Ruby on Rails 无法连接到 Docker 容器中的数据库主机

Question

My Docker setup is listed in this ticket for reference: Ruby on Rails 4.1.8 Gem::LoadError for mysql2 gem

Building a Ruby Docker image FROM ruby:2.1.3 works fine until I run bundle exec rails server . At this point an error is thrown:

/usr/local/bundle/gems/mysql2-0.3.20/lib/mysql2/client.rb:70:in `connect': Unknown MySQL server host 'dokku-mariadb-runrite-production' (25) (Mysql2::Error)

The MariaDB instance is a Docker container created using the MariaDB Dokku plugin. The MariaDB instance's image does not have OpenSSL installed but I've deployed more recent stacks using Dokku before and never had to worry about that.

What could be going on?

Answer 1

It looks like the hostname cannot be resolved. The likely causes are one of the following:

1. The instance name isn't what you think it is. You might need to make sure the --name flag when you started the MariaDB docker instance is correct, or override the --hostname of the MariaDB container to match the name you're referring to in the Ruby container.
2. The two containers aren't attached to the same network. You can make sure they're both on the same network by specifying it in the --network flag when using docker run.

You may also have something unusual related to your DNS resolver setup, but I'm guessing based on this question you haven't done anything that would have altered the defaults, so I'm guessing it's not a factor.

If you're using docker compose or another method of starting the containers, the same issues apply, but docker-compose has configuration options in the YAML for network and hostname, and docker-compose will quietly create a few hostname aliases for each instance that you bring up (usually something like project_containername, containername, and containername-replicanumber).