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The AllowPartialResults cursor method (DBQuery.Option.partial) does not have the desired effect when a shard backed by a REPLICA SET becomes unavailable. In first example below I have set up a sharded cluster where each shard is backed by a single MONGOD. AllowPartialResults works as expected - if I kill on MONGOD I get results back from the other MONGOD. In the second example below, I have set up exact same cluster except I have backed each shard with a [single mongod] replica set. AllowPartialResults now does not work, in the sense that if I kill the mongod that forms the replica set, the db.collection.find().allowPartialResults() fails reporting an error saying "could not find host matching read preference { mode: \"primary\", tags: [ {} ] } for set shard1".

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esha.maharishi@10gen.com commented on Wed, 2 Oct 2019 13:48:10 +0000: This was fixed (establishCursors was made to swallow FailedToSatisfyReadPreference errors from trying to find a particular host within a targeted shard) under SERVER-33597. Closing as duplicate. ramon.fernandez commented on Fri, 16 Nov 2018 00:29:56 +0000: Thanks for your report jg1 – this ticket has been sent to the Query team for evaluation. We'll post updates to the ticket as we find out more details about what's causing this behavior. Regards, Ramón. jg1 commented on Wed, 11 Oct 2017 10:55:16 +0000: I suspect the issue arises because the query does not get sent at all. Rather, the ReplicaSetMonitor (replica_set_monitor.cpp) fails in getHostOrRefresh (or possibly async_requests_sender.cpp) fails to connect to replica set before even the find gets sent. jg1 commented on Wed, 11 Oct 2017 10:26:31 +0000: THIS ONE PRETTY MUCH SAME EXCEPT SETS UP 2 SHARDS BACKED BY [single mongod] REPLICA SETS. partial DOES NOT HAVE DESIRED EFFECT IF ONE MONGOD KILLED. kill -2 $(pgrep -a mongo | grep " 400" | awk '{print $1}') sleep 1 cd rm PartialTest2 -r -f mkdir PartialTest2 cd PartialTest2/ mkdir config mkdir shard1 mkdir shard2 mkdir logs mongod --port 40001 --configsvr --replSet config --dbpath config --logpath logs/config.log --fork mongo localhost:40001 --quiet --eval 'rs.initiate( { _id: "config", configsvr:true, members:[{_id:0, host: "localhost:40001"}]} )' mongos --port 40002 --configdb config/localhost:40001 --logpath logs/mongos.log --fork mongod --port 40011 --shardsvr --replSet shard1 --dbpath shard1 --logpath logs/shard1.log --fork mongo localhost:40011 --quiet --eval 'rs.initiate( { _id: "shard1", members:[{_id:0, host: "localhost:40011"}]} )' mongod --port 40012 --shardsvr --replSet shard2 --dbpath shard2 --logpath logs/shard2.log --fork mongo localhost:40012 --quiet --eval 'rs.initiate( { _id: "shard2", members:[{_id:0, host: "localhost:40012"}]} )' mongo localhost:40002 --quiet --eval 'sh.addShard("shard1/localhost:40011")' mongo localhost:40002 --quiet --eval 'sh.addShard("shard2/localhost:40012")' mongo localhost:40002 --quiet --eval 'sh.enableSharding("partial")' mongo localhost:40002/partial --quiet --eval 'db.test.createIndex({"key" : 1})' mongo localhost:40002 --quiet --eval 'sh.shardCollection("partial.test", { "key" : 1 })' mongo localhost:40002 --quiet --eval 'sh.addShardTag("shard1", "firsthalf")' mongo localhost:40002 --quiet --eval 'sh.addShardTag("shard2", "secondhalf")' mongo localhost:40002 --quiet --eval 'sh.addTagRange("partial.test", { key : "A"}, {key: "L"}, "firsthalf")' mongo localhost:40002 --quiet --eval 'sh.addTagRange("partial.test", { key : "L"}, {key: "Z"}, "secondhalf")' mongo localhost:40002/partial --quiet --eval 'db.test.insert({ "key" : "B" })' mongo localhost:40002/partial --quiet --eval 'db.test.insert({ "key" : "Y" })' mongo localhost:40002/partial --quiet --eval 'db.test.find().addOption(DBQuery.Option.partial)' #should return 2 kill -2 $(pgrep -a mongo | grep "40011" | awk '{print $1}') db.test.find().addOption(DBQuery.Option.partial) mongo localhost:40002/partial --quiet --eval 'db.test.find().addOption(DBQuery.Option.partial)' #should return 1 but fails [mongod@BLAH PartialTest2]$ mongo localhost:40002/partial --quiet --eval 'db.test.find()' { "_id" : ObjectId("59dcc375f955bba2a5b15850"), "key" : "B" } { "_id" : ObjectId("59dcc375f90654100e5edb0e"), "key" : "Y" } [mongod@BLAH PartialTest2]$ mongo localhost:40002/partial --quiet --eval 'db.test.find().addOption(DBQuery.Option.partial)' { "_id" : ObjectId("59dcc375f955bba2a5b15850"), "key" : "B" } { "_id" : ObjectId("59dcc375f90654100e5edb0e"), "key" : "Y" } [mongod@BLAH PartialTest2]$ kill -2 $(pgrep -a mongo | grep "40011" | awk ' {print $1} ') [mongod@BLAHPartialTest2]$ mongo localhost:40002/partial --quiet --eval 'db.test.find().addOption(DBQuery.Option.partial)' Error: error: { "ok" : 0, "errmsg" : "could not find host matching read preference { mode: \"primary\", tags: [ {} ] } for set shard1", "code" : 133, "codeName" : "FailedToSatisfyReadPreference" } jg1 commented on Wed, 11 Oct 2017 10:26:07 +0000: THIS ONE SETS UP 2 SHARDS BACKED DIRECTLY BY SINGLE MONGODS AND partial WORKS FINE IF ONE MONGOD KILLED. kill -2 $(pgrep -a mongo | grep " 300" | awk '{print $1}') sleep 1 cd rm PartialTest -r -f mkdir PartialTest cd PartialTest/ mkdir config mkdir shard1 mkdir shard2 mkdir logs mongod --port 30001 --configsvr --replSet config --dbpath config --logpath logs/config.log --fork sleep 1 mongo localhost:30001 --quiet --eval 'rs.initiate( { _id: "config", configsvr:true, members:[{_id:0, host: "localhost:30001"}]} )' sleep 5 mongos --port 30002 --configdb config/localhost:30001 --logpath logs/mongos.log --fork mongod --port 30011 --shardsvr --dbpath shard1 --logpath logs/shard1.log --fork mongod --port 30012 --shardsvr --dbpath shard2 --logpath logs/shard2.log --fork mongo localhost:30002 --quiet --eval 'sh.addShard("localhost:30011")' mongo localhost:30002 --quiet --eval 'sh.addShard("localhost:30012")' mongo localhost:30002 --quiet --eval 'sh.enableSharding("partial")' mongo localhost:30002/partial --quiet --eval 'db.test.createIndex({"key" : 1})' mongo localhost:30002 --quiet --eval 'sh.shardCollection("partial.test", { "key" : 1 })' mongo localhost:30002 --quiet --eval 'sh.addShardTag("shard0000", "firsthalf")' mongo localhost:30002 --quiet --eval 'sh.addShardTag("shard0001", "secondhalf")' mongo localhost:30002 --quiet --eval 'sh.addTagRange("partial.test", { key : "A"}, {key: "L"}, "firsthalf")' mongo localhost:30002 --quiet --eval 'sh.addTagRange("partial.test", { key : "L"}, {key: "Z"}, "secondhalf")' mongo localhost:30002/partial --quiet --eval 'db.test.insert({ "key" : "B" })' mongo localhost:30002/partial --quiet --eval 'db.test.insert({ "key" : "Y" })' mongo localhost:30002/partial --quiet --eval 'db.test.find()' #should return 2 mongo localhost:30002/partial --quiet --eval 'db.test.find().addOption(DBQuery.Option.partial)' #should return 2 kill -2 $(pgrep -a mongo | grep "30011" | awk '{print $1}') mongo localhost:30002/partial --quiet --eval 'db.test.find().addOption(DBQuery.Option.partial)' #should return 1 mongo localhost:30002/partial --quiet --eval 'db.test.find()' #should error Results: mongos> db.test.find().addOption(DBQuery.Option.partial) { "_id" : ObjectId("59dcb0f38b49a86cdec9e780"), "key" : "B" } { "_id" : ObjectId("59dcb0f37950b2dc5ff66fd5"), "key" : "Y" } mongos> exit bye [mongod@BLAH PartialTest]$ kill -2 $(pgrep -a mongo | grep "30011" | awk ' {print $1} ') [mongod@BLAH PartialTest]$ mongo localhost:30002/partial --quiet --eval 'db.test.find().addOption(DBQuery.Option.partial)' #should return 1 { "_id" : ObjectId("59dcb0f37950b2dc5ff66fd5"), "key" : "Y" } [mongod@BLAH PartialTest]$ mongo localhost:30002/partial --quiet --eval 'db.test.find()' #should error because no partial Error: error: { "ok" : 0, "errmsg" : "Connection refused", "code" : 6, "codeName" : "HostUnreachable" }

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