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  1. Core Server
  2. SERVER-9858

After a chunk migration, requests on secondaries return multiple objects with same _id

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    Details

    • Type: Bug
    • Status: Closed
    • Priority: Major - P3
    • Resolution: Duplicate
    • Affects Version/s: 2.4.3
    • Fix Version/s: None
    • Component/s: Replication, Sharding
    • Environment:
      Ubuntu 12.10, 3 replica set shards, each replica set contains 3 machines
    • Operating System:
      Linux
    • Steps To Reproduce:
      Hide

      We can reproduce this bug everytime we try to retrieve a duplicated object, but we don't know how to reproduce the all situation.

      Show
      We can reproduce this bug everytime we try to retrieve a duplicated object, but we don't know how to reproduce the all situation.

      Description

      After a chunk migration, there are some objects that have been moved to another shard, that are still present on the old one.
      When we query these objects on a mongo shell, reading from the primaries, everything seems fine, but if we read from the secondaries, it appears that these objects are duplicated.

      Shell behavior:

      mongos> db.ig_medias.find({ mid: "421587931535525217_30569368" }, { mid: 1, "cnt.lke": 1 })
      { "_id" : ObjectId("51939a0041002e238ff315d2"), "mid" : "421587931535525217_30569368", "cnt" : { "lke" : 12 } }
       
      mongos> db.ig_medias.find({ mid: "421587931535525217_30569368" }, { mid: 1, "cnt.lke": 1 }).readPref('secondary')
      { "_id" : ObjectId("51939a0041002e238ff315d2"), "mid" : "421587931535525217_30569368", "cnt" : { "lke" : 12 } }
      { "_id" : ObjectId("51939a0041002e238ff315d2"), "mid" : "421587931535525217_30569368", "cnt" : { "lke" : 12 } }

      As you can see, there are two objects that are exactly the same when reading from secondaries.
      Here are the same queries with explain():

      mongos> db.ig_medias.find({ mid: "421587931535525217_30569368" }, { mid: 1, "cnt.lke": 1 }).explain()
      {
      	"clusteredType" : "ParallelSort",
      	"shards" : {
      		"str1/str1-1.nitrogr.am:27017,str1-2.nitrogr.am:27017,str1-3.nitrogr.am:27017" : [
      			{
      				"cursor" : "BtreeCursor mid_1",
      				"isMultiKey" : false,
      				"n" : 0,
      				"nscannedObjects" : 0,
      				"nscanned" : 0,
      				"nscannedObjectsAllPlans" : 0,
      				"nscannedAllPlans" : 0,
      				"scanAndOrder" : false,
      				"indexOnly" : false,
      				"nYields" : 0,
      				"nChunkSkips" : 0,
      				"millis" : 0,
      				"indexBounds" : {
      					"mid" : [
      						[
      							"421587931535525217_30569368",
      							"421587931535525217_30569368"
      						]
      					]
      				},
      				"server" : "str1-1.nitrogr.am:27017"
      			}
      		],
      		"str2/str2-1.nitrogr.am:27017,str2-2.nitrogr.am:27017,str2-3.nitrogr.am:27017" : [
      			{
      				"cursor" : "BtreeCursor mid_1",
      				"isMultiKey" : false,
      				"n" : 0,
      				"nscannedObjects" : 1,
      				"nscanned" : 1,
      				"nscannedObjectsAllPlans" : 1,
      				"nscannedAllPlans" : 1,
      				"scanAndOrder" : false,
      				"indexOnly" : false,
      				"nYields" : 0,
      				"nChunkSkips" : 1,
      				"millis" : 0,
      				"indexBounds" : {
      					"mid" : [
      						[
      							"421587931535525217_30569368",
      							"421587931535525217_30569368"
      						]
      					]
      				},
      				"server" : "str2-3.nitrogr.am:27017"
      			}
      		],
      		"str3/str3-1.nitrogr.am:27017,str3-2.nitrogr.am:27017,str3-3.nitrogr.am:27017" : [
      			{
      				"cursor" : "BtreeCursor mid_1",
      				"isMultiKey" : false,
      				"n" : 1,
      				"nscannedObjects" : 1,
      				"nscanned" : 1,
      				"nscannedObjectsAllPlans" : 1,
      				"nscannedAllPlans" : 1,
      				"scanAndOrder" : false,
      				"indexOnly" : false,
      				"nYields" : 0,
      				"nChunkSkips" : 0,
      				"millis" : 0,
      				"indexBounds" : {
      					"mid" : [
      						[
      							"421587931535525217_30569368",
      							"421587931535525217_30569368"
      						]
      					]
      				},
      				"server" : "str3-2.nitrogr.am:27017"
      			}
      		]
      	},
      	"cursor" : "BtreeCursor mid_1",
      	"n" : 1,
      	"nChunkSkips" : 1,
      	"nYields" : 0,
      	"nscanned" : 2,
      	"nscannedAllPlans" : 2,
      	"nscannedObjects" : 2,
      	"nscannedObjectsAllPlans" : 2,
      	"millisShardTotal" : 0,
      	"millisShardAvg" : 0,
      	"numQueries" : 3,
      	"numShards" : 3,
      	"millis" : 3
      }
       
      mongos> db.ig_medias.find({ mid: "421587931535525217_30569368" }, { mid: 1, "cnt.lke": 1 }).readPref('secondary').explain()
      {
      	"clusteredType" : "ParallelSort",
      	"shards" : {
      		"str1/str1-1.nitrogr.am:27017,str1-2.nitrogr.am:27017,str1-3.nitrogr.am:27017" : [
      			{
      				"cursor" : "BtreeCursor mid_1",
      				"isMultiKey" : false,
      				"n" : 0,
      				"nscannedObjects" : 0,
      				"nscanned" : 0,
      				"nscannedObjectsAllPlans" : 0,
      				"nscannedAllPlans" : 0,
      				"scanAndOrder" : false,
      				"indexOnly" : false,
      				"nYields" : 0,
      				"nChunkSkips" : 0,
      				"millis" : 0,
      				"indexBounds" : {
      					"mid" : [
      						[
      							"421587931535525217_30569368",
      							"421587931535525217_30569368"
      						]
      					]
      				},
      				"server" : "str1-3.nitrogr.am:27017"
      			}
      		],
      		"str2/str2-1.nitrogr.am:27017,str2-2.nitrogr.am:27017,str2-3.nitrogr.am:27017" : [
      			{
      				"cursor" : "BtreeCursor mid_1",
      				"isMultiKey" : false,
      				"n" : 1,
      				"nscannedObjects" : 1,
      				"nscanned" : 1,
      				"nscannedObjectsAllPlans" : 1,
      				"nscannedAllPlans" : 1,
      				"scanAndOrder" : false,
      				"indexOnly" : false,
      				"nYields" : 0,
      				"nChunkSkips" : 0,
      				"millis" : 0,
      				"indexBounds" : {
      					"mid" : [
      						[
      							"421587931535525217_30569368",
      							"421587931535525217_30569368"
      						]
      					]
      				},
      				"server" : "str2-1.nitrogr.am:27017"
      			}
      		],
      		"str3/str3-1.nitrogr.am:27017,str3-2.nitrogr.am:27017,str3-3.nitrogr.am:27017" : [
      			{
      				"cursor" : "BtreeCursor mid_1",
      				"isMultiKey" : false,
      				"n" : 1,
      				"nscannedObjects" : 1,
      				"nscanned" : 1,
      				"nscannedObjectsAllPlans" : 1,
      				"nscannedAllPlans" : 1,
      				"scanAndOrder" : false,
      				"indexOnly" : false,
      				"nYields" : 0,
      				"nChunkSkips" : 0,
      				"millis" : 0,
      				"indexBounds" : {
      					"mid" : [
      						[
      							"421587931535525217_30569368",
      							"421587931535525217_30569368"
      						]
      					]
      				},
      				"server" : "str3-1b.nitrogr.am:27017"
      			}
      		]
      	},
      	"cursor" : "BtreeCursor mid_1",
      	"n" : 2,
      	"nChunkSkips" : 0,
      	"nYields" : 0,
      	"nscanned" : 2,
      	"nscannedAllPlans" : 2,
      	"nscannedObjects" : 2,
      	"nscannedObjectsAllPlans" : 2,
      	"millisShardTotal" : 0,
      	"millisShardAvg" : 0,
      	"numQueries" : 3,
      	"numShards" : 3,
      	"millis" : 12
      }

      But there is another interesting thing: according to the previous queries, before the chunk migration, the object was on str2 and has been moved to str3. From what the first explained query returns, it is not present on str2:PRIMARY anymore. But here are the results of this same query on the replica set members:

      // str2-1 SECONDARY
      str2:SECONDARY> rs.slaveOk()
      str2:SECONDARY> db.ig_medias.find({ mid: "421587931535525217_30569368" }, { mid: 1, "cnt.lke": 1 })
      { "_id" : ObjectId("51939a0041002e238ff315d2"), "mid" : "421587931535525217_30569368", "cnt" : { "lke" : 12 } }
       
      // str2-2 SECONDARY
      str2:SECONDARY> rs.slaveOk()
      str2:SECONDARY> db.ig_medias.find({ mid: "421587931535525217_30569368" }, { mid: 1, "cnt.lke": 1 })
      { "_id" : ObjectId("51939a0041002e238ff315d2"), "mid" : "421587931535525217_30569368", "cnt" : { "lke" : 12 } }
       
      // str2-3 PRIMARY
      str2:PRIMARY> db.ig_medias.find({ mid: "421587931535525217_30569368" }, { mid: 1, "cnt.lke": 1 })
      { "_id" : ObjectId("51939a0041002e238ff315d2"), "mid" : "421587931535525217_30569368", "cnt" : { "lke" : 12 } }

      The slt (sharding key) value of the previous object is 393022. According to sh.status(true):

      { "slt" : 392653 } -->> { "slt" : 393077 } on : str3 { "t" : 530, "i" : 0 } 

      When we try to retrieve all the objects from this chunk:

      mongos> db.ig_medias.find({ slt: {$gte: 392653, $lt: 393077}}, { mid: 1, "cnt.lke": 1 }).readPref('primary').explain()
      {
      	"clusteredType" : "ParallelSort",
      	"shards" : {
      		"str3/str3-1.nitrogr.am:27017,str3-2.nitrogr.am:27017,str3-3.nitrogr.am:27017" : [
      			{
      				"cursor" : "BtreeCursor slt_1",
      				"isMultiKey" : false,
      				"n" : 43791,
      				"nscannedObjects" : 43791,
      				"nscanned" : 43791,
      				"nscannedObjectsAllPlans" : 43791,
      				"nscannedAllPlans" : 43791,
      				"scanAndOrder" : false,
      				"indexOnly" : false,
      				"nYields" : 59,
      				"nChunkSkips" : 0,
      				"millis" : 235,
      				"indexBounds" : {
      					"slt" : [
      						[
      							392653,
      							393077
      						]
      					]
      				},
      				"server" : "str3-2.nitrogr.am:27017"
      			}
      		]
      	},
      	"cursor" : "BtreeCursor slt_1",
      	"n" : 43791,
      	"nChunkSkips" : 0,
      	"nYields" : 59,
      	"nscanned" : 43791,
      	"nscannedAllPlans" : 43791,
      	"nscannedObjects" : 43791,
      	"nscannedObjectsAllPlans" : 43791,
      	"millisShardTotal" : 235,
      	"millisShardAvg" : 235,
      	"numQueries" : 1,
      	"numShards" : 1,
      	"indexBounds" : {
      		"slt" : [
      			[
      				392653,
      				393077
      			]
      		]
      	},
      	"millis" : 254
      }
      mongos> db.ig_medias.find({ slt: {$gte: 392653, $lt: 393077}}, { mid: 1, "cnt.lke": 1 }).readPref('secondary').explain()
      {
      	"clusteredType" : "ParallelSort",
      	"shards" : {
      		"str3/str3-1.nitrogr.am:27017,str3-2.nitrogr.am:27017,str3-3.nitrogr.am:27017" : [
      			{
      				"cursor" : "BtreeCursor slt_1",
      				"isMultiKey" : false,
      				"n" : 43791,
      				"nscannedObjects" : 43791,
      				"nscanned" : 43791,
      				"nscannedObjectsAllPlans" : 43791,
      				"nscannedAllPlans" : 43791,
      				"scanAndOrder" : false,
      				"indexOnly" : false,
      				"nYields" : 0,
      				"nChunkSkips" : 0,
      				"millis" : 65,
      				"indexBounds" : {
      					"slt" : [
      						[
      							392653,
      							393077
      						]
      					]
      				},
      				"server" : "str3-3.nitrogr.am:27017"
      			}
      		]
      	},
      	"cursor" : "BtreeCursor slt_1",
      	"n" : 43791,
      	"nChunkSkips" : 0,
      	"nYields" : 0,
      	"nscanned" : 43791,
      	"nscannedAllPlans" : 43791,
      	"nscannedObjects" : 43791,
      	"nscannedObjectsAllPlans" : 43791,
      	"millisShardTotal" : 65,
      	"millisShardAvg" : 65,
      	"numQueries" : 1,
      	"numShards" : 1,
      	"indexBounds" : {
      		"slt" : [
      			[
      				392653,
      				393077
      			]
      		]
      	},
      	"millis" : 102
      }

      Everything looks fine, but here is the result of the same queries, on str2 machines:

      // str2-1 SECONDARY
      str2:SECONDARY> db.ig_medias.find({ slt: {$gte: 392653, $lt: 393077}}, { mid: 1, "cnt.lke": 1 })
      { "_id" : ObjectId("5121d413710ea50b420debbe"), "mid" : "347964548473999677_9543635", "cnt" : { "lke" : 29 } }
      { "_id" : ObjectId("512393f541002e238ff4a859"), "mid" : "390217151503894124_28517528", "cnt" : { "lke" : 38 } }
      { "_id" : ObjectId("5131378341002e238f32144a"), "mid" : "383281284459672776_39604492", "cnt" : { "lke" : 24 } }
      { "_id" : ObjectId("5131fecc41002e238f3b943b"), "mid" : "238527467705375711_36829133", "cnt" : { "lke" : 38 } }
      { "_id" : ObjectId("512d19bc41002e238f07f548"), "mid" : "395567356503299509_279531956", "cnt" : { "lke" : 38 } }
      { "_id" : ObjectId("512c87de41002e238f0030d6"), "mid" : "369210183091623223_897180", "cnt" : { "lke" : 23 } }
      { "_id" : ObjectId("513a581d41002e238f94b99c"), "mid" : "407332724338286182_270796238", "cnt" : { "lke" : 10 } }
      { "_id" : ObjectId("512c396241002e238ffa969a"), "mid" : "398216305239938103_312516556", "cnt" : { "lke" : 18 } }
      { "_id" : ObjectId("515591ec41002e238f634c66"), "mid" : "422301314713088397_12787523", "cnt" : { "lke" : 13 } }
      { "_id" : ObjectId("512c614e41002e238ffd154a"), "mid" : "397802494119832758_313004538", "cnt" : { "lke" : 27 } }
      { "_id" : ObjectId("512e638541002e238f1974ad"), "mid" : "392182161891056449_306179277", "cnt" : { "lke" : 45 } }
      { "_id" : ObjectId("512f128741002e238f1dc17b"), "mid" : "401136119972715099_182925853", "cnt" : { "lke" : 22 } }
      { "_id" : ObjectId("5171d1b641002e238f630665"), "mid" : "190548751164070962_42200470", "cnt" : { "lke" : 6 } }
      { "_id" : ObjectId("513222e341002e238f421310"), "mid" : "364344438491857520_265458215", "cnt" : { "lke" : 12 } }
      { "_id" : ObjectId("5132b87241002e238f503297"), "mid" : "253952124281476138_47594148", "cnt" : { "lke" : 5 } }
      { "_id" : ObjectId("5132952041002e238f4c9046"), "mid" : "293163186055939480_174301408", "cnt" : { "lke" : 19 } }
      { "_id" : ObjectId("513336aa41002e238f55910a"), "mid" : "375606112677368687_199701635", "cnt" : { "lke" : 105 } }
      { "_id" : ObjectId("5136f27a41002e238f707851"), "mid" : "362900259412374239_4157811", "cnt" : { "lke" : 17 } }
      { "_id" : ObjectId("513c5cf241002e238fa0cb92"), "mid" : "351195940879395820_222487873", "cnt" : { "lke" : 46 } }
      { "_id" : ObjectId("513c97e641002e238fa22b02"), "mid" : "350248142066526553_245941897", "cnt" : { "lke" : 42 } }
      Type "it" for more
      str2:SECONDARY> db.ig_medias.find({ slt: {$gte: 392653, $lt: 393077}}, { mid: 1, "cnt.lke": 1 }).count()
      40347
       
      // str2-2 SECONDARY
      str2:SECONDARY> db.ig_medias.find({ slt: {$gte: 392653, $lt: 393077}}, { mid: 1, "cnt.lke": 1 })
      { "_id" : ObjectId("5121d413710ea50b420debbe"), "mid" : "347964548473999677_9543635", "cnt" : { "lke" : 29 } }
      { "_id" : ObjectId("512393f541002e238ff4a859"), "mid" : "390217151503894124_28517528", "cnt" : { "lke" : 38 } }
      { "_id" : ObjectId("5131fecc41002e238f3b943b"), "mid" : "238527467705375711_36829133", "cnt" : { "lke" : 38 } }
      { "_id" : ObjectId("512d19bc41002e238f07f548"), "mid" : "395567356503299509_279531956", "cnt" : { "lke" : 38 } }
      { "_id" : ObjectId("512c87de41002e238f0030d6"), "mid" : "369210183091623223_897180", "cnt" : { "lke" : 23 } }
      { "_id" : ObjectId("512e638541002e238f1974ad"), "mid" : "392182161891056449_306179277", "cnt" : { "lke" : 45 } }
      { "_id" : ObjectId("513a581d41002e238f94b99c"), "mid" : "407332724338286182_270796238", "cnt" : { "lke" : 10 } }
      { "_id" : ObjectId("512c396241002e238ffa969a"), "mid" : "398216305239938103_312516556", "cnt" : { "lke" : 18 } }
      { "_id" : ObjectId("512c614e41002e238ffd154a"), "mid" : "397802494119832758_313004538", "cnt" : { "lke" : 27 } }
      { "_id" : ObjectId("512f128741002e238f1dc17b"), "mid" : "401136119972715099_182925853", "cnt" : { "lke" : 22 } }
      { "_id" : ObjectId("5131378341002e238f32144a"), "mid" : "383281284459672776_39604492", "cnt" : { "lke" : 24 } }
      { "_id" : ObjectId("5171d1b641002e238f630665"), "mid" : "190548751164070962_42200470", "cnt" : { "lke" : 6 } }
      { "_id" : ObjectId("513222e341002e238f421310"), "mid" : "364344438491857520_265458215", "cnt" : { "lke" : 12 } }
      { "_id" : ObjectId("5132b87241002e238f503297"), "mid" : "253952124281476138_47594148", "cnt" : { "lke" : 5 } }
      { "_id" : ObjectId("5136f27a41002e238f707851"), "mid" : "362900259412374239_4157811", "cnt" : { "lke" : 17 } }
      { "_id" : ObjectId("5132952041002e238f4c9046"), "mid" : "293163186055939480_174301408", "cnt" : { "lke" : 19 } }
      { "_id" : ObjectId("513336aa41002e238f55910a"), "mid" : "375606112677368687_199701635", "cnt" : { "lke" : 105 } }
      { "_id" : ObjectId("513c5cf241002e238fa0cb92"), "mid" : "351195940879395820_222487873", "cnt" : { "lke" : 46 } }
      { "_id" : ObjectId("513c97e641002e238fa22b02"), "mid" : "350248142066526553_245941897", "cnt" : { "lke" : 42 } }
      { "_id" : ObjectId("513cea5241002e238fa32c6e"), "mid" : "349858663940795082_181939649", "cnt" : { "lke" : 54 } }
      Type "it" for more
      str2:SECONDARY> db.ig_medias.find({ slt: {$gte: 392653, $lt: 393077}}, { mid: 1, "cnt.lke": 1 }).count()
      40347
       
      // str2-3 PRIMARY
      str2:PRIMARY> db.ig_medias.find({ slt: {$gte: 392653, $lt: 393077}}, { mid: 1, "cnt.lke": 1 })
      { "_id" : ObjectId("5121d413710ea50b420debbe"), "mid" : "347964548473999677_9543635", "cnt" : { "lke" : 29 } }
      { "_id" : ObjectId("512393f541002e238ff4a859"), "mid" : "390217151503894124_28517528", "cnt" : { "lke" : 38 } }
      { "_id" : ObjectId("518a036141002e238fa5f264"), "mid" : "328357796146382089_13860576", "cnt" : { "lke" : 55 } }
      { "_id" : ObjectId("515f6e8841002e238fab2ac7"), "mid" : "326812285631217041_20784622", "cnt" : { "lke" : 6 } }
      { "_id" : ObjectId("513a581d41002e238f94b99c"), "mid" : "407332724338286182_270796238", "cnt" : { "lke" : 10 } }
      { "_id" : ObjectId("5131fecc41002e238f3b943b"), "mid" : "238527467705375711_36829133", "cnt" : { "lke" : 38 } }
      { "_id" : ObjectId("512d19bc41002e238f07f548"), "mid" : "395567356503299509_279531956", "cnt" : { "lke" : 38 } }
      { "_id" : ObjectId("512c87de41002e238f0030d6"), "mid" : "369210183091623223_897180", "cnt" : { "lke" : 23 } }
      { "_id" : ObjectId("512e638541002e238f1974ad"), "mid" : "392182161891056449_306179277", "cnt" : { "lke" : 45 } }
      { "_id" : ObjectId("512c396241002e238ffa969a"), "mid" : "398216305239938103_312516556", "cnt" : { "lke" : 18 } }
      { "_id" : ObjectId("512c614e41002e238ffd154a"), "mid" : "397802494119832758_313004538", "cnt" : { "lke" : 27 } }
      { "_id" : ObjectId("517afcac880084345c36eb36"), "mid" : "442869895293265303_32677776", "cnt" : { "lke" : 3 } }
      { "_id" : ObjectId("512f128741002e238f1dc17b"), "mid" : "401136119972715099_182925853", "cnt" : { "lke" : 22 } }
      { "_id" : ObjectId("5131378341002e238f32144a"), "mid" : "383281284459672776_39604492", "cnt" : { "lke" : 24 } }
      { "_id" : ObjectId("513222e341002e238f421310"), "mid" : "364344438491857520_265458215", "cnt" : { "lke" : 12 } }
      { "_id" : ObjectId("5132b87241002e238f503297"), "mid" : "253952124281476138_47594148", "cnt" : { "lke" : 5 } }
      { "_id" : ObjectId("513336aa41002e238f55910a"), "mid" : "375606112677368687_199701635", "cnt" : { "lke" : 105 } }
      { "_id" : ObjectId("5132952041002e238f4c9046"), "mid" : "293163186055939480_174301408", "cnt" : { "lke" : 19 } }
      { "_id" : ObjectId("5136f27a41002e238f707851"), "mid" : "362900259412374239_4157811", "cnt" : { "lke" : 17 } }
      { "_id" : ObjectId("513c5cf241002e238fa0cb92"), "mid" : "351195940879395820_222487873", "cnt" : { "lke" : 46 } }
      Type "it" for more
      str2:PRIMARY> db.ig_medias.find({ slt: {$gte: 392653, $lt: 393077}}, { mid: 1, "cnt.lke": 1 }).count()
      40347

      After checking all chunks, we found out that 56/1196 of them contain duplicates.

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              Reporter:
              n1t0 Anthony MOI
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