Creating a MongoDB Replicaset with the Rancher Kubernetes Catalog

Gray Calendar Icon Published: April 25, 2016
Gray Calendar Icon Updated: September 11, 2019

mongodb-logoOne of the key features of the Kubernetes integration in Rancher is the application catalog that Rancher provides. Rancher provides the ability to create Kubernetes templates that give users the ability to launch sophisticated multi-node applications with the click of a button. Rancher also adds the support of Application Services to Kubernetes, which leverage the use of Rancher’s meta-data services, DNS, and Load Balancers. All of this comes with a consistent and easy to use UI. In this post, I am going to show an example of creating a Kubernetes catalog template that can be integrated later in complex stacks. This example will be a MongoDB replicaset. In the past I’ve shown how to create a MongoDB replicaset using Docker Compose, which works in environments using cattle and Docker Swarm as orchesrators.

MongoDB replicaset

MongoDB replicaset is a group of MongoDB processes that maintain the same data set. This provides high availability and redundancy between your replication nodes. Each MongoDB replicaset consists of one primary node and several secondary nodes. Galal
K1 Note: this image is from the official documentation of MongoDB. The primary node in MongoDB receives all the write operations. The secondary nodes can handle the read operations. The primary records all the changes in operation logs, or “oplogs,\” which will be replicated to each secondary node and then apply to the operations. If the primary is unavailable, an eligible secondary will hold an election to elect itself the new primary. For more information about MongoDB replicaset, please refer the official documentation.

Kubernetes Catalog Item

The Kubernetes catalog item consists of several yaml files. These files will be responsible for creating pods, services, endpoints, and load balancers. The files will be executed to create these objects in a Kubernetes environment. You can create your own private catalog of templates, or you can add a new catalog template in the public community catalog. To create the catalog item, you need to place the template in a separate directory under kubernetes-templates. This directory will contain a minimum of three items:

  • config.yml
  • catalogIcon-entry.svg
  • 0 folder

The 0 folder will contain the kubernetes yaml files and an additional rancher-compose.yml file. The rancher-compose file will contain additional information to help you customize your catalog entry. This will include a “questions” section that will let the user enter values for some variables that can be used later in the Kubernetes yaml files. We will see in the later section how to customize the rancher-compose file to serve your needs.

MongoDB Replica Set with Kubernetes

In order to make MongoDB work with kubernetes, I had to make a custom MongoDB docker container that will communicate with its peers and construct the replicaset. This custom solution will be represented by the following diagram: Galal
K2 The primary pod will be a single MongoDB container that will initiate the replicaset and add the secondary pods. The master and secondary pods are identical. The only difference is that the primary pod has the PRIMARY environment variable set to “true.” The Pod will have a special script that will check for this environment variable. If it was set to primary, then the pod will initiate the MongoDB replicaset with one IP which is the private IP of the primary pod. The secondary pod will check the same variable and then attempt to connect to the primary node and add itself to the replicaset using rs.add(). This script will look like the following:


set -x


exit 1




function initiate_rs() {

sleep 5

IP=$(ip -o -4 addr list eth0 | awk '{print $4}' | cut -d/ -f1 | sed -n 2p)

CONFIG="{\"_id\" : \"rs0\",\"version\" : 1,\"members\" : [{\"_id\" : 0,\"host\" : \"$IP:27017\"}]}"

mongo --eval "printjson(rs.initiate($CONFIG))"


function add_rs_member() {

sleep 5

# get pod ip address

IP=$(ip -o -4 addr list eth0 | awk '{print $4}' | cut -d/ -f1 | sed -n 2p)

while true; do

ismaster=$(mongo --host ${primary} --port 27017 --eval "printjson(db.isMaster())" | grep ismaster |grep -o true)

if [[ "$ismaster" == "true" ]]; then

mongo --host ${primary} --port 27017 --eval "printjson(rs.add('$IP:27017'))"


new_primary=$(mongo --host ${primary} --port 27017 --eval "printjson(db.isMaster())" | grep primary | cut -d"\"" -f4 | cut -d":" -f1)

mongo --host ${new_primary} --port 27017 --eval "printjson(rs.add('$IP:27017'))"


if [[ "$?" == "0" ]]; then



echo "Connecting to primary failed. Waiting..."

sleep 10



if [[ $PRIMARY == "true" ]]; then

initiate_rs &


add_rs_member &


/ $@

As described in the previous script, the pod will check if the MongoDB master pod is still primary or not. If it is not the primary node in the replicaset, the pod will search for the master node using the rs.isMaster() function and then try to connect and add itself to the new master. This simple script will also give the users the ability to scale this replicaset as needed. Every new node will try to connect to the same replicaset.

Kubernetes Template Files

The template files consists of 3 yaml files:

  • Mongo-master.yaml

This is the file that contains the instruction to create the master pod and the master service that will hold the IP of the primary MongoDB container. Galal

  • Mongo-controller.yaml

This file contains the instruction to create replication controller with the default of 2 pods. These pods will represent the secondary containers. Galal
K4 Note that the variable \${sec_no} is used to specify the number of replicas in the replicaset. This should be an even number because it will be responsible for creating secondary containers. This value, by default, is 2.

  • Mongo-sec-service.yaml

This file contains the instruction to create a kubernetes service for the secondary MongoDB containers. Galal
K5 The final file inside the template directory. This is the rancher-compose file that contains some information about the template and the questions that will be used as a variable. Galal
K6 Running MongoDB replicaset You can find the template in the catalog section in your Kubernetes environment: Galal
K7 Click on view details, and then specify the number of replicas you want to start. As mentioned before, the default value is two secondary nodes. Galal
K8 Then click launch. You should be able to see, after few minutes, the fully functional replicaset. Galal
K9 Also, you should be able to see the logs of the master container which should look something like the following: ……

4/5/2016 1:02:42 AM2016-04-04T22:02:42.518+0000 I NETWORK  [conn8] end connection (2 connections now open)
4/5/2016 1:02:42 AM2016-04-04T22:02:42.519+0000 I NETWORK  [initandlisten] connection accepted from #11 (3 connections now open)
4/5/2016 1:02:42 AM2016-04-04T22:02:42.521+0000 I NETWORK  [conn11] end connection (2 connections now open)
4/5/2016 1:02:42 AM2016-04-04T22:02:42.948+0000 I NETWORK  [initandlisten] connection accepted from #12 (3 connections now open)
4/5/2016 1:02:42 AM2016-04-04T22:02:42.956+0000 I NETWORK  [conn12] end connection (2 connections now open)
4/5/2016 1:02:43 AM2016-04-04T22:02:43.962+0000 I NETWORK  [initandlisten] connection accepted from #13 (3 connections now open)
4/5/2016 1:02:43 AM2016-04-04T22:02:43.969+0000 I NETWORK  [conn13] end connection (2 connections now open)
4/5/2016 1:02:44 AM2016-04-04T22:02:44.513+0000 I REPL     [ReplicationExecutor] Member is now in state SECONDARY
4/5/2016 1:02:44 AM2016-04-04T22:02:44.515+0000 I REPL     [ReplicationExecutor] Member is now in state SECONDARY

…… This shows that two members are added to the replicaset and are in the secondary state. Conclusion Rancher is expanding its capabilities by adding Kubernetes support by adding all the key features of Kubernetes to Rancher management platform. This gives the users the ability to use and create their own intricate designs using features like the Application Catalog. Join us for our next meetup later this month to learn more about running Kubernetes environments in Rancher.

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