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Multi-User Testing<\/span><\/span><\/p>\n \u00a0<\/p>\n David Glasser of <\/span>Meteor<\/span><\/a> wrote <\/span>a blog on an MongoDB query missing matching documents issue<\/span><\/a> he ran into. \u00a0It is straightforward to reproduce the issue on both MongoDB MMAPv1 and MongoDB WiredTiger engine. Here are his conclusions from his article (emphasis is mine)<\/span><\/span><\/p>\n \u00a0<\/p>\n Long story short\u2026<\/span><\/span><\/p>\n This issue doesn\u2019t affect queries that don\u2019t use an index, such as queries that just look up a document by ID.<\/span><\/span><\/p>\n<\/li>\n It doesn\u2019t affect queries which explicitly do a single value equality match on all fields used in the index key.<\/span><\/span><\/p>\n<\/li>\n It doesn\u2019t affect queries that use indexes whose fields are never modified after the document is originally inserted.<\/span><\/span><\/p>\n<\/li>\n But any other kind of MongoDB query can fail to include all the matching documents!<\/span><\/span><\/p>\n<\/li>\n<\/ul>\n \u00a0<\/p>\n Here\u2019s another way to look at it. In MongoDB, if the query can retrieve two documents using a secondary index (index on something other than _id) when concurrent operations are going on, the results could be wrong. \u00a0This is a common scenario in many database applications. <\/span><\/span><\/p>\n \u00a0<\/p>\n Here is the test:<\/span><\/span><\/p>\n Create a Container: Bucket, table, or collection.<\/span><\/span><\/p>\n<\/li>\n Load the data with a small dataset, say 300K documents.<\/span><\/span><\/p>\n<\/li>\n Create an index on the field you want to filter on (predicates).<\/span><\/span><\/p>\n<\/li>\n In one session, update the indexed field in one session and query on the other. <\/span><\/span><\/p>\n<\/li>\n<\/ol>\n \n\u00a0<\/p>\n \n\u00a0<\/p>\n Steps to reproduce the issue on MongoDB:<\/span><\/span><\/p>\n \u00a0<\/p>\n Install MongoDB 3.2.<\/span><\/span><\/p>\n<\/li>\n Bring up mongod with either MMAPv1 or WiredTiger.<\/span><\/span><\/p>\n<\/li>\n Load data using tpcc.py<\/span><\/span><\/p>\n<\/li>\n python tpcc.py –warehouses 1 –no-execute mongodb<\/span><\/span><\/p>\n<\/li>\n Get the count<\/span><\/span><\/p>\n<\/li>\n<\/ol>\n > use tpcc<\/span><\/span><\/p>\n > db.ORDER_LINE.find().count();<\/span><\/span><\/p>\n 299890<\/span><\/span><\/p>\n db.ORDER_LINE.ensureIndex({state:1});<\/span><\/span><\/p>\n<\/li>\n<\/ol>\n \u00a0<\/p>\n \u00a0<\/p>\n Setup the state field with the value aaaaaa and then concurrently update this value to zzzzzz and query for total number of documents with the two values ['aaaaaa','zzzzzz'] matching the field. \u00a0When the value of the indexed field moves from lower (aaaaaa) to higher (zzzzzz) value, these entries are moving from one side of the B-tree to the other. \u00a0Now, we\u2019re trying to see if the scan returns duplicate value, translated to higher count() value.<\/span><\/span><\/p>\n > db.ORDER_LINE.update({OL_DIST_INFO:{$gt:””}}, {$set: {state:”aaaaaa”}}, {multi:true});<\/span><\/span><\/p>\n WriteResult({ “nMatched” : 299890, “nUpserted” : 0, “nModified” : 299890 })<\/span><\/span><\/p>\n > db.ORDER_LINE.find({state:{$in:['aaaaaa','zzzzzz']}}).count();<\/span><\/span><\/p>\n 299890<\/span><\/span><\/p>\n \u00a0<\/p>\n > db.ORDER_LINE.find({state:{$in:['aaaaaa','zzzzzz']}}).explain();<\/span><\/span><\/p>\n {<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>“queryPlanner” : {<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>“plannerVersion” : 1,<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>“namespace” : “tpcc.ORDER_LINE”,<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>“indexFilterSet” : false,<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>“parsedQuery” : {<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>“state” : {<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>“$in” : [<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>“aaaaaa”,<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>“zzzzzz”<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>]<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>}<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>},<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>“winningPlan” : {<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>“stage” : “FETCH”,<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>“inputStage” : {<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>“stage” : “IXSCAN”,<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>“keyPattern” : {<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>“state” : 1<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>},<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>“indexName” : “state_1”,<\/span><\/span><\/p>\n \u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>\u00a0\u00a0 \u00a0<\/span>“isMultiKey” : false,<\/span><\/span><\/p>\n\n
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MongoDB Testing<\/span><\/span><\/h3>\n
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MongoDB Experiment 1: Update to higher value<\/span><\/span><\/h4>\n
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