Demonstration of a mongo cluster using terraform, ansible, and docker This assumes that the network has already been created, i.e. there is already a working VPC with subnets and Route53 domain. The terraform aws ec2 module and security group is adapted

• Terraform - Provision EC2 instances, security groups, EBS volumes, and DNS
• Docker - Used to deploy versioned Mongo image
• Ansible - Automated deployment of Docker images to cluster
• Mongo - Use mongo 3.4

$ terraform --version
Terraform v0.11.2
$ docker --version
Docker version 17.09.0-ce, build afdb6d4
$ ansible --version
ansible 2.0.0.2

1. Terraform is used to initially provision resources

$ terraform init
$ terraform plan
$ terraform apply 

This provisions a mongo cluster of ${var.count} nodes, each having a ${var.ebs_volume_size} EBS volume, and a route53 A record for each node. User data mounts EBS to /data for persistence of mongo data.

2. The mongo3.4 container is edited such that it mounts the /data/db directory and outputs logs to /data/logs for persistence. An automatic build can be set up for this container on dockerhub/dockercloud such that each repository commit re-builds the docker image. Otherwise, this can be done manually with

$ docker build -t test/mongo-cluster .
$ docker login
$ docker push test/mongo-cluster

3. Although it is often preferrable to use a container orchestrator/scheduler for container deploys, the ansible docker_container module can suffice for one off deployments. Run the following command in the playbooks directory once the mongo-cluster image has been pushed. The ansible playbook uses the ec2.py tag grouping in order to reach all nodes tagged with ansibleGroup:mongoCluster key:value.

$ ansible-playbook mongo-cluster.yml

4. Clustering the mongo cluster is not yet automated and can be done by ssh-ing into a mongo instance and running the following commands. This will create a leader node and two secondary nodes. Note: You may have to use the IP address if the DNS record is not live yet.

$ docker exec -it mongo bash
$ mongo
$ rs.initiate( {
   _id : "rs",
   members: [               
      { _id: 0, host: "mongo-0.test.com:27017" }, 
      { _id: 1, host: "mongo-1.test.com:27017" },
      { _id: 2, host: "mongo-2.test.com:27017" }
   ]
})

Since this was a demonstration, there are multiple areas of improvement

• Store the terraform state file in an S3 bucket
• Automatic clustering of mongo nodes
• container orchestration for deployments
• for high IOPS optimization the host network can be used for the mongo