Mongos unable to find document after migration I have set up a mongo cluster as follows:


Four shards => mongoshard1 (Disk1), mongoshard2 (Disk2), mongoshard3(Disk3), mongoshard4(Disk4)
Replica set of one configuration server => mongoshardconfig (Disk5)
One mongos server => mongos
Shard key is a hashed index of _id


The entire setup is on a single server and the various mongod instances have been setup on different ports. (Setting up on a single server instead of different nodes has been a conscious decision though against recommended practices)

My server got terminated and I created a new cluster on another server with exact same settings as above using the old disks. The sharded collection document has a field: url . For a particular id, if I search using _id, mongos returns null but returns the object on finding using the url. Connecting individually to shard2 returns the object on querying using same _id. Some of the documents are found by querying on _id through mongos and some are not.

Why is mongos unable to find the document using id? Can this be fixed?
When querying with _id thru mongos, system directs that query to right node using config -databases chunks -collection information. It sounds like that "order" of your shards has been changed. (meaning that disk1 is not anymore at mongoshard1 -machine). Connect shard directly  and 

use local
db.me.find()

to check that nodes are who they should be.