SERVER-50423 Change memberConfig's slaveDelay field to secondaryDelaySecs

15 files changed, 415 insertions, 51 deletions

diff --git a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
index 7f73024c99d..cbb1a42b858 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
@@ -50,6 +50,8 @@
 #include "mongo/db/ops/write_ops.h"
 #include "mongo/db/read_write_concern_defaults.h"
 #include "mongo/db/repl/repl_client_info.h"
+#include "mongo/db/repl/repl_server_parameters_gen.h"
+#include "mongo/db/repl/repl_set_config.h"
 #include "mongo/db/repl/replication_coordinator.h"
 #include "mongo/db/s/active_migrations_registry.h"
 #include "mongo/db/s/config/sharding_catalog_manager.h"
@@ -239,11 +241,47 @@ public:
         FeatureCompatibilityVersion::validateSetFeatureCompatibilityVersionRequest(
             actualVersion, requestedVersion, isFromConfigServer);
 
+        auto replCoord = repl::ReplicationCoordinator::get(opCtx);
+        const bool isReplSet =
+            replCoord->getReplicationMode() == repl::ReplicationCoordinator::modeReplSet;
         if (actualVersion < requestedVersion) {
             checkInitialSyncFinished(opCtx);
 
             FeatureCompatibilityVersion::updateFeatureCompatibilityVersionDocument(
                 opCtx, actualVersion, requestedVersion, isFromConfigServer);
+
+            // If the 'useSecondaryDelaySecs' feature flag is enabled in the upgraded FCV, issue a
+            // reconfig to change the 'slaveDelay' field to 'secondaryDelaySecs'.
+            if (isReplSet && repl::feature_flags::gUseSecondaryDelaySecs.isEnabledAndIgnoreFCV() &&
+                requestedVersion == ServerGlobalParams::FeatureCompatibility::kLatest) {
+                auto getNewConfig = [&](const repl::ReplSetConfig& oldConfig, long long term) {
+                    auto newConfig = oldConfig.getMutable();
+                    newConfig.setConfigVersion(newConfig.getConfigVersion() + 1);
+                    for (auto mem = oldConfig.membersBegin(); mem != oldConfig.membersEnd();
+                         mem++) {
+                        newConfig.useSecondaryDelaySecsFieldName(mem->getId());
+                    }
+                    return repl::ReplSetConfig(std::move(newConfig));
+                };
+                auto status = replCoord->doReplSetReconfig(opCtx, getNewConfig, true /* force */);
+                uassertStatusOKWithContext(status, "Failed to upgrade the replica set config");
+
+                LOGV2(5042301,
+                      "Waiting for the upgraded replica set config to propagate to a majority");
+                // If a write concern is given, we'll use its wTimeout. It's kNoTimeout by default.
+                WriteConcernOptions writeConcern(
+                    repl::ReplSetConfig::kConfigMajorityWriteConcernModeName,
+                    WriteConcernOptions::SyncMode::NONE,
+                    opCtx->getWriteConcern().wTimeout);
+                writeConcern.checkCondition = WriteConcernOptions::CheckCondition::Config;
+                repl::OpTime fakeOpTime(Timestamp(1, 1), replCoord->getTerm());
+                uassertStatusOKWithContext(
+                    replCoord->awaitReplication(opCtx, fakeOpTime, writeConcern).status,
+                    "Failed to wait for the upgraded replica set config to propagate to a "
+                    "majority");
+                LOGV2(5042302, "The upgraded replica set config has been propagated to a majority");
+            }
+
             {
                 // Take the global lock in S mode to create a barrier for operations taking the
                 // global IX or X locks. This ensures that either
@@ -322,6 +360,41 @@ public:
             FeatureCompatibilityVersion::updateFeatureCompatibilityVersionDocument(
                 opCtx, actualVersion, requestedVersion, isFromConfigServer);
 
+            // If the 'useSecondaryDelaySecs' feature flag is disabled in the downgraded FCV, issue
+            // a reconfig to change the 'secondaryDelaySecs' field to 'slaveDelay'.
+            if (isReplSet && repl::feature_flags::gUseSecondaryDelaySecs.isEnabledAndIgnoreFCV() &&
+                requestedVersion < repl::feature_flags::gUseSecondaryDelaySecs.getVersion()) {
+                auto getNewConfig = [&](const repl::ReplSetConfig& oldConfig, long long term) {
+                    auto newConfig = oldConfig.getMutable();
+                    newConfig.setConfigVersion(newConfig.getConfigVersion() + 1);
+                    for (auto mem = oldConfig.membersBegin(); mem != oldConfig.membersEnd();
+                         mem++) {
+                        newConfig.useSlaveDelayFieldName(mem->getId());
+                    }
+
+                    return repl::ReplSetConfig(std::move(newConfig));
+                };
+
+                auto status = replCoord->doReplSetReconfig(opCtx, getNewConfig, true /* force */);
+                uassertStatusOKWithContext(status, "Failed to downgrade the replica set config");
+
+                LOGV2(5042303,
+                      "Waiting for the downgraded replica set config to propagate to a majority");
+                // If a write concern is given, we'll use its wTimeout. It's kNoTimeout by default.
+                WriteConcernOptions writeConcern(
+                    repl::ReplSetConfig::kConfigMajorityWriteConcernModeName,
+                    WriteConcernOptions::SyncMode::NONE,
+                    opCtx->getWriteConcern().wTimeout);
+                writeConcern.checkCondition = WriteConcernOptions::CheckCondition::Config;
+                repl::OpTime fakeOpTime(Timestamp(1, 1), replCoord->getTerm());
+                uassertStatusOKWithContext(
+                    replCoord->awaitReplication(opCtx, fakeOpTime, writeConcern).status,
+                    "Failed to wait for the downgraded replica set config to propagate to a "
+                    "majority");
+                LOGV2(5042304,
+                      "The downgraded replica set config has been propagated to a majority");
+            }
+
             {
                 // Take the global lock in S mode to create a barrier for operations taking the
                 // global IX or X locks. This ensures that either
diff --git a/src/mongo/db/repl/SConscript b/src/mongo/db/repl/SConscript
index f8d16936ad9..ce657d65aad 100644
--- a/src/mongo/db/repl/SConscript
+++ b/src/mongo/db/repl/SConscript
@@ -853,6 +853,7 @@ env.Library(
         '$BUILD_DIR/mongo/client/connection_string',
         '$BUILD_DIR/mongo/db/common',
         '$BUILD_DIR/mongo/db/server_options_core',
+        '$BUILD_DIR/mongo/idl/feature_flag',
         '$BUILD_DIR/mongo/rpc/command_status',
         '$BUILD_DIR/mongo/rpc/metadata',
         '$BUILD_DIR/mongo/transport/transport_layer_common',
diff --git a/src/mongo/db/repl/member_config.cpp b/src/mongo/db/repl/member_config.cpp
index 7e962d57267..6569a8f73b9 100644
--- a/src/mongo/db/repl/member_config.cpp
+++ b/src/mongo/db/repl/member_config.cpp
@@ -91,7 +91,7 @@ MemberConfig::MemberConfig(const BSONObj& mcfg) {
         if (!isVoter()) {
             uasserted(ErrorCodes::BadValue, "priority must be 0 when non-voting (votes:0)");
         }
-        if (getSlaveDelay() > Seconds(0)) {
+        if (getSecondaryDelay() > Seconds(0)) {
             uasserted(ErrorCodes::BadValue, "priority must be 0 when slaveDelay is used");
         }
         if (isHidden()) {
@@ -211,7 +211,13 @@ BSONObj MemberConfig::toBSON(bool omitNewlyAddedField) const {
 
     _splitHorizon.toBSON(configBuilder);
 
-    configBuilder.append(kSlaveDelaySecsFieldName, getSlaveDelaySecs());
+    if (getSecondaryDelaySecs()) {
+        configBuilder.append(kSecondaryDelaySecsFieldName, getSecondaryDelaySecs().get());
+    }
+    if (getSlaveDelaySecs()) {
+        configBuilder.append(kSlaveDelaySecsFieldName, getSlaveDelaySecs().get());
+    }
+
     configBuilder.append(kVotesFieldName, MemberConfigBase::getVotes() ? 1 : 0);
     return configBuilder.obj();
 }
diff --git a/src/mongo/db/repl/member_config.h b/src/mongo/db/repl/member_config.h
index d6bb75622f3..0122b1f05a0 100644
--- a/src/mongo/db/repl/member_config.h
+++ b/src/mongo/db/repl/member_config.h
@@ -63,6 +63,7 @@ public:
     using MemberConfigBase::kIdFieldName;
     using MemberConfigBase::kNewlyAddedFieldName;
     using MemberConfigBase::kPriorityFieldName;
+    using MemberConfigBase::kSecondaryDelaySecsFieldName;
     using MemberConfigBase::kSlaveDelaySecsFieldName;
     using MemberConfigBase::kTagsFieldName;
     using MemberConfigBase::kVotesFieldName;
@@ -134,8 +135,14 @@ public:
      * Gets the amount of time behind the primary that this member will atempt to
      * remain.  Zero seconds means stay as caught up as possible.
      */
-    Seconds getSlaveDelay() const {
-        return Seconds(getSlaveDelaySecs());
+    Seconds getSecondaryDelay() const {
+        if (getSecondaryDelaySecs()) {
+            return Seconds(getSecondaryDelaySecs().get());
+        }
+        if (getSlaveDelaySecs()) {
+            return Seconds(getSlaveDelaySecs().get());
+        }
+        return Seconds(0);
     }
 
     /**
@@ -161,6 +168,20 @@ public:
     }
 
     /**
+     * Returns true if this member has the field 'slaveDelay'.
+     */
+    bool hasSlaveDelay() const {
+        return getSlaveDelaySecs().has_value();
+    }
+
+    /**
+     * Returns true if this member has the field 'secondaryDelaySecs'.
+     */
+    bool hasSecondaryDelaySecs() const {
+        return getSecondaryDelaySecs().has_value();
+    }
+
+    /**
      * Returns true if this member is newly added from reconfig. This indicates that this node
      * should be treated as non-voting.
      */
diff --git a/src/mongo/db/repl/member_config.idl b/src/mongo/db/repl/member_config.idl
index 9f937c39eb8..4340d8957e9 100644
--- a/src/mongo/db/repl/member_config.idl
+++ b/src/mongo/db/repl/member_config.idl
@@ -95,10 +95,19 @@ structs:
             slaveDelay:
                 cpp_name: slaveDelaySecs
                 type: safeInt64
-                default: 0
+                optional: true
+                validator: { gte: 0, lte: 31622400 }
+                description: "The number of seconds 'behind' the primary that this replica set
+                              member should 'lag'. This is a legacy field kept for backwards
+                              compatibility, and 5.0+ nodes should use secondaryDelaySecs"
+            secondaryDelaySecs:
+                cpp_name: secondaryDelaySecs
+                type: safeInt64
+                optional: true
                 validator: { gte: 0, lte: 31622400 }
                 description: "The number of seconds 'behind' the primary that this replica set
-                              member should 'lag'"
+                              member should 'lag'. Only 5.0+ nodes will use this field,
+                              and older nodes will use slaveDelay"
             arbiterOnly:
                 type: safeBool
                 default: false
diff --git a/src/mongo/db/repl/member_config_test.cpp b/src/mongo/db/repl/member_config_test.cpp
index 00c9fb36765..a9cad26e77c 100644
--- a/src/mongo/db/repl/member_config_test.cpp
+++ b/src/mongo/db/repl/member_config_test.cpp
@@ -49,7 +49,7 @@ TEST(MemberConfig, ParseMinimalMemberConfigAndCheckDefaults) {
     ASSERT_EQUALS(MemberId(0), mc.getId());
     ASSERT_EQUALS(HostAndPort("localhost", 12345), mc.getHostAndPort());
     ASSERT_EQUALS(1.0, mc.getPriority());
-    ASSERT_EQUALS(Seconds(0), mc.getSlaveDelay());
+    ASSERT_EQUALS(Seconds(0), mc.getSecondaryDelay());
     ASSERT_TRUE(mc.isVoter());
     ASSERT_FALSE(mc.isHidden());
     ASSERT_FALSE(mc.isArbiter());
@@ -675,7 +675,7 @@ TEST(MemberConfig, ParseSlaveDelay) {
                                    << "h"
                                    << "priority" << 0 << "slaveDelay" << 100),
                         &tagConfig);
-        ASSERT_EQUALS(Seconds(100), mc.getSlaveDelay());
+        ASSERT_EQUALS(Seconds(100), mc.getSecondaryDelay());
     }
     {
         ASSERT_THROWS(MemberConfig(BSON("_id" << 0 << "host"
@@ -715,6 +715,54 @@ TEST(MemberConfig, ParseSlaveDelay) {
     }
 }
 
+TEST(MemberConfig, ParseSecondaryDelay) {
+    ReplSetTagConfig tagConfig;
+    {
+        MemberConfig mc(BSON("_id" << 0 << "host"
+                                   << "h"
+                                   << "priority" << 0 << "secondaryDelaySecs" << 100),
+                        &tagConfig);
+        ASSERT_EQUALS(Seconds(100), mc.getSecondaryDelay());
+    }
+    {
+        ASSERT_THROWS(MemberConfig(BSON("_id" << 0 << "host"
+                                              << "h"
+                                              << "secondaryDelaySecs" << 100),
+                                   &tagConfig),
+                      ExceptionFor<ErrorCodes::BadValue>);
+    }
+    {
+        MemberConfig mc(BSON("_id" << 0 << "host"
+                                   << "h"
+                                   << "priority" << 0 << "secondaryDelaySecs" << 0),
+                        &tagConfig);
+    }
+    {
+        MemberConfig mc(BSON("_id" << 0 << "host"
+                                   << "h"
+                                   << "priority" << 0 << "secondaryDelaySecs" << 3600 * 10),
+                        &tagConfig);
+    }
+    {
+        ASSERT_THROWS_CODE(
+            MemberConfig(BSON("_id" << 0 << "host"
+                                    << "h"
+                                    << "priority" << 0 << "secondaryDelaySecs" << -1),
+                         &tagConfig),
+            AssertionException,
+            51024);
+    }
+    {
+        ASSERT_THROWS_CODE(
+            MemberConfig(BSON("_id" << 0 << "host"
+                                    << "h"
+                                    << "priority" << 0 << "secondaryDelaySecs" << 3600 * 24 * 400),
+                         &tagConfig),
+            AssertionException,
+            51024);
+    }
+}
+
 TEST(MemberConfig, ParseAcceptsAnyNumberSlaveDelay) {
     ReplSetTagConfig tagConfig;
     {
@@ -722,28 +770,28 @@ TEST(MemberConfig, ParseAcceptsAnyNumberSlaveDelay) {
                                    << "h"
                                    << "priority" << 0 << "slaveDelay" << 0),
                         &tagConfig);
-        ASSERT_EQUALS(mc.getSlaveDelay(), Seconds(0));
+        ASSERT_EQUALS(mc.getSecondaryDelay(), Seconds(0));
     }
     {
         MemberConfig mc(BSON("_id" << 1 << "host"
                                    << "h"
                                    << "priority" << 0 << "slaveDelay" << 0.5),
                         &tagConfig);
-        ASSERT_EQUALS(mc.getSlaveDelay(), Seconds(0));
+        ASSERT_EQUALS(mc.getSecondaryDelay(), Seconds(0));
     }
     {
         MemberConfig mc(BSON("_id" << 1 << "host"
                                    << "h"
                                    << "priority" << 0 << "slaveDelay" << -0.5),
                         &tagConfig);
-        ASSERT_EQUALS(mc.getSlaveDelay(), Seconds(0));
+        ASSERT_EQUALS(mc.getSecondaryDelay(), Seconds(0));
     }
     {
         MemberConfig mc(BSON("_id" << 1 << "host"
                                    << "h"
                                    << "priority" << 0 << "slaveDelay" << 1),
                         &tagConfig);
-        ASSERT_EQUALS(mc.getSlaveDelay(), Seconds(1));
+        ASSERT_EQUALS(mc.getSecondaryDelay(), Seconds(1));
     }
     {
         ASSERT_THROWS_CODE(MemberConfig(BSON("_id" << 1 << "host"
@@ -758,14 +806,69 @@ TEST(MemberConfig, ParseAcceptsAnyNumberSlaveDelay) {
                                    << "h"
                                    << "priority" << 0 << "slaveDelay" << 1.6),
                         &tagConfig);
-        ASSERT_EQUALS(mc.getSlaveDelay(), Seconds(1));
+        ASSERT_EQUALS(mc.getSecondaryDelay(), Seconds(1));
     }
     {
         MemberConfig mc(BSON("_id" << 1 << "host"
                                    << "h"
                                    << "priority" << 0 << "slaveDelay" << 4),
                         &tagConfig);
-        ASSERT_EQUALS(mc.getSlaveDelay(), Seconds(4));
+        ASSERT_EQUALS(mc.getSecondaryDelay(), Seconds(4));
+    }
+}
+
+TEST(MemberConfig, ParseAcceptsAnyNumberSecondaryDelay) {
+    ReplSetTagConfig tagConfig;
+    {
+        MemberConfig mc(BSON("_id" << 1 << "host"
+                                   << "h"
+                                   << "priority" << 0 << "secondaryDelaySecs" << 0),
+                        &tagConfig);
+        ASSERT_EQUALS(mc.getSecondaryDelay(), Seconds(0));
+    }
+    {
+        MemberConfig mc(BSON("_id" << 1 << "host"
+                                   << "h"
+                                   << "priority" << 0 << "secondaryDelaySecs" << 0.5),
+                        &tagConfig);
+        ASSERT_EQUALS(mc.getSecondaryDelay(), Seconds(0));
+    }
+    {
+        MemberConfig mc(BSON("_id" << 1 << "host"
+                                   << "h"
+                                   << "priority" << 0 << "secondaryDelaySecs" << -0.5),
+                        &tagConfig);
+        ASSERT_EQUALS(mc.getSecondaryDelay(), Seconds(0));
+    }
+    {
+        MemberConfig mc(BSON("_id" << 1 << "host"
+                                   << "h"
+                                   << "priority" << 0 << "secondaryDelaySecs" << 1),
+                        &tagConfig);
+        ASSERT_EQUALS(mc.getSecondaryDelay(), Seconds(1));
+    }
+    {
+        ASSERT_THROWS_CODE(
+            MemberConfig(BSON("_id" << 1 << "host"
+                                    << "h"
+                                    << "priority" << 0 << "secondaryDelaySecs" << -1.5),
+                         &tagConfig),
+            AssertionException,
+            51024);
+    }
+    {
+        MemberConfig mc(BSON("_id" << 0 << "host"
+                                   << "h"
+                                   << "priority" << 0 << "secondaryDelaySecs" << 1.6),
+                        &tagConfig);
+        ASSERT_EQUALS(mc.getSecondaryDelay(), Seconds(1));
+    }
+    {
+        MemberConfig mc(BSON("_id" << 1 << "host"
+                                   << "h"
+                                   << "priority" << 0 << "secondaryDelaySecs" << 4),
+                        &tagConfig);
+        ASSERT_EQUALS(mc.getSecondaryDelay(), Seconds(4));
     }
 }
 
@@ -990,6 +1093,15 @@ TEST(MemberConfig, ValidatePriorityAndSlaveDelayRelationship) {
                   ExceptionFor<ErrorCodes::BadValue>);
 }
 
+TEST(MemberConfig, ValidatePriorityAndSecondaryDelayRelationship) {
+    ReplSetTagConfig tagConfig;
+    ASSERT_THROWS(MemberConfig(BSON("_id" << 0 << "host"
+                                          << "h"
+                                          << "priority" << 1 << "secondaryDelaySecs" << 60),
+                               &tagConfig),
+                  ExceptionFor<ErrorCodes::BadValue>);
+}
+
 TEST(MemberConfig, ValidatePriorityAndHiddenRelationship) {
     ReplSetTagConfig tagConfig;
     {
diff --git a/src/mongo/db/repl/repl_server_parameters.idl b/src/mongo/db/repl/repl_server_parameters.idl
index 0fcbefb7a60..a1c56d2c6f7 100644
--- a/src/mongo/db/repl/repl_server_parameters.idl
+++ b/src/mongo/db/repl/repl_server_parameters.idl
@@ -450,3 +450,9 @@ feature_flags:
             When enabled, tenant migration commands are supported.
         cpp_varname: feature_flags::gTenantMigrations
         default: false
+
+    featureFlagUseSecondaryDelaySecs:
+        description: >-
+            If enabled, use field secondaryDelaySecs instead of slaveDelay in MemberConfig.
+        cpp_varname: feature_flags::gUseSecondaryDelaySecs
+        default: false
\ No newline at end of file
diff --git a/src/mongo/db/repl/repl_set_config.cpp b/src/mongo/db/repl/repl_set_config.cpp
index 9fb2c00aea6..c40b7428203 100644
--- a/src/mongo/db/repl/repl_set_config.cpp
+++ b/src/mongo/db/repl/repl_set_config.cpp
@@ -97,6 +97,18 @@ ReplSetConfig ReplSetConfig::parseForInitiate(const BSONObj& cfg, OID newReplica
     return result;
 }
 
+void ReplSetConfig::setDefaultDelayFieldForMember(MemberConfig mem) {
+    // We should only be setting the default values if the config has neither delay field set.
+    if (!mem.hasSecondaryDelaySecs() && !mem.hasSlaveDelay()) {
+        if (feature_flags::gUseSecondaryDelaySecs.isEnabled(
+                serverGlobalParams.featureCompatibility)) {
+            useSecondaryDelaySecsFieldName(mem.getId());
+        } else {
+            useSlaveDelayFieldName(mem.getId());
+        }
+    }
+}
+
 void ReplSetConfig::_setRequiredFields() {
     // The three required fields need to be set to something valid to avoid a potential
     // invariant if the uninitialized object is ever used with toBSON().
@@ -144,6 +156,8 @@ Status ReplSetConfig::_initialize(bool forInitiate,
         // The const_cast is necessary because "non_const_getter" in the IDL doesn't work for
         // arrays.
         const_cast<MemberConfig&>(member).addTagInfo(&_tagConfig);
+
+        setDefaultDelayFieldForMember(member);
     }
 
     //
@@ -252,6 +266,12 @@ Status ReplSetConfig::validate() const {
             }
         }
 
+        if (memberI.hasSlaveDelay() && memberI.hasSecondaryDelaySecs()) {
+            return Status(ErrorCodes::BadValue,
+                          "Cannot specify both secondaryDelaySecs and slaveDelay as fields in "
+                          "member configuration.");
+        }
+
         if (memberI.getHostAndPort().isLocalHost()) {
             ++localhostCount;
         }
@@ -377,10 +397,10 @@ Status ReplSetConfig::validate() const {
                               "Members in replica set configurations being used for config "
                               "servers must build indexes");
             }
-            if (mem->getSlaveDelay() != Seconds(0)) {
+            if (mem->getSecondaryDelay() != Seconds(0)) {
                 return Status(ErrorCodes::BadValue,
                               "Members in replica set configurations being used for config "
-                              "servers cannot have a non-zero slaveDelay");
+                              "servers cannot have a non-zero secondaryDelaySecs");
             }
         }
         if (serverGlobalParams.clusterRole != ClusterRole::ConfigServer &&
@@ -709,5 +729,23 @@ void MutableReplSetConfig::removeNewlyAddedFieldForMember(MemberId memberId) {
     _findMemberByID(memberId)->setNewlyAdded(boost::none);
 }
 
+void MutableReplSetConfig::useSecondaryDelaySecsFieldName(MemberId memberId) {
+    auto mem = _findMemberByID(memberId);
+    if (mem->hasSecondaryDelaySecs()) {
+        return;
+    }
+    mem->setSecondaryDelaySecs(mem->hasSlaveDelay() ? mem->getSlaveDelaySecs() : 0LL);
+    mem->setSlaveDelaySecs(boost::none);
+}
+
+void MutableReplSetConfig::useSlaveDelayFieldName(MemberId memberId) {
+    auto mem = _findMemberByID(memberId);
+    if (mem->hasSlaveDelay()) {
+        return;
+    }
+    mem->setSlaveDelaySecs(mem->hasSecondaryDelaySecs() ? mem->getSecondaryDelaySecs() : 0LL);
+    mem->setSecondaryDelaySecs(boost::none);
+}
+
 }  // namespace repl
 }  // namespace mongo
diff --git a/src/mongo/db/repl/repl_set_config.h b/src/mongo/db/repl/repl_set_config.h
index 8f2ae9d04be..5148d0fa8ea 100644
--- a/src/mongo/db/repl/repl_set_config.h
+++ b/src/mongo/db/repl/repl_set_config.h
@@ -131,6 +131,20 @@ public:
      */
     void removeNewlyAddedFieldForMember(MemberId memberId);
 
+    /**
+     * Sets the member config's 'secondaryDelaySecs' to the value of 'slaveDelay' and removes the
+     * 'slaveDelay' field entirely. If 'slaveDelay' is not set, then sets 'secondaryDelaySecs' to
+     * the default value.
+     */
+    void useSecondaryDelaySecsFieldName(MemberId memberId);
+
+    /**
+     * Sets the member config's 'slaveDelay' to the value of 'secondaryDelaySecs' and removes the
+     * 'secondaryDelaySecs' field entirely. If 'secondaryDelaySecs' is not set, then sets
+     * 'slaveDelay' to the default value.
+     */
+    void useSlaveDelayFieldName(MemberId memberId);
+
 protected:
     MutableReplSetConfig() = default;
 
@@ -218,6 +232,13 @@ public:
     static ReplSetConfig parseForInitiate(const BSONObj& cfg, OID newReplicaSetId);
 
     /**
+     * Sets the default delay field name for a member config based on feature compatibility version,
+     * but only if the member config has neither 'secondaryDelaySecs' nor 'slaveDelay' already set.
+     * This function is used when constructing 'ReplSetConfigs' for initiate and reconfig.
+     */
+    void setDefaultDelayFieldForMember(MemberConfig mem);
+
+    /**
      * Returns true if this object has been successfully initialized or copied from
      * an initialized object.
      */
diff --git a/src/mongo/db/repl/repl_set_config_checks.cpp b/src/mongo/db/repl/repl_set_config_checks.cpp
index b768317f041..20ad4106e2f 100644
--- a/src/mongo/db/repl/repl_set_config_checks.cpp
+++ b/src/mongo/db/repl/repl_set_config_checks.cpp
@@ -34,6 +34,7 @@
 #include <algorithm>
 #include <iterator>
 
+#include "mongo/db/repl/repl_server_parameters_gen.h"
 #include "mongo/db/repl/repl_set_config.h"
 #include "mongo/db/repl/replication_coordinator_external_state.h"
 #include "mongo/db/service_context.h"
@@ -98,6 +99,54 @@ Status ensureNoNewlyAddedMembers(const ReplSetConfig& config) {
 }
 
 /**
+ * Checks that the current feature compatibility version is compatible with
+ * each member config's delay field name. If the feature flag 'featureFlagUseSecondaryDelaySecs' is
+ * enabled, the nodes must use the 'secondaryDelaySecs' field instead of the 'slaveDelay' field.
+ */
+Status isFCVCompatible(const ReplSetConfig& config) {
+    auto version = serverGlobalParams.featureCompatibility.getVersion();
+    // TODO (SERVER-53354) If we are currently upgrading, we check if the feature flag is enabled
+    // for the target version. We use the generic FCV references here to avoid having to update the
+    // FCV constants used after each continuous release. After release, we should make sure to
+    // remove these references while removing the feature flag.
+    //
+    //(Generic FCV reference): feature flag support
+    if (version == ServerGlobalParams::FeatureCompatibility::kUpgradingFromLastLTSToLatest ||
+        version == ServerGlobalParams::FeatureCompatibility::kUpgradingFromLastContinuousToLatest) {
+        version = ServerGlobalParams::FeatureCompatibility::kLatest;
+    }
+
+    ServerGlobalParams::FeatureCompatibility targetFCV;
+    targetFCV.setVersion(version);
+    bool isEnabled = feature_flags::gUseSecondaryDelaySecs.isEnabled(targetFCV);
+    // We must check that every member config has a valid delay field name.
+    for (auto iter = config.membersBegin(); iter != config.membersEnd(); ++iter) {
+        if ((isEnabled && iter->hasSlaveDelay()) || (!isEnabled && iter->hasSecondaryDelaySecs())) {
+            // TODO (SERVER-53354) If the feature flag is disabled, getVersion() will throw. In this
+            // case, the version should default to kLatest. We use the generic FCV references here
+            // to avoid having to update the FCV constants used after each continuous release. After
+            // release, we should make sure to remove these references while removing the feature
+            // flag.
+            //
+            //(Generic FCV reference): feature flag support
+            auto featureFlagVersion = isEnabled
+                ? FeatureCompatibilityVersionParser::toString(
+                      feature_flags::gUseSecondaryDelaySecs.getVersion())
+                : FeatureCompatibilityVersionParser::toString(
+                      ServerGlobalParams::FeatureCompatibility::kLatest);
+            return Status(ErrorCodes::BadValue,
+                          str::stream() << "If the node is in FCV " << featureFlagVersion
+                                        << ", we must use the secondaryDelaySecs field name "
+                                           "instead of slaveDelay. Only nodes below FCV "
+                                        << featureFlagVersion << " should use slaveDelay.");
+        }
+    }
+
+
+    return Status::OK();
+}
+
+/**
  * Compares two initialized and validated replica set configurations and checks to see if the
  * transition from 'oldConfig' to 'newConfig' adds or removes at most 1 voting node.
  *
@@ -331,6 +380,11 @@ StatusWith<int> validateConfigForInitiate(ReplicationCoordinatorExternalState* e
         return StatusWith<int>(status);
     }
 
+    status = isFCVCompatible(newConfig);
+    if (!status.isOK()) {
+        return StatusWith<int>(status);
+    }
+
     status = newConfig.checkIfWriteConcernCanBeSatisfied(newConfig.getDefaultWriteConcern());
     if (!status.isOK()) {
         return status.withContext(
@@ -370,6 +424,11 @@ Status validateConfigForReconfig(const ReplSetConfig& oldConfig,
         return status;
     }
 
+    status = isFCVCompatible(newConfig);
+    if (!status.isOK()) {
+        return status;
+    }
+
     status = newConfig.checkIfWriteConcernCanBeSatisfied(newConfig.getDefaultWriteConcern());
     if (!status.isOK()) {
         return status.withContext(
diff --git a/src/mongo/db/repl/repl_set_config_test.cpp b/src/mongo/db/repl/repl_set_config_test.cpp
index 16618f56cbf..2606bd06c42 100644
--- a/src/mongo/db/repl/repl_set_config_test.cpp
+++ b/src/mongo/db/repl/repl_set_config_test.cpp
@@ -820,6 +820,23 @@ TEST(ReplSetConfig, ValidateFailsWithDuplicateMemberId) {
     ASSERT_EQUALS(ErrorCodes::BadValue, status);
 }
 
+TEST(ReplSetConfig, ValidateFailsWithBothDelaySecsFieldNames) {
+    ReplSetConfig config(ReplSetConfig::parse(
+        BSON("_id"
+             << "rs0"
+             << "protocolVersion" << 1 << "version" << 1 << "configsvr" << true << "members"
+             << BSON_ARRAY(BSON("_id" << 0 << "host"
+                                      << "localhost:12345")
+                           << BSON("_id" << 1 << "host"
+                                         << "localhost:54321"
+                                         << "priority" << 0 << "secondaryDelaySecs" << 10
+                                         << "slaveDelay" << 10)))));
+    Status status = config.validate();
+    ASSERT_EQUALS(ErrorCodes::BadValue, status);
+    ASSERT_STRING_CONTAINS(status.reason(),
+                           "Cannot specify both secondaryDelaySecs and slaveDelay");
+}
+
 TEST(ReplSetConfig, InitializeFailsWithInvalidMember) {
     ReplSetConfig config;
     ASSERT_THROWS(ReplSetConfig::parse(BSON("_id"
@@ -1255,7 +1272,7 @@ bool operator==(const MemberConfig& a, const MemberConfig& b) {
         }
     }
     return a.getId() == b.getId() && a.getHostAndPort() == b.getHostAndPort() &&
-        a.getPriority() == b.getPriority() && a.getSlaveDelay() == b.getSlaveDelay() &&
+        a.getPriority() == b.getPriority() && a.getSecondaryDelay() == b.getSecondaryDelay() &&
         a.isVoter() == b.isVoter() && a.isArbiter() == b.isArbiter() &&
         a.isNewlyAdded() == b.isNewlyAdded() && a.isHidden() == b.isHidden() &&
         a.shouldBuildIndexes() == b.shouldBuildIndexes() && a.getNumTags() == b.getNumTags() &&
@@ -1365,8 +1382,8 @@ TEST(ReplSetConfig, toBSONRoundTripAbilityLarge) {
                       << BSON("_id" << 3 << "host"
                                     << "localhost:3828"
                                     << "arbiterOnly" << false << "hidden" << true << "buildIndexes"
-                                    << false << "priority" << 0 << "slaveDelay" << 17 << "votes"
-                                    << 0 << "newlyAdded" << true << "tags"
+                                    << false << "priority" << 0 << "secondaryDelaySecs" << 17
+                                    << "votes" << 0 << "newlyAdded" << true << "tags"
                                     << BSON("coast"
                                             << "east"
                                             << "ssd"
@@ -1558,21 +1575,20 @@ TEST(ReplSetConfig, CheckConfigServerMustBuildIndexes) {
     ASSERT_STRING_CONTAINS(status.reason(), "must build indexes");
 }
 
-TEST(ReplSetConfig, CheckConfigServerCantHaveSlaveDelay) {
+TEST(ReplSetConfig, CheckConfigServerCantHaveSecondaryDelaySecs) {
     ReplSetConfig configA;
-    configA = ReplSetConfig::parse(BSON("_id"
-                                        << "rs0"
-                                        << "protocolVersion" << 1 << "version" << 1 << "configsvr"
-                                        << true << "members"
-                                        << BSON_ARRAY(BSON("_id" << 0 << "host"
-                                                                 << "localhost:12345")
-                                                      << BSON("_id" << 1 << "host"
-                                                                    << "localhost:54321"
-                                                                    << "priority" << 0
-                                                                    << "slaveDelay" << 3))));
+    configA = ReplSetConfig::parse(
+        BSON("_id"
+             << "rs0"
+             << "protocolVersion" << 1 << "version" << 1 << "configsvr" << true << "members"
+             << BSON_ARRAY(BSON("_id" << 0 << "host"
+                                      << "localhost:12345")
+                           << BSON("_id" << 1 << "host"
+                                         << "localhost:54321"
+                                         << "priority" << 0 << "secondaryDelaySecs" << 3))));
     Status status = configA.validate();
     ASSERT_EQUALS(ErrorCodes::BadValue, status);
-    ASSERT_STRING_CONTAINS(status.reason(), "cannot have a non-zero slaveDelay");
+    ASSERT_STRING_CONTAINS(status.reason(), "cannot have a non-zero secondaryDelaySecs");
 }
 
 TEST(ReplSetConfig, CheckConfigServerMustHaveTrueForWriteConcernMajorityJournalDefault) {
diff --git a/src/mongo/db/repl/replication_coordinator_impl.cpp b/src/mongo/db/repl/replication_coordinator_impl.cpp
index cde642c1fe9..fe58a8fc366 100644
--- a/src/mongo/db/repl/replication_coordinator_impl.cpp
+++ b/src/mongo/db/repl/replication_coordinator_impl.cpp
@@ -1041,7 +1041,7 @@ Seconds ReplicationCoordinatorImpl::getSecondaryDelaySecs() const {
         // queue of work.
         return Seconds(0);
     }
-    return _rsConfig.getMemberAt(_selfIndex).getSlaveDelay();
+    return _rsConfig.getMemberAt(_selfIndex).getSecondaryDelay();
 }
 
 void ReplicationCoordinatorImpl::clearSyncSourceBlacklist() {
diff --git a/src/mongo/db/repl/topology_coordinator.cpp b/src/mongo/db/repl/topology_coordinator.cpp
index e98b19e20f8..e7bf9e67d2e 100644
--- a/src/mongo/db/repl/topology_coordinator.cpp
+++ b/src/mongo/db/repl/topology_coordinator.cpp
@@ -483,13 +483,14 @@ bool TopologyCoordinator::_isEligibleSyncSource(int candidateIndex,
             return false;
         }
         // Candidate must not have a configured delay larger than ours.
-        if (_selfConfig().getSlaveDelay() < memberConfig.getSlaveDelay()) {
+        if (_selfConfig().getSecondaryDelay() < memberConfig.getSecondaryDelay()) {
             LOGV2_DEBUG(3873111,
                         2,
-                        "Cannot select sync source with larger slaveDelay than ours",
+                        "Cannot select sync source with larger secondaryDelaySecs than ours",
                         "syncSourceCandidate"_attr = syncSourceCandidate,
-                        "syncSourceCandidateSlaveDelay"_attr = memberConfig.getSlaveDelay(),
-                        "slaveDelay"_attr = _selfConfig().getSlaveDelay());
+                        "syncSourceCandidateSecondaryDelaySecs"_attr =
+                            memberConfig.getSecondaryDelay(),
+                        "secondaryDelaySecs"_attr = _selfConfig().getSecondaryDelay());
             return false;
         }
     }
@@ -2096,7 +2097,7 @@ void TopologyCoordinator::fillHelloForReplSet(std::shared_ptr<HelloResponse> res
     invariant(!_rsConfig.members().empty());
 
     for (const auto& member : _rsConfig.members()) {
-        if (member.isHidden() || member.getSlaveDelay() > Seconds{0}) {
+        if (member.isHidden() || member.getSecondaryDelay() > Seconds{0}) {
             continue;
         }
         auto hostView = member.getHostAndPort(horizonString);
@@ -2130,8 +2131,8 @@ void TopologyCoordinator::fillHelloForReplSet(std::shared_ptr<HelloResponse> res
     } else if (selfConfig.getPriority() == 0) {
         response->setIsPassive(true);
     }
-    if (selfConfig.getSlaveDelay() > Seconds(0)) {
-        response->setSecondaryDelaySecs(selfConfig.getSlaveDelay());
+    if (selfConfig.getSecondaryDelay() > Seconds(0)) {
+        response->setSecondaryDelaySecs(selfConfig.getSecondaryDelay());
     }
     if (selfConfig.isHidden()) {
         response->setIsHidden(true);
@@ -3098,8 +3099,8 @@ bool TopologyCoordinator::shouldChangeSyncSourceDueToPingTime(const HostAndPort&
         return false;
     }
 
-    // If we are configured with slaveDelay, do not re-evaluate our sync source.
-    if (_selfIndex == -1 || _selfConfig().getSlaveDelay() > Seconds(0)) {
+    // If we are configured with secondaryDelaySecs, do not re-evaluate our sync source.
+    if (_selfIndex == -1 || _selfConfig().getSecondaryDelay() > Seconds(0)) {
         return false;
     }
 
diff --git a/src/mongo/db/repl/topology_coordinator_v1_test.cpp b/src/mongo/db/repl/topology_coordinator_v1_test.cpp
index 83d8af17643..bef6dc6238d 100644
--- a/src/mongo/db/repl/topology_coordinator_v1_test.cpp
+++ b/src/mongo/db/repl/topology_coordinator_v1_test.cpp
@@ -434,7 +434,7 @@ TEST_F(TopoCoordTest, NodeReturnsClosestValidSyncSourceAsSyncSource) {
                                          << "arbiterOnly" << true)
                            << BSON("_id" << 50 << "host"
                                          << "h5"
-                                         << "slaveDelay" << 1 << "priority" << 0)
+                                         << "secondaryDelaySecs" << 1 << "priority" << 0)
                            << BSON("_id" << 60 << "host"
                                          << "h6")
                            << BSON("_id" << 70 << "host"
@@ -4360,13 +4360,13 @@ TEST_F(TopoCoordTest, DontChangeDueToPingTimeWhenCandidatePingTimeIsMissing) {
                                                                     ReadPreference::Nearest));
 }
 
-TEST_F(ReevalSyncSourceTest, NoChangeWhenNodeConfiguredWithSlaveDelay) {
+TEST_F(ReevalSyncSourceTest, NoChangeWhenNodeConfiguredWithSecondaryDelaySecs) {
     updateConfig(BSON("_id"
                       << "rs0"
                       << "version" << 5 << "term" << 1 << "members"
                       << BSON_ARRAY(BSON("_id" << 0 << "host"
                                                << "host1:27017"
-                                               << "slaveDelay" << 1 << "priority" << 0)
+                                               << "secondaryDelaySecs" << 1 << "priority" << 0)
                                     << BSON("_id" << 1 << "host"
                                                   << "host2:27017")
                                     << BSON("_id" << 2 << "host"
@@ -4374,7 +4374,8 @@ TEST_F(ReevalSyncSourceTest, NoChangeWhenNodeConfiguredWithSlaveDelay) {
                       << "protocolVersion" << 1),
                  0);
 
-    // Set up so that without slaveDelay, the node otherwise would have changed sync sources.
+    // Set up so that without secondaryDelaySecs, the node otherwise would have changed sync
+    // sources.
     getTopoCoord().setPing_forTest(HostAndPort("host2"), pingTime);
     getTopoCoord().setPing_forTest(HostAndPort("host3"), significantlyCloserPingTime);
 
@@ -4391,7 +4392,7 @@ TEST_F(ReevalSyncSourceTest, NoChangeWhenNodeNotFoundInConfig) {
                       << "version" << 5 << "term" << 1 << "members"
                       << BSON_ARRAY(BSON("_id" << 0 << "host"
                                                << "host1:27017"
-                                               << "slaveDelay" << 1 << "priority" << 0)
+                                               << "secondaryDelaySecs" << 1 << "priority" << 0)
                                     << BSON("_id" << 1 << "host"
                                                   << "host2:27017")
                                     << BSON("_id" << 2 << "host"
@@ -4399,8 +4400,8 @@ TEST_F(ReevalSyncSourceTest, NoChangeWhenNodeNotFoundInConfig) {
                       << "protocolVersion" << 1),
                  -1);
 
-    // Set up so that without slaveDelay and not being in the config, the node otherwise would have
-    // changed sync sources.
+    // Set up so that without secondaryDelaySecs and not being in the config, the node otherwise
+    // would have changed sync sources.
     getTopoCoord().setPing_forTest(HostAndPort("host2"), pingTime);
     getTopoCoord().setPing_forTest(HostAndPort("host3"), significantlyCloserPingTime);
 
diff --git a/src/mongo/dbtests/mock/mock_replica_set.cpp b/src/mongo/dbtests/mock/mock_replica_set.cpp
index d7dfb75aec1..53fd2d0a8cd 100644
--- a/src/mongo/dbtests/mock/mock_replica_set.cpp
+++ b/src/mongo/dbtests/mock/mock_replica_set.cpp
@@ -248,9 +248,9 @@ void MockReplicaSet::mockIsMasterCmd() {
                 builder.append("passive", true);
             }
 
-            if (member->getSlaveDelay().count()) {
-                builder.appendIntOrLL("slaveDelay",
-                                      durationCount<Seconds>(member->getSlaveDelay()));
+            if (member->getSecondaryDelay().count()) {
+                builder.appendIntOrLL("secondaryDelaySecs",
+                                      durationCount<Seconds>(member->getSecondaryDelay()));
             }
 
             if (member->isHidden()) {