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OPERATIONAL DEFECT DATABASE
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While performing the applyOps command, duplicateKey exception are ignored during insertion but not on update operations. This could result in a situation where oplog would not be idempotent with a unique index as update could fail where they should not. For example: db.test.createIndex({a: 1}, {unique: true}) db.test.insert({a: 1}) db.test.update({a: 1}, {$set: {a: 2, b: 1}}) db.test.remove({a: 2}) db.test.insert({a: 2}) if you replay all oplog entries whereas the last insert has already been done ({a: 2} is present in the collection), mongod will fail with: db.test.update({a: 1}, {$set: {a: 2}}) applyOps: E11000 duplicate key error collection: test.test index: a_1 dup key: { : 2.0 } This is specially painful while you are performing point in time restore as mongorestore is stopping at the first error encountered. Cheers, Damien
JIRAUSER1254842 commented on Tue, 4 Aug 2020 17:51:57 +0000: Now with https://jira.mongodb.org/browse/SERVER-31507 done it is possible to specify applyOps parameter: `oplogApplicationMode: "InitialSync"` or `oplogApplicationMode: "Recovering"` or `oplogApplicationMode: "Secondary"`. One of this modes should be suitable for mongorestore. https://github.com/mongodb/mongo/commit/ced3d3341a2aac6a11297f7dcc2c3c6d2c0e3bec milkie commented on Fri, 20 May 2016 14:39:48 +0000: Thanks for filing this; it's a known issue and we are tracking the fix at TOOLS-176 and the related SERVER tickets linked to that one.
#!/bin/sh pkill mongod sleep 5 mkdir db1 rm -rf db1/* db1_backup/ dump/ mongod --dbpath ./db1 --fork --syslog --replSet test sleep 2 # initializing the db mongo --eval 'rs.initiate()' && sleep 2 mongo --eval 'db.getCollection("test").createIndex({a: 1}, {unique: true})' mongo --eval 'db.getCollection("test").insert({a: 1})' mongo --eval 'db.getCollection("test").update({a: 1}, {$set: {a: 2}})' mongo --eval 'db.getCollection("test").remove({a: 2})' mongo --eval 'db.getCollection("test").insert({a: 2})' # let's perform a point in time restore using the oplog now mongodump -d local -c oplog.rs mv dump/local/oplog.rs.bson dump/oplog.bson rm -rf dump/local mongorestore --oplogReplay # failed: restore error: error applying oplog: applyOps: E11000 duplicate key error collection: test.test index: a_1 dup key: { : 2.0 }