Notes.md

Demo 2 - Connected Containers

Goal: Add another service to our local development setup.

Steps

1. Install dependancies and run

   docker-compose run app npm install
   docker-compose up

   Go hit up port 3000 and you will see that we are upserting timestamps to a MongoDB collection. Since we are not using data volumes however, all of this will be wiped out when the container is removed.

   docker-compose down
   docker-compose up app # The data has been wiped out

2. The magic of depends_on

   See that entry for depends_on inside of the docker-compose.yml file? That tells docker to link the mongo and app services together.

   That does things like letting us only have to declare one service when we start everything up

   docker-compose up app   # starts both app and mongo

   And it also creates network mappings so that our containers can communicate with each other using their service names.

   // app.js
   const url = 'mongodb://mongo:27017'; 

3. Overrides!

   Everytime that we run docker-compose up, it looks for a docker-compose.yml file, but it will also pull in services from docker-compose.overrides.yml. This is useful if we might want to have different configurations per environment, for example.

   In this case, since the api service is defined in both files, the .override configuration will win out. Thus, both app and api will be built.

   # Look for both docker-compose.yml and docker-compose.overrides.yml
   docker-compose up app api
   
   # Only look at docker-compose.yml
   docker-compose -f docker-compose.yml up app api
   
   # Override with production configuration
   docker-compose -f docker-compose.yml -f docker-compose.prod.yml up app api

4. (Bonus) "Works on my machine"

   So now that we are up and running, let's show why Docker is so helpful. Notice what version of node.js out containers are running?

   FROM node:10-alpine

   This is a really lightweight version of node 10 that we can use as the base of our Dockerfiles. And while node 10 supports async/await functions, those did not exist inside of node 6.

   If you change that line in the Dockerfile to say FROM node:6-alpine and then rebuild, you will notice that things don't seem quite right

   docker-compose build app
   docker-compose up
   
   # /usr/src/app/app.js:8
   # app_1    | async function upsertDocuments(req, res, next) {
   # app_1    |       ^^^^^^^^
   # app_1    | 
   # app_1    | SyntaxError: Unexpected token function

   Sure enough, the app crashed. This is the same thing that could happen without Docker if "Developer A" had node 10 installed locally, but another teammate had node 6 installed. But by source controlling our runtime with our code, that should not happen anymore!