Sharing precompiled assets across docker containers

Question

I have an nginx container separate from my rails container and want to be able to serve precompiled assets from rails with the nginx container. This sounds like a job for a volume container but I have got myself confused after having quickly needing to learn docker and reading the documentation endlessly. Has anybody had to deal with a similar situation?

Answer 1

I'm having the same issue. Here's what I'm currently working on:

Option 1: use a single image for both nginx and the app

This way, I can build the image once (with the app, precompiled assets and nginx), then run two instances of it: one running the app server, and another for the nginx frontend:

docker build -t hello . docker run --name hello-app hello rackup docker run --name hello-web -p 80:80 --link hello-app:app hello nginx 

Not pretty, but very easy to set up and upgrade.

Option 2: use a shared volume, and precompile assets as a job

Shared volumes cannot be updated in the build process, but can be updated by a container instance. So we can run our rake task to precompile the assets just before running our app:

docker build -t hello . docker run -v /apps/hello/assets:/app/public/assets hello rake assets:precompile docker run --name hello-app hello rackup docker run --name hello-web -p 80:80 --link hello-app:app -v /apps/hello/assets:/usr/share/nginx/html/assets nginx 

This looks like a more robust option, but will require a more complex instrumentation. I'm leaning towards this option, however, since we'll need a separate job for database migrations anyway.

Option 3: distribute the assets to a CDN on build-time

Your Dockerfile can upload the resulting assets directly to a CDN. Then you configure your Rails app to use it as the asset_host. Something like:

RUN rake assets:precompile && aws s3 sync public/assets s3://test-assets/ 

I'm currently experimenting with this option. Since I'm using Amazon CloudFront, looks like I can just sync the resulting assets to S3 using the AWS CLI. There's also a gem for that (asset_sync), but it looks stale.

The downside is that you'll have to send the needed AWS credentials to the build context or the Dockerfile itself – this might need committing them to your source repository, if you're using an automated build.