Attempting to set up a simple sharded cluster with MongoDB 3.1.9 for training and learning purposes is now far harder and more confusing than it was and appears to fail at first in may ways . This provides a very bad first impression of MongoDB. Here are some points as to why.

3.1.9 requires a single node replica set - this itself causes issues as it's a breaking change and there are hundreds of getting started with MongoDB tutiorials that no longer work.

If you do make this mistake then add --replSet config to your config server you get an uninitialised Replica set which you can't use - but it's not obvious why. Even logging on you get a blank prompt not a config[NOT YET REPLICA] or something similar.

IF you try to start mongos with

mongos --configdb localhost:27019 --port 27017

You get

BadValue Must have either 3 node legacy config servers, or a replica set config server
try 'mongos --help' for more information

but mongos --help gives you Zero help as it says

Sharding options:
--configdb arg 1 or 3 comma separated config servers

There are no clues that you need

mongos --configdb config/localhost:27109

NOR does it infer 27019 as it did before or infer a default replica set name

Worst part - and this is a BUG I think

For approximately a minute after you get the mongos up, attempts to contact get

MacPro:~ jlp$ mongo --port 27017
MongoDB shell version: 3.1.9
connecting to: 127.0.0.1:27017/test
2015-10-16T09:47:27.251+0100 W NETWORK [thread1] Failed to connect to 127.0.0.1:27017, reason: errno:61 Connection refused
2015-10-16T09:47:27.251+0100 E QUERY [thread1] Error: couldn't connect to server 127.0.0.1:27017, connection attempt failed :
connect@src/mongo/shell/mongo.js:216:14
@(connect):1:6

exception: connect failed
MacPro:~ jlp$

in which time people stop it and try again as it appears to have failed

If you wait approximately three minutes then you get the following in the mongos log

2015-10-16T09:49:07.855+0100 W SHARDING [thread1] pinging failed for distributed lock pinger :: caused by :: findAndModify query predicate didn't match any lock document
2015-10-16T09:49:08.273+0100 I SHARDING [Balancer] about to contact config servers and shards
2015-10-16T09:49:08.273+0100 I NETWORK [mongosMain] waiting for connections on port 27017
2015-10-16T09:49:08.274+0100 I SHARDING [Balancer] config servers and shards contacted successfully
2015-10-16T09:49:08.274+0100 I SHARDING [Balancer] balancer id: MacPro.local:27017 started
2015-10-16T09:49:08.308+0100 I SHARDING [Balancer] distributed lock 'balancer' acquired, ts : 5620ba04600c6c11c7053c0b
2015-10-16T09:49:08.394+0100 I SHARDING [Balancer] distributed lock with ts: 5620ba04600c6c11c7053c0b' unlocked.

and you can then connect.

Just the kind of UX that would turn me and many others away from MongoDB in that golden 30 minutes when I first try it.