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?
Best Answer
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.