Rails on Elasticbeanstalk: caching assets for deployment

18 March 2018 • Tags: ruby on rails — AWS — Elasticbeanstalk

Our Elasticbeanstalk adventure continues and one main task was to get deployment times down. Vanilla it did take way more than 10 minutes: 200 gems and quite a few js/css libs (incl. compass) did slow it down that much.

The previous setup with just AutoScaling kept the app on basically the same instances/AMI and especially the asset cache kicked in every time without me even remember that was a thing.

Elasticbeanstalk is a bit different, you won’t have the asset cache automatically. Thankfully Sebastian wrote a little ebextension to rectify this and speed up deployment. But quickly I ran into the same problem: when the scaling launched a new instance this instance still took ages to deploy. Sebianstan’s extension didn’t cover that but luckily we can simple use S3 (where we have a bucket anyways for the appdeploy) to copy a gzipped asset file to and from:

      if aws s3 ls s3://elasticbeanstalk-my-region-my-accountid/cache/assets.tar.gz ; then
        aws s3 cp --quiet "$(/opt/elasticbeanstalk/bin/s3-assets-cache.sh)" /tmp/assets.tar.gz
        [ -f /tmp/assets.tar.gz ] && tar -xf /tmp/assets.tar.gz -C $EB_APP_STAGING_DIR \
          && chown webapp:webapp -R "$EB_APP_STAGING_DIR/tmp"
      fi

and when we build the cache we store the file to S3:

      tar -zcf /tmp/cache/assets.tar.gz       tmp/cache/assets
      aws s3 cp /tmp/assets.tar.gz s3://elasticbeanstalk-my-region-my-accountid/cache/assets.tar.gz

Done. Well, fo rmost of you, but I’m not there yet. As I had to find out the hard way, we have one js gem (tinymce) that insists on copying the ASSET_HOST into the js output. Usually not a problem, but we have different assets hosts for different copies of the application, mostly for test purposes or keeping bigger features available for testing. We just rename the assets.tar.gz file to assets-my-environment.tar.gz, right? Yes, and to get that tiny big of information about the environment’s name you have to setup a policy for the IAM role and query via aws… This gist by Melvin did the trick for me, so much I put it into its own file that gets created from the ebextension:

  "/opt/elasticbeanstalk/bin/s3-assets-cache.sh":
    mode: "000755"
    owner: root
    group: root
    content: |
      #!/bin/bash
      set -xe

      INSTANCE_ID=$(/opt/aws/bin/ec2-metadata -i | awk '{print $2}')
      REGION=$(/opt/aws/bin/ec2-metadata -z | awk '{print substr($2, 0, length($2)-1)}')
      TAG="elasticbeanstalk:environment-name"

      ENVIRONMENT_NAME=$(aws ec2 describe-tags \
        --output text \
        --filters "Name=resource-id,Values=${INSTANCE_ID}" \
                  "Name=key,Values=${TAG}" \
        --region "${REGION}" \
        --query "Tags[*].Value")
      echo "s3://elasticbeanstalk-my-region-my-accountid/cache/assets-${ENVIRONMENT_NAME}.tar.gz"

Rails on Elasticbeanstalk: caching assets for deployment

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