This articles describes the steps to take when a cluster is unable to discover its nodes. Steps in this articles assume Mongo is hosted as an add-on on Nex!™ PaaS but could in essence be applied to any Mongo hosting.

1 - Context

It may happen upon doing multiple restarts that a Mongo addon replica set becomes stale. In that very specific case all nodes will recognise there is a replica set but each node will have a list of outdated members in the replica set. In this case the cluster becomes stale with no node actually taking leadership for cluster.

In that kind of situation you will likely observe the following error when querying the replica set through the mongo shell:

> rs.status()
{
 "startupStatus" : 1,
 "ok" : 0,
 "errmsg" : "loading local.system.replset config (LOADINGCONFIG)"
}

2 - Preferred steps

The steps described in this article are quite radical and must only be used in the event where the cluster cannot be reconfigured.

We recommend first trying to reconfigure the Mongo cluster using the steps on this link.

If things don't work then proceed to section 3.

3 - Reset the replica set

If this happens you can follow the instructions below to recover the replica set:

First choose a container in your mongo addon - preferrably the last master if you can identify it
```
nex-cli cubes --addon name_of_mongo_addon
```
Stop all other mongo containers
```
nex-cli cubes:stop other_container_id
```
SSH to the rack hosting the container that is still alive
```
nex-cli racks:ssh 10.1.1.1
```
Connect to the container
```
sudo nex-attach my_container_id
```
Connect to Mongo using the Shell

mongo -u $MONGO_USER -p $MONGO_PASSWORD admin
Delete the local replication information. This command does NOT delete your actual data but will destroy uncommitted replication transactions.
```
use local
db.dropDatabase()
```
Exit the MongoShell and the container
On the rack issue a restart of the container. Upon restart the mongo instance will initiate a new replicaSet (previous one being deleted)
```
# Restart
nex-restart --name my_container_id

# Monitor logs
docker logs -f my_container_id
```
Exit the rack and bring up the other nodes using the nex-cli
```
nex-cli cubes:start other_container_id
```

Addon - Recover a stale Mongo ReplicaSet

1 - Context

2 - Preferred steps

3 - Reset the replica set