AWS: By what IP address should I reference on an EC2 instance where is running Redis?

Question

I have 2 EC2 instances - on one is the production app, on the other one is Redis. I connect from the first one to Redis.

This is how I set Sidekiq:

Sidekiq.configure_server do |config|
    config.redis = { url:       "redis://deployer_redis@PUBLIC_IP:6379/#{env_num}", 
                     namespace: "rails_app_#{Rails.env}" }
  end

  Sidekiq.configure_client do |config|
    config.redis = { url:       "redis://deployer_redis@PUBLIC_IP:6379/#{env_num}", 
                     namespace: "rails_app_#{Rails.env}" }
  end

When I do restart the Redis instance, the PUBLIC_IP will be different and the Rails app will not be able to connect to Redis.

I tried to use the PRIVATE_IP instead of the PUBLIC_IP (as PRIVATE_IP doesn't change while ), but it didn't work. There were no errors, but simply the jobs were not processed.

What's the recommended way to set the IP of EC2 instances in applications?

EDIT:

Also, when I ssh to the server, which IP should I use?

Answer 1

If you use ElastiCache with Redis then you should use DNS of your cluster. It usually looks like this redis.j5k8wa.0001.use1.cache.amazonaws.com and you can get it from your node configuration. Amazon recommends creating CNAME record in your DNS as an alias to the name provided by them.

If you run Redis on your own instance then the easiest way is using private static IP address for it. Another alternative is using dynamic DNS but this setup is fragile because of DNS propagation latency.

Update: Using Elastic IP would be your third option but using public IP address for internal communications between your app and Redis is not very secure and it is wasteful.