<center><img src="https://www.docker.com/sites/default/files/vertical.png"></center>
<center><sup><a href="https://www.docker.com/legal/brand-guidelines">The original Docker logo</a></center>

***

We continue to learn Docker. See the [first part](https://steemit.com/docker/@emrebeyler/docker-notes-2), [second part](https://steemit.com/docker/@emrebeyler/docker-notes-2) on the learning notes.

#### docker compose

A tool for running multi-container application. An application may need postgresql, redis, or any other 3rd party servers.

Building all of them inside a single container doesn't make any sense in practical.

Example: Running two containers; first one is the web app, second one is the mongodb instance. (filename convention: ```docker-compose.yml```.

***
```
services:
     web:
         image: "emrebeyler/my-web-app"
     database:
         image: "mongodb"
```
***

use ```docker compose up``` to run the containers.


###### Port mapping
***

***
```
services:
     web:
         image: "emre/my-web-app"
         ports:
              - "80:5000"
```
***

###### Volume mapping

***
```
services:
        database:
             image: "mongodb"
             volumes:
                 - /opt/data:/var/lib/mongodb
```
***

###### Stop or Remove containers
***

- ```docker-compose stop``` to stop containers.

- ```docker-compose down``` to stop everything and remove the containers entirely.

###### Passing environment files to containers
***

```
services:
        database:
             image: "mongodb"
             environment:
                - MYSQL_ROOT_PASSWORD=password
```
***

It's also possible to set a environment file:

***
```
web:
  env_file:
    - web-variables.env
```
***

It's a good practice to share the configuration information via environment variables.

##### Placing dependencies on containers
***

It's not a good idea to fire up the web app before the database.

***
```
version: '3'
services:
  web:
    depends_on:
      - db
  db:
    image: postgres
```
***

docker compose will start the postgres first, and web later since it. It follows the dependency order.

##### Networking between services
***

By default, every service can reach each other by their image names. No need to know which IP they operate on.

***
```
version: '3'
services:
  web:
    depends_on:
      - db
  db:
    image: postgres
```
***

web service can reach the postgres server by using ```postgres://db:5432``` as a connection string.