Run the following JS script: // Tests that SecondaryPreferred queries routed to the primary of a shard will handle stale metadata (function() { 'use strict' ; // Start a sharding cluster with a single shard, which has one node var st = new ShardingTest({ mongos: 2, shards: 2, other: { rs: { nodes: 1 } } }); // Shard a collection by the first mongos assert .commandWorked(st.s0.adminCommand({ enableSharding: 'TestDB' })); st.ensurePrimaryShard( 'TestDB' , st.shard0.shardName); assert .commandWorked(st.s0.adminCommand({ shardCollection: 'TestDB.TestColl' , key: { Key: 1 } })); // Insert some documents assert .writeOK(st.s0.getDB( 'TestDB' ).TestColl.insert({ Key: 0, Value: 'Value 0' })); assert .writeOK(st.s0.getDB( 'TestDB' ).TestColl.insert({ Key: 1, Value: 'Value 1' })); // Make sure the second mongos has the most up-to-date metadata assert .eq(2, st.s0.getDB( 'TestDB' ).TestColl.find().itcount()); assert .eq(2, st.s1.getDB( 'TestDB' ).TestColl.find().itcount()); // Make sure the second mongos has cached a versioned collection with the stale version var slaveOkConnection = new Mongo(st.s1.host); slaveOkConnection.setSlaveOk(); assert .eq(2, slaveOkConnection.getDB( 'TestDB' ).TestColl.distinct( 'Value' ).length); // Split the chunk on the first mongos assert .commandWorked(st.s0.adminCommand({ split: 'TestDB.TestColl' , find: { Key: 0 } })); assert .commandWorked(st.s0.adminCommand({ moveChunk: 'TestDB.TestColl' , find: { Key: 1 }, to: st.shard1.shardName })); // Now do the distinct again and use the same connection as the one on which we ran distinct // earlier, because sharded/versioned connections are cached per thread. assert .eq(2, slaveOkConnection.getDB( 'TestDB' ).TestColl.distinct( 'Value' ).length); st.stop(); })();