insert the featureCompatibilityVersion document into the server configuration
     // collection. The server parameter will be updated on commit by the op observer.
     uassertStatusOK(storageInterface->insertDocument(
         opCtx,
@@ -210,7 +208,7 @@ void FeatureCompatibilityVersion::_validateVersion(StringData version) {
 void FeatureCompatibilityVersion::_runUpdateCommand(OperationContext* opCtx,
                                                     UpdateBuilder builder) {
     DBDirectClient client(opCtx);
-    NamespaceString nss(FeatureCompatibilityVersion::kCollection);
+    NamespaceString nss(NamespaceString::kServerConfigurationNamespace);
 
     BSONObjBuilder updateCmd;
     updateCmd.append("update", nss.coll());
@@ -231,7 +229,7 @@ void FeatureCompatibilityVersion::_runUpdateCommand(OperationContext* opCtx,
     }
     updateCmd.append(WriteConcernOptions::kWriteConcernField, WriteConcernOptions::Majority);
 
-    // Update the featureCompatibilityVersion document stored in the "admin.system.version"
+    // Update the featureCompatibilityVersion document stored in the server configuration
     // collection.
     BSONObj updateResult;
     client.runCommand(nss.db().toString(), updateCmd.obj(), updateResult);
diff --git a/src/mongo/db/commands/feature_compatibility_version.h b/src/mongo/db/commands/feature_compatibility_version.h
index 727ef8529fe..e41530edddd 100644
--- a/src/mongo/db/commands/feature_compatibility_version.h
+++ b/src/mongo/db/commands/feature_compatibility_version.h
@@ -47,9 +47,6 @@ extern bool internalValidateFeaturesAsMaster;
 
 class FeatureCompatibilityVersion {
 public:
-    static constexpr StringData kCollection = "admin.system.version"_sd;
-    static constexpr StringData kDatabase = "admin"_sd;
-
     /**
      * Should be taken in shared mode by any operations that should not run while
      * setFeatureCompatibilityVersion is running.
@@ -96,9 +93,9 @@ public:
     static bool isCleanStartUp();
 
     /**
-     * Examines a document inserted or updated in admin.system.version. If it is the
-     * featureCompatibilityVersion document, validates the document and on commit, updates
-     * the server parameter.
+     * Examines a document inserted or updated in the server configuration collection
+     * (admin.system.version). If it is the featureCompatibilityVersion document, validates the
+     * document and on commit, updates the server parameter.
      */
     static void onInsertOrUpdate(OperationContext* opCtx, const BSONObj& doc);
 
diff --git a/src/mongo/db/commands/feature_compatibility_version_command_parser.cpp b/src/mongo/db/commands/feature_compatibility_version_command_parser.cpp
index ddb479f75f0..f9a66bd6f34 100644
--- a/src/mongo/db/commands/feature_compatibility_version_command_parser.cpp
+++ b/src/mongo/db/commands/feature_compatibility_version_command_parser.cpp
@@ -35,8 +35,6 @@
 #include "mongo/db/commands.h"
 #include "mongo/db/commands/feature_compatibility_version_documentation.h"
 #include "mongo/db/commands/feature_compatibility_version_parser.h"
-#include "mongo/db/query/query_request.h"
-#include "mongo/util/version.h"
 
 namespace mongo {
 
diff --git a/src/mongo/db/commands/feature_compatibility_version_parser.cpp b/src/mongo/db/commands/feature_compatibility_version_parser.cpp
index 8aea75ac1a9..30eeca1bf4c 100644
--- a/src/mongo/db/commands/feature_compatibility_version_parser.cpp
+++ b/src/mongo/db/commands/feature_compatibility_version_parser.cpp
@@ -33,6 +33,7 @@
 #include "mongo/base/status.h"
 #include "mongo/bson/bsonobj.h"
 #include "mongo/db/commands/feature_compatibility_version_documentation.h"
+#include "mongo/db/namespace_string.h"
 
 namespace mongo {
 
@@ -61,42 +62,37 @@ FeatureCompatibilityVersionParser::parse(const BSONObj& featureCompatibilityVers
             if (elem.type() != BSONType::String) {
                 return Status(
                     ErrorCodes::TypeMismatch,
-                    str::stream()
-                        << fieldName
-                        << " must be of type String, but was of type "
-                        << typeName(elem.type())
-                        << ". Contents of "
-                        << kParameterName
-                        << " document in "
-                        << "admin.system.version"  // TODO: NamespaceString constant (SERVER-33562)
-                        << ": "
-                        << featureCompatibilityVersionDoc
-                        << ". See "
-                        << feature_compatibility_version_documentation::kCompatibilityLink
-                        << ".");
+                    str::stream() << fieldName << " must be of type String, but was of type "
+                                  << typeName(elem.type())
+                                  << ". Contents of "
+                                  << kParameterName
+                                  << " document in "
+                                  << NamespaceString::kServerConfigurationNamespace.toString()
+                                  << ": "
+                                  << featureCompatibilityVersionDoc
+                                  << ". See "
+                                  << feature_compatibility_version_documentation::kCompatibilityLink
+                                  << ".");
             }
 
             if (elem.String() != kVersion40 && elem.String() != kVersion36) {
                 return Status(
                     ErrorCodes::BadValue,
-                    str::stream()
-                        << "Invalid value for "
-                        << fieldName
-                        << ", found "
-                        << elem.String()
-                        << ", expected '"
-                        << kVersion40
-                        << "' or '"
-                        << kVersion36
-                        << "'. Contents of "
-                        << kParameterName
-                        << " document in "
-                        << "admin.system.version"  // TODO: NamespaceString constant (SERVER-33562)
-                        << ": "
-                        << featureCompatibilityVersionDoc
-                        << ". See "
-                        << feature_compatibility_version_documentation::kCompatibilityLink
-                        << ".");
+                    str::stream() << "Invalid value for " << fieldName << ", found "
+                                  << elem.String()
+                                  << ", expected '"
+                                  << kVersion40
+                                  << "' or '"
+                                  << kVersion36
+                                  << "'. Contents of "
+                                  << kParameterName
+                                  << " document in "
+                                  << NamespaceString::kServerConfigurationNamespace.toString()
+                                  << ": "
+                                  << featureCompatibilityVersionDoc
+                                  << ". See "
+                                  << feature_compatibility_version_documentation::kCompatibilityLink
+                                  << ".");
             }
 
             if (fieldName == kVersionField) {
@@ -107,18 +103,15 @@ FeatureCompatibilityVersionParser::parse(const BSONObj& featureCompatibilityVers
         } else {
             return Status(
                 ErrorCodes::BadValue,
-                str::stream()
-                    << "Unrecognized field '"
-                    << fieldName
-                    << "'. Contents of "
-                    << kParameterName
-                    << " document in "
-                    << "admin.system.version"  // TODO: NamespaceString constant (SERVER-33562)
-                    << ": "
-                    << featureCompatibilityVersionDoc
-                    << ". See "
-                    << feature_compatibility_version_documentation::kCompatibilityLink
-                    << ".");
+                str::stream() << "Unrecognized field '" << fieldName << "'. Contents of "
+                              << kParameterName
+                              << " document in "
+                              << NamespaceString::kServerConfigurationNamespace.toString()
+                              << ": "
+                              << featureCompatibilityVersionDoc
+                              << ". See "
+                              << feature_compatibility_version_documentation::kCompatibilityLink
+                              << ".");
         }
     }
 
@@ -134,34 +127,28 @@ FeatureCompatibilityVersionParser::parse(const BSONObj& featureCompatibilityVers
         if (targetVersionString == kVersion40 || targetVersionString == kVersion36) {
             return Status(
                 ErrorCodes::BadValue,
-                str::stream()
-                    << "Invalid state for "
-                    << kParameterName
-                    << " document in "
-                    << "admin.system.version"  // TODO: NamespaceString constant (SERVER-33562)
-                    << ": "
-                    << featureCompatibilityVersionDoc
-                    << ". See "
-                    << feature_compatibility_version_documentation::kCompatibilityLink
-                    << ".");
+                str::stream() << "Invalid state for " << kParameterName << " document in "
+                              << NamespaceString::kServerConfigurationNamespace.toString()
+                              << ": "
+                              << featureCompatibilityVersionDoc
+                              << ". See "
+                              << feature_compatibility_version_documentation::kCompatibilityLink
+                              << ".");
         } else {
             version = ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo40;
         }
     } else {
         return Status(
             ErrorCodes::BadValue,
-            str::stream()
-                << "Missing required field '"
-                << kVersionField
-                << "''. Contents of "
-                << kParameterName
-                << " document in "
-                << "admin.system.version"  // TODO: NamespaceString constant (SERVER-33562)
-                << ": "
-                << featureCompatibilityVersionDoc
-                << ". See "
-                << feature_compatibility_version_documentation::kCompatibilityLink
-                << ".");
+            str::stream() << "Missing required field '" << kVersionField << "''. Contents of "
+                          << kParameterName
+                          << " document in "
+                          << NamespaceString::kServerConfigurationNamespace.toString()
+                          << ": "
+                          << featureCompatibilityVersionDoc
+                          << ". See "
+                          << feature_compatibility_version_documentation::kCompatibilityLink
+                          << ".");
     }
 
     return version;
diff --git a/src/mongo/db/commands/feature_compatibility_version_parser.h b/src/mongo/db/commands/feature_compatibility_version_parser.h
index a1382623b6f..ac36f73556a 100644
--- a/src/mongo/db/commands/feature_compatibility_version_parser.h
+++ b/src/mongo/db/commands/feature_compatibility_version_parser.h
@@ -50,8 +50,9 @@ public:
     static constexpr StringData kTargetVersionField = "targetVersion"_sd;
 
     /**
-     * Parses the featureCompatibilityVersion document from admin.system.version, and returns the
-     * state represented by the combination of the targetVersion and version.
+     * Parses the featureCompatibilityVersion document from the server configuration collection
+     * (admin.system.version), and returns the state represented by the combination of the
+     * targetVersion and version.
      */
     static StatusWith<ServerGlobalParams::FeatureCompatibility::Version> parse(
         const BSONObj& featureCompatibilityVersionDoc);
diff --git a/src/mongo/db/commands/rename_collection_cmd.cpp b/src/mongo/db/commands/rename_collection_cmd.cpp
index 3f05d9cd2b9..391b7ac388e 100644
--- a/src/mongo/db/commands/rename_collection_cmd.cpp
+++ b/src/mongo/db/commands/rename_collection_cmd.cpp
@@ -144,11 +144,12 @@ public:
             return false;
         }
 
-        if (source.isAdminDotSystemDotVersion()) {
-            CommandHelpers::appendCommandStatus(
-                result,
-                Status(ErrorCodes::IllegalOperation,
-                       "renaming admin.system.version is not allowed"));
+        if (source.isServerConfigurationCollection()) {
+            CommandHelpers::appendCommandStatus(result,
+                                                Status(ErrorCodes::IllegalOperation,
+                                                       "renaming the server configuration "
+                                                       "collection (admin.system.version) is not "
+                                                       "allowed"));
             return false;
         }
 
diff --git a/src/mongo/db/namespace_string.h b/src/mongo/db/namespace_string.h
index ad55b40aff6..a8fac0a2a58 100644
--- a/src/mongo/db/namespace_string.h
+++ b/src/mongo/db/namespace_string.h
@@ -72,7 +72,7 @@ public:
     // running as a replica set. Documents in this collection should represent some configuration
     // state of the server, which needs to be recovered/consulted at startup. Each document in this
     // namespace should have its _id set to some string, which meaningfully describes what it
-    // represents.
+    // represents. For example, 'shardIdentity' and 'featureCompatibilityVersion'.
     static const NamespaceString kServerConfigurationNamespace;
 
     // Namespace for storing the transaction information for each session
@@ -224,7 +224,7 @@ public:
     bool isSystemDotViews() const {
         return coll() == kSystemDotViewsCollectionName;
     }
-    bool isAdminDotSystemDotVersion() const {
+    bool isServerConfigurationCollection() const {
         return (db() == kAdminDb) && (coll() == "system.version");
     }
     bool isConfigDB() const {
diff --git a/src/mongo/db/op_observer_impl.cpp b/src/mongo/db/op_observer_impl.cpp
index c8a470f5b91..865fa0f09a7 100644
--- a/src/mongo/db/op_observer_impl.cpp
+++ b/src/mongo/db/op_observer_impl.cpp
@@ -395,7 +395,9 @@ void OpObserverImpl::onInserts(OperationContext* opCtx,
         Scope::storedFuncMod(opCtx);
     } else if (nss.coll() == DurableViewCatalog::viewsCollectionName()) {
         DurableViewCatalog::onExternalChange(opCtx, nss);
-    } else if (nss.ns() == FeatureCompatibilityVersion::kCollection) {
+    } else if (nss == NamespaceString::kServerConfigurationNamespace) {
+        // We must check server configuration collection writes for featureCompatibilityVersion
+        // document changes.
         for (auto it = first; it != last; it++) {
             FeatureCompatibilityVersion::onInsertOrUpdate(opCtx, it->doc);
         }
@@ -457,7 +459,9 @@ void OpObserverImpl::onUpdate(OperationContext* opCtx, const OplogUpdateEntryArg
         Scope::storedFuncMod(opCtx);
     } else if (args.nss.coll() == DurableViewCatalog::viewsCollectionName()) {
         DurableViewCatalog::onExternalChange(opCtx, args.nss);
-    } else if (args.nss.ns() == FeatureCompatibilityVersion::kCollection) {
+    } else if (args.nss == NamespaceString::kServerConfigurationNamespace) {
+        // We must check server configuration collection writes for featureCompatibilityVersion
+        // document changes.
         FeatureCompatibilityVersion::onInsertOrUpdate(opCtx, args.updatedDoc);
     } else if (args.nss == NamespaceString::kSessionTransactionsTableNamespace &&
                !opTime.writeOpTime.isNull()) {
@@ -511,7 +515,7 @@ void OpObserverImpl::onDelete(OperationContext* opCtx,
         Scope::storedFuncMod(opCtx);
     } else if (nss.coll() == DurableViewCatalog::viewsCollectionName()) {
         DurableViewCatalog::onExternalChange(opCtx, nss);
-    } else if (nss.isAdminDotSystemDotVersion()) {
+    } else if (nss.isServerConfigurationCollection()) {
         auto _id = deleteState.documentKey["_id"];
         if (_id.type() == BSONType::String &&
             _id.String() == FeatureCompatibilityVersionParser::kParameterName)
@@ -666,9 +670,8 @@ void OpObserverImpl::onDropDatabase(OperationContext* opCtx, const std::string&
                  kUninitializedStmtId,
                  {});
 
-    uassert(50714,
-            "dropping the admin database is not allowed.",
-            dbName != FeatureCompatibilityVersion::kDatabase);
+    uassert(
+        50714, "dropping the admin database is not allowed.", dbName != NamespaceString::kAdminDb);
 
     if (dbName == NamespaceString::kSessionTransactionsTableNamespace.db()) {
         SessionCatalog::get(opCtx)->invalidateSessions(opCtx, boost::none);
@@ -702,8 +705,8 @@ repl::OpTime OpObserverImpl::onDropCollection(OperationContext* opCtx,
     }
 
     uassert(50715,
-            "dropping the admin.system.version collection is not allowed.",
-            collectionName.ns() != FeatureCompatibilityVersion::kCollection);
+            "dropping the server configuration collection (admin.system.version) is not allowed.",
+            collectionName != NamespaceString::kServerConfigurationNamespace);
 
     if (collectionName.coll() == DurableViewCatalog::viewsCollectionName()) {
         DurableViewCatalog::onExternalChange(opCtx, collectionName);
diff --git a/src/mongo/db/repair_database_and_check_version.cpp b/src/mongo/db/repair_database_and_check_version.cpp
index d86c0da16af..214028ee19b 100644
--- a/src/mongo/db/repair_database_and_check_version.cpp
+++ b/src/mongo/db/repair_database_and_check_version.cpp
@@ -70,9 +70,10 @@ const std::string mustDowngradeErrorMsg = str::stream()
 
 Status restoreMissingFeatureCompatibilityVersionDocument(OperationContext* opCtx,
                                                          const std::vector<std::string>& dbNames) {
-    NamespaceString fcvNss(FeatureCompatibilityVersion::kCollection);
+    NamespaceString fcvNss(NamespaceString::kServerConfigurationNamespace);
 
-    // If the admin database does not exist, create it.
+    // If the admin database, which contains the server configuration collection with the
+    // featureCompatibilityVersion document, does not exist, create it.
     Database* db = dbHolder().get(opCtx, fcvNss.db());
     if (!db) {
         log() << "Re-creating admin database that was dropped.";
@@ -80,14 +81,16 @@ Status restoreMissingFeatureCompatibilityVersionDocument(OperationContext* opCtx
     db = dbHolder().openDb(opCtx, fcvNss.db());
     invariant(db);
 
-    // If admin.system.version does not exist, create it.
-    if (!db->getCollection(opCtx, FeatureCompatibilityVersion::kCollection)) {
-        log() << "Re-creating admin.system.version collection that was dropped.";
+    // If the server configuration collection, which contains the FCV document, does not exist, then
+    // create it.
+    if (!db->getCollection(opCtx, NamespaceString::kServerConfigurationNamespace)) {
+        log() << "Re-creating the server configuration collection (admin.system.version) that was "
+                 "dropped.";
         uassertStatusOK(
             createCollection(opCtx, fcvNss.db().toString(), BSON("create" << fcvNss.coll())));
     }
 
-    Collection* fcvColl = db->getCollection(opCtx, FeatureCompatibilityVersion::kCollection);
+    Collection* fcvColl = db->getCollection(opCtx, NamespaceString::kServerConfigurationNamespace);
     invariant(fcvColl);
 
     // Restore the featureCompatibilityVersion document if it is missing.
@@ -284,12 +287,12 @@ StatusWith<bool> repairDatabasesAndCheckVersion(OperationContext* opCtx) {
         repairVerifiedAllCollectionsHaveUUIDs = true;
 
         // Attempt to restore the featureCompatibilityVersion document if it is missing.
-        NamespaceString nss(FeatureCompatibilityVersion::kCollection);
+        NamespaceString fcvNSS(NamespaceString::kServerConfigurationNamespace);
 
-        Database* db = dbHolder().get(opCtx, nss.db());
+        Database* db = dbHolder().get(opCtx, fcvNSS.db());
         Collection* versionColl;
         BSONObj featureCompatibilityVersion;
-        if (!db || !(versionColl = db->getCollection(opCtx, nss)) ||
+        if (!db || !(versionColl = db->getCollection(opCtx, fcvNSS)) ||
             !Helpers::findOne(opCtx,
                               versionColl,
                               BSON("_id" << FeatureCompatibilityVersionParser::kParameterName),
@@ -407,11 +410,12 @@ StatusWith<bool> repairDatabasesAndCheckVersion(OperationContext* opCtx) {
             MONGO_UNREACHABLE;
         }
 
-        // Check if admin.system.version contains an invalid featureCompatibilityVersion.
-        // If a valid featureCompatibilityVersion is present, cache it as a server parameter.
+
+        // If the server configuration collection already contains a valid
+        // featureCompatibilityVersion document, cache it in-memory as a server parameter.
         if (dbName == "admin") {
             if (Collection* versionColl =
-                    db->getCollection(opCtx, FeatureCompatibilityVersion::kCollection)) {
+                    db->getCollection(opCtx, NamespaceString::kServerConfigurationNamespace)) {
                 BSONObj featureCompatibilityVersion;
                 if (Helpers::findOne(
                         opCtx,
diff --git a/src/mongo/db/repl/SConscript b/src/mongo/db/repl/SConscript
index d09428bf75d..68fb340da4b 100644
--- a/src/mongo/db/repl/SConscript
+++ b/src/mongo/db/repl/SConscript
@@ -18,7 +18,7 @@ env.Library(
         '$BUILD_DIR/mongo/base',
         '$BUILD_DIR/mongo/db/background',
         '$BUILD_DIR/mongo/db/catalog/catalog_helpers',
-        '$BUILD_DIR/mongo/db/commands/dcommands_fcv',
+        '$BUILD_DIR/mongo/db/commands/feature_compatibility_version_parser',
         '$BUILD_DIR/mongo/db/db_raii',
         '$BUILD_DIR/mongo/db/dbdirectclient',
         '$BUILD_DIR/mongo/db/dbhelpers',
@@ -1435,7 +1435,7 @@ env.Library(
         'rollback_checker',
         'storage_interface',
         '$BUILD_DIR/mongo/client/fetcher',
-        '$BUILD_DIR/mongo/db/commands/dcommands_fcv',
+        '$BUILD_DIR/mongo/db/commands/feature_compatibility_version_parser',
     ],
 )
 
diff --git a/src/mongo/db/repl/initial_syncer.cpp b/src/mongo/db/repl/initial_syncer.cpp
index 7fac9df71c1..bec5848652e 100644
--- a/src/mongo/db/repl/initial_syncer.cpp
+++ b/src/mongo/db/repl/initial_syncer.cpp
@@ -41,7 +41,6 @@
 #include "mongo/bson/util/bson_extract.h"
 #include "mongo/client/fetcher.h"
 #include "mongo/client/remote_command_retry_scheduler.h"
-#include "mongo/db/commands/feature_compatibility_version.h"
 #include "mongo/db/commands/feature_compatibility_version_parser.h"
 #include "mongo/db/commands/server_status_metric.h"
 #include "mongo/db/concurrency/d_concurrency.h"
@@ -439,7 +438,7 @@ void InitialSyncer::_startInitialSyncAttemptCallback(
 
     LOG(2) << "Resetting feature compatibility version to last-stable. If the sync source is in "
               "latest feature compatibility version, we will find out when we clone the "
-              "admin.system.version collection.";
+              "server configuration collection (admin.system.version).";
     serverGlobalParams.featureCompatibility.reset();
 
     // Clear the oplog buffer.
@@ -611,7 +610,7 @@ void InitialSyncer::_lastOplogEntryFetcherCallbackForBeginTimestamp(
     const auto& lastOpTimeWithHash = opTimeWithHashResult.getValue();
 
     BSONObjBuilder queryBob;
-    queryBob.append("find", nsToCollectionSubstring(FeatureCompatibilityVersion::kCollection));
+    queryBob.append("find", NamespaceString::kServerConfigurationNamespace.coll());
     auto filterBob = BSONObjBuilder(queryBob.subobjStart("filter"));
     filterBob.append("_id", FeatureCompatibilityVersionParser::kParameterName);
     filterBob.done();
@@ -619,7 +618,7 @@ void InitialSyncer::_lastOplogEntryFetcherCallbackForBeginTimestamp(
     _fCVFetcher = stdx::make_unique<Fetcher>(
         _exec,
         _syncSource,
-        nsToDatabaseSubstring(FeatureCompatibilityVersion::kCollection).toString(),
+        NamespaceString::kServerConfigurationNamespace.db().toString(),
         queryBob.obj(),
         [=](const StatusWith<mongo::Fetcher::QueryResponse>& response,
             mongo::Fetcher::NextAction*,
diff --git a/src/mongo/db/repl/initial_syncer_test.cpp b/src/mongo/db/repl/initial_syncer_test.cpp
index c4c8e26d72c..246c582a9f9 100644
--- a/src/mongo/db/repl/initial_syncer_test.cpp
+++ b/src/mongo/db/repl/initial_syncer_test.cpp
@@ -34,9 +34,9 @@
 
 #include "mongo/client/fetcher.h"
 #include "mongo/db/client.h"
-#include "mongo/db/commands/feature_compatibility_version.h"
 #include "mongo/db/commands/feature_compatibility_version_parser.h"
 #include "mongo/db/json.h"
+#include "mongo/db/namespace_string.h"
 #include "mongo/db/query/getmore_request.h"
 #include "mongo/db/repl/base_cloner_test_fixture.h"
 #include "mongo/db/repl/data_replicator_external_state_mock.h"
@@ -571,9 +571,9 @@ void InitialSyncerTest::processSuccessfulLastOplogEntryFetcherResponse(std::vect
 }
 
 void assertFCVRequest(RemoteCommandRequest request) {
-    ASSERT_EQUALS(nsToDatabaseSubstring(FeatureCompatibilityVersion::kCollection), request.dbname)
+    ASSERT_EQUALS(NamespaceString::kServerConfigurationNamespace.db(), request.dbname)
         << request.toString();
-    ASSERT_EQUALS(nsToCollectionSubstring(FeatureCompatibilityVersion::kCollection),
+    ASSERT_EQUALS(NamespaceString::kServerConfigurationNamespace.coll(),
                   request.cmdObj.getStringField("find"));
     ASSERT_BSONOBJ_EQ(BSON("_id" << FeatureCompatibilityVersionParser::kParameterName),
                       request.cmdObj.getObjectField("filter"));
@@ -589,8 +589,8 @@ void InitialSyncerTest::processSuccessfulFCVFetcherResponse(std::vector<BSONObj>
     auto net = getNet();
     auto request = assertRemoteCommandNameEquals(
         "find",
-        net->scheduleSuccessfulResponse(makeCursorResponse(
-            0LL, NamespaceString(FeatureCompatibilityVersion::kCollection), docs)));
+        net->scheduleSuccessfulResponse(
+            makeCursorResponse(0LL, NamespaceString::kServerConfigurationNamespace, docs)));
     assertFCVRequest(request);
     net->runReadyNetworkOperations();
 }
@@ -1350,7 +1350,7 @@ TEST_F(InitialSyncerTest, InitialSyncerPassesThroughFCVFetcherScheduleError) {
         [&request](const executor::RemoteCommandRequest& requestToSend) {
             request = requestToSend;
             return "find" == requestToSend.cmdObj.firstElement().fieldNameStringData() &&
-                nsToCollectionSubstring(FeatureCompatibilityVersion::kCollection) ==
+                NamespaceString::kServerConfigurationNamespace.coll() ==
                 requestToSend.cmdObj.firstElement().str();
         };
 
diff --git a/src/mongo/db/repl/oplog.cpp b/src/mongo/db/repl/oplog.cpp
index 32eda65ce6b..f0ea23b3b22 100644
--- a/src/mongo/db/repl/oplog.cpp
+++ b/src/mongo/db/repl/oplog.cpp
@@ -56,7 +56,6 @@
 #include "mongo/db/catalog/uuid_catalog.h"
 #include "mongo/db/client.h"
 #include "mongo/db/commands.h"
-#include "mongo/db/commands/feature_compatibility_version.h"
 #include "mongo/db/commands/feature_compatibility_version_parser.h"
 #include "mongo/db/concurrency/write_conflict_exception.h"
 #include "mongo/db/db_raii.h"
@@ -1052,10 +1051,10 @@ Status applyOperation_inlock(OperationContext* opCtx,
         collection = db->getCollection(opCtx, requestNss);
     }
 
-    // During upgrade from 3.4 to 3.6, the feature compatibility version cannot change during
-    // initial sync because we cannot do some operations with UUIDs and others without.
+    // The feature compatibility version in the server configuration collection must not change
+    // during initial sync.
     if ((mode == OplogApplication::Mode::kInitialSync) &&
-        requestNss.ns() == FeatureCompatibilityVersion::kCollection) {
+        requestNss == NamespaceString::kServerConfigurationNamespace) {
         std::string oID;
         auto status = bsonExtractStringField(o, "_id", &oID);
         if (status.isOK() && oID == FeatureCompatibilityVersionParser::kParameterName) {
@@ -1480,7 +1479,8 @@ Status applyCommand_inlock(OperationContext* opCtx,
         }
     }
 
-    // The feature compatibility version cannot change during initial sync.
+    // The feature compatibility version in the server configuration collection cannot change during
+    // initial sync.
     // We do not attempt to parse the whitelisted ops because they do not have a collection
     // namespace. If we drop the 'admin' database we will also log a 'drop' oplog entry for each
     // collection dropped. 'applyOps' will try to apply each individual operation, and those
@@ -1489,7 +1489,7 @@ Status applyCommand_inlock(OperationContext* opCtx,
     if ((mode == OplogApplication::Mode::kInitialSync) &&
         (std::find(whitelistedOps.begin(), whitelistedOps.end(), o.firstElementFieldName()) ==
          whitelistedOps.end()) &&
-        parseNs(nss.ns(), o).ns() == FeatureCompatibilityVersion::kCollection) {
+        parseNs(nss.ns(), o) == NamespaceString::kServerConfigurationNamespace) {
         return Status(ErrorCodes::OplogOperationUnsupported,
                       str::stream() << "Applying command to feature compatibility version "
                                        "collection not supported in initial sync: "
diff --git a/src/mongo/db/repl/sync_tail_test.cpp b/src/mongo/db/repl/sync_tail_test.cpp
index 68359c36229..063ee21205f 100644
--- a/src/mongo/db/repl/sync_tail_test.cpp
+++ b/src/mongo/db/repl/sync_tail_test.cpp
@@ -41,7 +41,6 @@
 #include "mongo/db/catalog/database_holder.h"
 #include "mongo/db/catalog/document_validation.h"
 #include "mongo/db/client.h"
-#include "mongo/db/commands/feature_compatibility_version.h"
 #include "mongo/db/commands/feature_compatibility_version_parser.h"
 #include "mongo/db/concurrency/d_concurrency.h"
 #include "mongo/db/concurrency/write_conflict_exception.h"
@@ -1653,15 +1652,14 @@ TEST_F(SyncTailTest, FailOnDropFCVCollection) {
     ASSERT_OK(
         ReplicationCoordinator::get(_opCtx.get())->setFollowerMode(MemberState::RS_RECOVERING));
 
-    auto fcvNS = NamespaceString(FeatureCompatibilityVersion::kCollection);
+    auto fcvNS(NamespaceString::kServerConfigurationNamespace);
     auto cmd = BSON("drop" << fcvNS.coll());
-    auto op = makeCommandOplogEntry(
-        nextOpTime(), NamespaceString(FeatureCompatibilityVersion::kCollection), cmd);
+    auto op = makeCommandOplogEntry(nextOpTime(), fcvNS, cmd);
     ASSERT_EQUALS(runOpInitialSync(op), ErrorCodes::OplogOperationUnsupported);
 }
 
 TEST_F(SyncTailTest, FailOnInsertFCVDocument) {
-    auto fcvNS = NamespaceString(FeatureCompatibilityVersion::kCollection);
+    auto fcvNS(NamespaceString::kServerConfigurationNamespace);
     ::mongo::repl::createCollection(_opCtx.get(), fcvNS, CollectionOptions());
     ASSERT_OK(
         ReplicationCoordinator::get(_opCtx.get())->setFollowerMode(MemberState::RS_RECOVERING));
@@ -1672,7 +1670,7 @@ TEST_F(SyncTailTest, FailOnInsertFCVDocument) {
 }
 
 TEST_F(IdempotencyTest, InsertToFCVCollectionBesidesFCVDocumentSucceeds) {
-    auto fcvNS = NamespaceString(FeatureCompatibilityVersion::kCollection);
+    auto fcvNS(NamespaceString::kServerConfigurationNamespace);
     ::mongo::repl::createCollection(_opCtx.get(), fcvNS, CollectionOptions());
     ASSERT_OK(
         ReplicationCoordinator::get(_opCtx.get())->setFollowerMode(MemberState::RS_RECOVERING));
diff --git a/src/mongo/db/s/config/configsvr_shard_collection_command.cpp b/src/mongo/db/s/config/configsvr_shard_collection_command.cpp
index 179e0212794..a6e50420b6b 100644
--- a/src/mongo/db/s/config/configsvr_shard_collection_command.cpp
+++ b/src/mongo/db/s/config/configsvr_shard_collection_command.cpp
@@ -38,7 +38,6 @@
 #include "mongo/db/auth/authorization_session.h"
 #include "mongo/db/auth/privilege.h"
 #include "mongo/db/commands.h"
-#include "mongo/db/commands/feature_compatibility_version.h"
 #include "mongo/db/hasher.h"
 #include "mongo/db/index/index_descriptor.h"
 #include "mongo/db/namespace_string.h"
diff --git a/src/mongo/db/server_options.h b/src/mongo/db/server_options.h
index 7765a3579a0..e9ea5c752d1 100644
--- a/src/mongo/db/server_options.h
+++ b/src/mongo/db/server_options.h
@@ -146,8 +146,9 @@ struct ServerGlobalParams {
 
     struct FeatureCompatibility {
         /**
-         * The combination of the fields in the admin.system.version document in the format
-         * (version, targetVersion) are represented by this enum and determine this node's behavior.
+         * The combination of the fields (version, targetVersion) in the featureCompatiiblityVersion
+         * document in the server configuration collection (admin.system.version) are represented by
+         * this enum and determine this node's behavior.
          *
          * Features can be gated for specific versions, or ranges of versions above or below some
          * minimum or maximum version, respectively.