How to setup database.yml to connect to Postgres Docker container?

Question

I have a Rails app. In the development and test environments, I want the Rails app to connect to a dockerized Postgres. The Rails app itself will not be in a container though - just Postgres.

What should my database.yml look like?

I have a docker default machine running. I created docker-compose.yml:

postgres:
  image: postgres
  ports:
    - "5432:5432"
  environment:
    - POSTGRES_USER=timbuktu
    - POSTGRES_PASSWORD=mysecretpassword

I ran docker-compose up to get Postgres running.

Then I ran docker-machine ip default to get the IP address of the Docker virtual machine, and I updated database.yml accordingly:

...
development: 
  adapter: postgresql
  host: 192.168.99.100
  port: 5432
  database: timbuktu_development
  username: timbuktu
  password: mysecretpassword
  encoding: unicode
  pool: 5
...

So all is well and I can connect to Postgres in its container.

But, if someone else pulls the repo, they won't be able to connect to Postgres using my database.yml, because the IP address of their Docker default machine will be different from mine.

So how can I change my database.yml to account for this?

One idea I have is to ask them to get the IP address of their Docker default machine by running docker-machine env default, and pasting the env DOCKER_HOST line into their bash_rc. For example,

export DOCKER_HOST="tcp://192.168.99.100:2376"

Then my database.yml host can include the line

host: <%= ENV['DOCKER_HOST'].match(/tcp:\/\/(.+):\d{3,}/)[1] %>

But this feels ugly and hacky. Is there a better way?

Your question really heped me with my docker setup :)

theDrifter
– theDrifter

2016-06-09 20:55:21 +00:00
Commented Jun 9, 2016 at 20:55 — theDrifter
– theDrifter, Commented Jun 9, 2016 at 20:55

VonC · Accepted Answer · 2024-01-10 09:37:48Z

2015: You could set a correct environment variable first, and access it from your database.yml:

host: <%= ENV['POSTGRES_IP'] %>

With a bashrc like (using bash substring removal):

export DOCKER_HOST=$(docker-machine env default)
export POSTGRES_IP=${DOCKER_HOST#tcp://}

2023: Reminder, the docker machine command, which was part of the Docker Machine tool, has been replaced by Docker Desktop's built-in features and Docker Compose.
Docker Machine was primarily used to provision and manage Docker hosts (or machines) on virtual environments.

The docker-machine env default command was used to set up the shell environment for interacting with a specific Docker machine, named default in this case. That command was particularly useful when managing multiple Docker hosts or when working with Docker running on a virtual machine.

In the modern Docker ecosystem, especially with Docker Desktop, the need for such a command is greatly reduced. Docker Desktop runs Docker directly on your host (either Windows or macOS), negating the need for a separate virtual machine to host Docker. So the Docker client is configured by default to communicate with Docker Desktop's daemon without additional environment setup.

However, if you are looking to replicate a similar setup where Docker runs in a virtual machine or a remote host, you would configure your Docker client to communicate with that Docker daemon. That is commonly done through environment variables, particularly DOCKER_HOST, DOCKER_CERT_PATH, and DOCKER_TLS_VERIFY.

You would need to:

set environment variables manually:
Determine the IP address and the port of your remote Docker daemon.
Set the DOCKER_HOST environment variable, and if necessary, DOCKER_CERT_PATH and DOCKER_TLS_VERIFY.
```
export DOCKER_HOST="tcp://[IP_ADDRESS]:[PORT]"
export DOCKER_CERT_PATH="[path/to/cert]"
export DOCKER_TLS_VERIFY=1
```
use SSH to connect to remote docker:
Docker also supports using SSH to connect to a remote Docker daemon.
```
export DOCKER_HOST="ssh://user@remote-host"
```
use Docker contexts:
Docker contexts can be used to manage different Docker endpoints.
Create a new context for your remote Docker daemon:
```
docker context create my-remote-docker --docker "host=tcp://[IP_ADDRESS]:[PORT]"
```
And use the context:
```
docker context use my-remote-docker
```

Again, in a modern setup, especially for local development, Docker Desktop eliminates the need for most of this configuration.
The steps above are more relevant if you are working with Docker in remote or specialized environments. However, with the evolution of Docker's ecosystem, especially the enhancements in Docker Desktop and Docker Compose, the functionalities of Docker Machine became redundant and less relevant.

Docker Desktop now includes features that cover most use cases of Docker Machine. It allows you to manage Docker containers, images, networks, and volumes, as well as build and run multi-container applications defined with Docker Compose. Docker Desktop's integration with cloud providers and its ability to handle local Kubernetes clusters further extend its capabilities beyond what Docker Machine offered.

% docker-machine env default zsh: command not found: docker-machine
@stackdave 8+ years later, I am not surprised: docker/machine has been archived 2 years ago (Sept. 2021). It has been replaced by Docker Desktop's built-in features and Docker Compose. I have edited the answer to make that clearer.

Sandy · Accepted Answer · 2015-10-11 17:31:00Z

3

I found a simpler way:

host: <%= `docker-machine ip default` %>

answered Oct 11, 2015 at 17:31

Sandy

2233 silver badges9 bronze badges

2 Comments

VonC Over a year ago

That should work too, I just prefer separating the config value (environment variable) from how that value is acquired/set. Still, +1.

sites Over a year ago

or host: <%= ENV['POSTGRES_IP'] || `docker-machine ip default`%>

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