mongodb upgrade 3.6 to 4.0

1)  To Upgrade Mongo 3.6 to 4.0 The 3.6 instance must have
featureCompatibilityVersion set to 3.6. 

2) To check featureCompatibilityVersion:

>db.adminCommand( { getParameter: 1, featureCompatibilityVersion: 1 } )

       The operation should return a result that includes: 

"featureCompatibilityVersion" : { "version" : "3.6" }.

3) if not then set featureCompatibilityVersion as below

 > db.adminCommand( { setFeatureCompatibilityVersion: "3.6" } )

Upgrade Process

1) now upgrade repo to 4  with name mongodb-org-4.0.repo

[mongodb-org-3.6]

name=MongoDB Repository

baseurl=https://repo.mongodb.org/yum/amazon/2013.03/mongodb-org/3.6/
x86_64/

gpgcheck=1

enabled=1

gpgkey=https://www.mongodb.org/static/pgp/server-3.6.asc

# yum repolist all 

# yum upgrade mongodb-org

Remove pv0 for Replica Sets

# Starting in version 4.0, MongoDB removes the deprecated replica
set protocol version 0 pv0.

# Before upgrading to MongoDB 4.0, you must upgrade to pv1.

# To upgrade to pv1, connect a mongo shell to the replica set primary
and perform the following sequence of operations:

Steps i followed as below #

cfg = rs.conf();

cfg.protocolVersion=1;

rs.reconfig(cfg);

# Set future compatibility version to 4.0 go to primary and fire below
command on mongo shell 

>db.adminCommand( { setFeatureCompatibilityVersion: "3.6" } )

# Then check all secondary it must show 3.6 as compatible version to
upgrade it 4.0

# Now stop mongod on secondary

# Then upgrade repo to 4.0

# And restart mongod after restart it must be secondary 

# Now go to primary do the same up gradation procedure now all
replica set is in 4.0 

# Last step set future compatibility version to 4.0 go to mongo
primary and fire this command

>db.adminCommand( { setFeatureCompatibilityVersion: "4.0" } )

# Last Step check record count in both dbs go to mongo shell use db to check 

>mongo

>use locationdataV3

>db.LocationData.count()

#################### Done ###############################