Connect to MongoDB in separate docker container (AWS ECS)

Question

I am using AWS ECS and have a container for my frontend (Node app) and for my backend (mongo database).

The mongo container is exposing port 27017, but I cannot figure out how to connect to it from my frontend container. If I try to connect to the db using 'mongodb://localhost:27017/db_name' I get an ECONNREFUSED error.

I have a service running for both of these task definitions with an ALB for the frontend. I don't have them in the same task definition because it doesn't seem optimal to have to scale them together.

I have tried multiple variations of the url

• mongodb://0.0.0.0:27017/db_name
• mongodb://localhost:27017/db_name

If I "curl" the mongo container from within the EC2 instance, I get an empty reply from server.

Database Task Definition:

{
  "executionRoleArn": null,
  "containerDefinitions": [
    {
      "dnsSearchDomains": null,
      "logConfiguration": null,
      "entryPoint": null,
      "portMappings": [
        {
          "hostPort": 27017,
          "protocol": "tcp",
          "containerPort": 27017
        }
      ],
      "command": null,
      "linuxParameters": null,
      "cpu": 0,
      "environment": [
        {
          "name": "MONGODB_ADMIN_PASS",
          "value": <PASSWORD>
        },
        {
          "name": "MONGODB_APPLICATION_DATABASE",
          "value": <DB NAME>
        },
        {
          "name": "MONGODB_APPLICATION_PASS",
          "value": <PASSWORD>
        },
        {
          "name": "MONGODB_APPLICATION_USER",
          "value": <USERNAME>
        }
      ],
      "ulimits": null,
      "dnsServers": null,
      "mountPoints": [],
      "workingDirectory": null,
      "dockerSecurityOptions": null,
      "memory": null,
      "memoryReservation": 128,
      "volumesFrom": [],
      "image": "registry.hub.docker.com/library/mongo:latest",
      "disableNetworking": null,
      "healthCheck": null,
      "essential": true,
      "links": null,
      "hostname": null,
      "extraHosts": null,
      "user": null,
      "readonlyRootFilesystem": null,
      "dockerLabels": null,
      "privileged": null,
      "name": "mongo"
    }
  ],
  "placementConstraints": [],
  "memory": null,
  "taskRoleArn": null,
  "compatibilities": [
    "EC2"
  ],
  "taskDefinitionArn": "arn:aws:ecs:us-east-2:821819063141:task-definition/dappy_coin_database:2",
  "family": "dappy_coin_database",
  "requiresAttributes": [
    {
      "targetId": null,
      "targetType": null,
      "value": null,
      "name": "com.amazonaws.ecs.capability.docker-remote-api.1.21"
    }
  ],
  "requiresCompatibilities": null,
  "networkMode": null,
  "cpu": null,
  "revision": 2,
  "status": "ACTIVE",
  "volumes": []
}

Answer 1

OLD:

You have to add, in the task definition for node, which I assume you have: links: ["mongo"] Then you can reference mongo://...

NEW:

Just saw that you want them in separate task definitions. That's a lot of complexity and I want to dissuade you from this path, because you are facing options like: ELB, service discovery via DNS, ambassador container pattern (per this answer - which, if that is all you wanted, this question is a dupe). If you have to do it, see that answer, and weep.

Maybe you would consider deploying your Node app as a single-container Elastic Beanstalk app, and connecting it to MongoDB Atlas? That way you get load balancing, auto-scaling, monitoring, all built in, instead of needing to do it yourself.

Or, at least you could use AWS Fargate. It is a launch mode of ECS that handles more of the infrastructure and networking for you. To quote the docs,

links are not allowed as they are a property of the “bridge” network mode (and are now a legacy feature of Docker). Instead, containers share a network namespace and communicate with each other over the localhost interface. They can be referenced using the following:

localhost/127.0.0.1:<some_port_number>

Where in this case, some_port_number = 27017.