Communication in Docker - Networks / Requests (section 3)

In many application, you'll need more than one container - for two main reasons:

1. It's considered a good practice to focus each container on one main task(e.g. run a web server, run a database, ...)
2. It's very hard to configure a Container that does more than one main thing (e.g. run a web server AND a database)

Multi-Container apps are quite common, especially if you're working on real applications.

Often, some of these Containers need to communicate though:

• either with each other
• or with the host machine
• or with the world wide web

Communicating with the World Wide Web (WWW)

Communicating with the WWW (i.e. sending Http request or other kinds of requests to other servers) is thankfully very easy.

Consider this JavaScript example - though it'll always work, no matter which technology you're using:

fetch('https://some-api.com/my-data').then(...)

This very basic code snippet tries to send a GET request to some-api.com/my-data.

This will work out of the box, no extra configuration is required! The application, running in a Container, will have no problems sending this request.

Communicating with the Host Machine

Communicating with the Host Machine (e.g. because you have a database running on the Host Machine) is also quite simple, though it doesn't work without any changes.

One important note: If you deploy a Container onto a server (i.e. another machine), it's very unlikely that you'll need to communicate with that machine. Communicating to the Host Machine typically is a requirement during development - for example because you're running some development database on your machine.

Again, consider this JS example:

fetch('localhost:3000/demo').then(...)

This code snippet tries to send a GET request to some web server running on the local host machine (i.e. outside of the Container but not the WWW).

On your local machine, this would work - inside of a Container, it will fail. Because localhost inside of the Container refers to the Container environment, not to your local host machine which is running the Container / Docker!

But Docker has got you covered!

You just need to change this snippet like this:

fetch('host.docker.internal:3000/demo').then(...)

host.docker.internal is a special address / identifier which is translated to the IP address of the machine hosting the Container by Docker.

Important: "Translated" does not mean that Docker goes ahead and changes the source code. Instead, it simply detects the outgoing request and is able to resolve the IP address for that request.

Communicating with Other Containers

Communicating with other Containers is also quite straightforward. You have two main options:

1. Manually find out the IP of the other Container (it may change though)
2. Use Docker Networks and put the communicating Containers into the same Network

Option 1 is not great since you need to search for the IP on your own and it might change over time.

Option 2 is perfect though. With Docker, you can create Networks via docker network create SOME_NAME and you can then attach multiple Containers to one and the same Network.

Like this:

docker run -network my-network --name cont1 my-image
docker run -network my-network --name cont2 my-other-image

Both cont1 and cont2 will be in the container names under same Network.

Now, you can simply use the Container names to let them communicate with each other - again, Docker will resolve the IP for you (see above).

fetch('cont1/my-data').then(...)

Commands used in this project

Here are the commands used in this project

Using host machine's DB

1. Connecting mongo (docker) from host machine

Replace mongodb://localhost:27017 by mongodb://host.docker.internal:27017

2. Building the project

docker build . -t actionanand/docker_communication:tagName

3. Running the container

docker run -d -p 3000:80 --rm --name docker_communication actionanand/docker_communication:tagName

Using docker's DB (with dedicated IP)

1. Running mongo image

docker run -d --rm --name mongoCommunication mongo:7.0

2. Finding IP of the mongo (running inside docker)

docker container inspect mongoCommunication

or

docker inspect mongoCommunication

• Look for the IPAddress property under the object NetworkSettings
• Just consider, 172.17.0.3 is the IP, then
• Replace mongodb://localhost:27017 by mongodb://172.17.0.3:27017

3. Building the project

docker build . -t actionanand/docker_communication:tagName

4. Running the container

docker run -d -p 3000:80 --rm --name docker_communication actionanand/docker_communication:tagName

Using docker's DB (docker network)

For this, All the containers should be under the same network

1. Create a new docker network

docker network create network_name

docker network create mongo-net

• You can view the existing networks by docker network ls
• You can remove the unused networks by docker network rm network_name
• docker network --help is for help related to docker network

2. Running mongo image with volume and docker network

docker run -d --rm -v mongo-com-vol:/data/db --name mongoCommunication --network mongo-net mongo:7.0

• -v mongo-com-vol:/data/db is named volume with the name mongo-com-vol

• You can view the existing volumes by docker volume ls

• You can remove the unused volume by docker volume rm volume_name

• docker volume --help is for help related to docker volume

• Change the DB connection in the project as below:

mongodb://<mongo db container name>:27017

mongodb://mongoCommunication:27017

3. Building the project

docker build . -t actionanand/docker_communication:tagName

4. Running the container with docker network

docker run -d -p 3000:80 --rm --name docker_communication --network mongo-net actionanand/docker_communication:tagName

5. You can open the postman app(or any other similar client) and try the following API end points

   1. GET request for http://localhost:3000/movies

   2. GET request for http://localhost:3000/people

   3. POST request for http://localhost:3000/favorites with the following body as example

      {
        "name": "A New Hope",
        "type": "movie",
        "url": "https://swapi.dev/api/films/1/"
      }

      And make sure to keep the type as either movie or character. If you use anything apart from this, error will be thrown from backend

   4. GET request for http://localhost:3000/favorites

      

Bonus

Accessing DB running inside docker from host machine (our local computer with localhost)

1. Exposing mongo(docker) port(27017) to host machine's local host

docker run -d --rm -p 20243:27017 -v mongo-local-vol:/data/db --name mongoDbLocal mongo:7.0

2. Connect to the following URl using MongoDB Compass, Studio 3T or NoSQL manager free - for multi connection

mongodb://localhost:20243

Docker Image's public URL

• actionanand/docker_communication

Associated repos:

1. Docker Basics
2. Managing Data and working with volumes
3. Docker Communication
4. Docker Multi-container with docker-compose
5. Docker Utility Containers & Executing Commands
6. Laravel with Docker
7. Angular with Docker