Operational Defect Database
BugZero found this defect 1210 days ago.
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MongoDB | 1599668
Session migration from moveChunk can lead to higher 'n' and 'nModified' for retryable updates by _id
Last update date:
3/13/2024
Affected products:
MongoDB Server
Affected releases:
No affected releases provided.
Fixed releases:
No fixed releases provided.
Description:
Info
Mongos combining the write results from multiple shards can lead to ambiguity due to how shards may report 'n' and 'nModified' for the same statements. [js_test:repro_retryable_update_by_id_nmodified] 2021-01-24T15:46:37.631+0000 s20023| {"t":{"$date":"2021-01-24T15:46:37.631+00:00"},"s":"I", "c":"COMMAND", "id":51803, "ctx":"conn6","msg":"Slow query","attr":{"type":"command","ns":"test.mycoll","appName":"MongoDB Shell","command":{"update":"mycoll","txnNumber":0,"lsid":{"id":{"$uuid":"5bb5fa37-09a3-4036-9442-83fa45776750"}},"$clusterTime":{"clusterTime":{"$timestamp":{"t":1611503197,"i":122}},"signature":{"hash":{"$binary":{"base64":"AAAAAAAAAAAAAAAAAAAAAAAAAAA=","subType":"0"}},"keyId":0}},"$db":"test"},"nShards":2,"nMatched":2,"nModified":2,"numYields":0,"reslen":185,"remote":"127.0.0.1:53194","protocol":"op_msg","durationMillis":2}} [js_test:repro_retryable_update_by_id_nmodified] 2021-01-24T15:46:37.632+0000 uncaught exception: Error: [{ "n" : 2, "nModified" : 2 }] != [{ "n" : 1, "nModified" : 1 }] are not equal :
Top User Comments
JIRAUSER1269163 commented on Wed, 13 Mar 2024 18:13:18 +0000: This ticket is not possible to be fixed under PM-3190 completely. The issue is fixed for all ordered: true writes of all batch sizes and ordered: false of batch size 1. However this bug remains for ordered: false batches of size more than 1 containing multiple update one with _id without shard key. xgen-internal-githook commented on Mon, 20 Nov 2023 22:04:20 +0000: Author: {'name': 'Abdul Qadeer', 'email': 'abdul.qadeer@mongodb.com', 'username': 'zorro786'} Message: SERVER-81594 SERVER-54019 Add tests Branch: master https://github.com/mongodb/mongo/commit/a1c223397a0f64e9eb5a730c2d502904d60c1c23 JIRAUSER1269163 commented on Thu, 16 Nov 2023 00:22:01 +0000: This will be fixed by the virtue of returning early in PM-3190 for updateOne with _id without shard key as soon as any one shard returns {'n' : 1, 'nModified' : 1} .
Steps to Reproduce
python buildscripts/resmoke.py run --suite=sharding repro_retryable_update_by_id_nmodified.js repro_retryable_update_by_id_nmodified.js (function() { "use strict"; load("jstests/sharding/libs/create_sharded_collection_util.js"); const st = new ShardingTest({mongos: 1, config: 1, shards: 2, rs: {nodes: 1}}); const db = st.s.getDB("test"); const collection = db.getCollection("mycoll"); CreateShardedCollectionUtil.shardCollectionWithChunks(collection, {x: 1}, [ {min: {x: MinKey}, max: {x: 0}, shard: st.shard0.shardName}, {min: {x: 0}, max: {x: 10}, shard: st.shard0.shardName}, {min: {x: 10}, max: {x: 20}, shard: st.shard1.shardName}, {min: {x: 20}, max: {x: MaxKey}, shard: st.shard1.shardName}, ]); assert.commandWorked(collection.insert({_id: 0, x: 5, counter: 0})); const sessionCollection = st.s.startSession({causalConsistency: false, retryWrites: false}) .getDatabase(db.getName()) .getCollection(collection.getName()); // Updates by _id are broadcasted to all shards which own chunks for the collection. After the // session information is migrated to shard1 from the moveChunk command, both shard0 and shard1 // will report {n: 1, nModified: 1} for stmtId=0. const updateCmd = { updates: [ {q: {_id: 0}, u: {$inc: {counter: 1}}}, {q: {_id: 10000}, u: {$inc: {counter: 1}}}, ], txnNumber: NumberLong(0), }; const firstRes = sessionCollection.runCommand("update", updateCmd); assert.eq({n: firstRes.n, nModified: firstRes.nModified}, {n: 1, nModified: 1}); assert.commandWorked( db.adminCommand({moveChunk: collection.getFullName(), find: {x: 5}, to: st.shard1.shardName})); const secondRes = sessionCollection.runCommand("update", updateCmd); assert.eq({n: secondRes.n, nModified: secondRes.nModified}, {n: 1, nModified: 1}); st.stop(); })();
