SERVER-44818 Change the balancerCollectionStatus command response format

jstests/sharding/balancer_collection_status.js

@@ -44,7 +44,7 @@ assert.commandFailedWithCode(st.s0.adminCommand({balancerCollectionStatus: 'db.c
 var result = assert.commandWorked(st.s0.adminCommand({balancerCollectionStatus: 'db.col'}));
 
 // new collections must be balanced
-assert.eq(result.status, 'balanced');
+assert.eq(result.balancerCompliant, true);
 
 // get shardIds
 var shards = st.s0.getDB('config').shards.find().toArray();
@@ -60,7 +60,8 @@ assert.commandWorked(st.s0.adminCommand({moveChunk: 'db.col', find: {key: 20}, t
 result = assert.commandWorked(st.s0.adminCommand({balancerCollectionStatus: 'db.col'}));
 
 // chunksImbalanced expected
-assert.eq(result.status, 'chunksImbalance');
+assert.eq(result.balancerCompliant, false);
+assert.eq(result.firstComplianceViolation, 'chunksImbalance');
 
 // run balancer with 3 rounds
 runBalancer(3);
@@ -68,7 +69,7 @@ runBalancer(3);
 // the chunks must be balanced now
 result = assert.commandWorked(st.s0.adminCommand({balancerCollectionStatus: 'db.col'}));
 
-assert.eq(result.status, 'balanced');
+assert.eq(result.balancerCompliant, true);
 
 // manually move a chunk to a shard before creating zones (this will help
 // testing the zone violation)
@@ -86,7 +87,8 @@ assert.commandWorked(st.s0.adminCommand(
 result = assert.commandWorked(st.s0.adminCommand({balancerCollectionStatus: 'db.col'}));
 
 // having a chunk on a different zone will cause a zone violation
-assert.eq(result.status, 'zoneViolation');
+assert.eq(result.balancerCompliant, false);
+assert.eq(result.firstComplianceViolation, 'zoneViolation');
 
 // run balancer, we don't know exactly where the first run moved the chunks
 // so lets run 3 rounds just in case
@@ -95,8 +97,8 @@ runBalancer(3);
 // the chunks must be balanced now
 result = assert.commandWorked(st.s0.adminCommand({balancerCollectionStatus: 'db.col'}));
 
-// final check: all chunks are balanced and in the correct zone
-assert.eq(result.status, 'balanced');
+// All chunks are balanced and in the correct zone
+assert.eq(result.balancerCompliant, true);
 
 st.stop();
 })();

src/mongo/db/s/balancer/balancer.cpp

@@ -83,7 +83,6 @@ const auto getBalancer = ServiceContext::declareDecoration<std::unique_ptr<Balan
 /**
  * Balancer status response
  */
-static constexpr StringData kBalancerPolicyStatusBalanced = "balanced"_sd;
 static constexpr StringData kBalancerPolicyStatusDraining = "draining"_sd;
 static constexpr StringData kBalancerPolicyStatusZoneViolation = "zoneViolation"_sd;
 static constexpr StringData kBalancerPolicyStatusChunksImbalance = "chunksImbalance"_sd;
@@ -700,27 +699,28 @@ void Balancer::notifyPersistedBalancerSettingsChanged() {
     _condVar.notify_all();
 }
 
-StringData Balancer::getBalancerStatusForNs(OperationContext* opCtx, const NamespaceString& ns) {
+Balancer::BalancerStatus Balancer::getBalancerStatusForNs(OperationContext* opCtx,
+                                                          const NamespaceString& ns) {
     auto splitChunks = uassertStatusOK(_chunkSelectionPolicy->selectChunksToSplit(opCtx, ns));
     if (!splitChunks.empty()) {
-        return kBalancerPolicyStatusZoneViolation;
+        return {false, kBalancerPolicyStatusZoneViolation.toString()};
     }
     auto chunksToMove = uassertStatusOK(_chunkSelectionPolicy->selectChunksToMove(opCtx, ns));
     if (chunksToMove.empty()) {
-        return kBalancerPolicyStatusBalanced;
+        return {true, boost::none};
     }
     const auto& migrationInfo = chunksToMove.front();
 
     switch (migrationInfo.reason) {
         case MigrateInfo::drain:
-            return kBalancerPolicyStatusDraining;
+            return {false, kBalancerPolicyStatusDraining.toString()};
         case MigrateInfo::zoneViolation:
-            return kBalancerPolicyStatusZoneViolation;
+            return {false, kBalancerPolicyStatusZoneViolation.toString()};
         case MigrateInfo::chunksImbalance:
-            return kBalancerPolicyStatusChunksImbalance;
+            return {false, kBalancerPolicyStatusChunksImbalance.toString()};
     }
 
-    return kBalancerPolicyStatusBalanced;
+    return {true, boost::none};
 }
 
 }  // namespace mongo

src/mongo/db/s/balancer/balancer.h

@@ -149,11 +149,15 @@ public:
      */
     void notifyPersistedBalancerSettingsChanged();
 
+    struct BalancerStatus {
+        bool balancerCompliant;
+        boost::optional<std::string> firstComplianceViolation;
+    };
     /**
      * Returns if a given collection is draining due to a removed shard, has chunks on an invalid
      * zone or the number of chunks is imbalanced across the cluster
      */
-    StringData getBalancerStatusForNs(OperationContext* opCtx, const NamespaceString& nss);
+    BalancerStatus getBalancerStatusForNs(OperationContext* opCtx, const NamespaceString& nss);
 
 private:
     /**

src/mongo/db/s/config/configsvr_balancer_collection_status_command.cpp

@@ -75,8 +75,14 @@ public:
             uassert(ErrorCodes::InvalidNamespace,
                     str::stream() << "Invalid namespace specified '" << nss.ns() << "'",
                     nss.isValid());
-
-            return Response(Balancer::get(opCtx)->getBalancerStatusForNs(opCtx, nss).toString());
+            const auto& balancerStatus = Balancer::get(opCtx)->getBalancerStatusForNs(opCtx, nss);
+            Response response(balancerStatus.balancerCompliant);
+            response.setFirstComplianceViolation(
+                balancerStatus.firstComplianceViolation.is_initialized()
+                    ? boost::optional<StringData>(
+                          StringData(*balancerStatus.firstComplianceViolation))
+                    : boost::optional<StringData>(boost::none));
+            return response;
         }
 
     private:

src/mongo/s/request_types/balancer_collection_status.idl

@@ -39,9 +39,13 @@ structs:
         description: "Response of the config server command"
         strict: false
         fields:
-            status:
+            balancerCompliant:
+                type: bool
+                description: "true if there are no actions needed, if false, then firstComplianceViolation will contain the violation with the highest priority which will be addressed next"
+            firstComplianceViolation:
                 type: string
-                description: "One of the following: balanced, draining, zoneViolation or chunksImbalance"
+                optional: true
+                description: "One of the following: draining, zoneViolation or chunksImbalance"
 
 commands:
     balancerCollectionStatus: