Get Heroku Release Number / Code Version From a Running Dyno - heroku

Question

Is it possible to get a unique key per slug / release from a running dyno? I was following this article to setup RAILS_CACHE_ID (for expiring etags after deployments) but found that the dynos no longer ship with GIT configured (which causes this error):
fatal: Not a git repository (or any parent up to mount point /app)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
I also considered setting it in a config/initializers to the current Time but obviously that wouldn't work across multiple dynos. Any ideas?

Answer 1

There's a new (Nov 2015) labs feature which does just what you need "Dyno metadata" https://devcenter.heroku.com/changelog-items/768
heroku labs:enable runtime-dyno-metadata -a <app name>
Then on heroku:
~ $ env
HEROKU_APP_ID: 9daa2797-e49b-4624-932f-ec3f9688e3da
HEROKU_APP_NAME: example-app
HEROKU_DYNO_ID: 1vac4117-c29f-4312-521e-ba4d8638c1ac
HEROKU_RELEASE_VERSION: v42
HEROKU_SLUG_COMMIT: 2c3a0b24069af49b3de35b8e8c26765c1dba9ff0
HEROKU_SLUG_DESCRIPTION: Deploy 2c3a0b2

Answer 2

One solution is to use a git pre-push hook to set a heroku config value. Since it's done before the push and slug compilation, the config variable will be available as an ENV var to your rails app.
.git/hooks/pre-push:
#!/bin/sh
remote="$1"
url="$2"
while read local_ref local_sha remote_ref remote_sha
do
if [[ $url =~ heroku ]] ; then
app=$(git remote show -n $remote | sed -n -E -e 's/[[:space:]]+(Push[[:space:]]+URL)(\/|:).+(:|\/)(.*)\.git$/\4/gp')
echo Setting RAILS_CACHE_ID to $local_sha on app $app
heroku config:set RAILS_CACHE_ID=$local_sha --app $app
fi
done
exit 0
The pre-push.sample file has some documentation about the parameters that the hook is called with. I'm using verbose output of git remote to determine which app to set the config value on. The '-E' option for sed is for Mac OS X -- if you are using GNU sed replace that with '-r'.
Another solution is to use the heroku-api via a profile.d script to get the unique release id. This example uses curl to get the latest release id using the RANGE header. It's not the commit reference but it will be unique to every release, including rollbacks and config changes. You'll want to set the API_KEY and the APP_NAME as heroku config variables.
.profile.d/release.sh
# get release id and set as RAILS_CACHE_ID
# Heroku config variables that need to be set
# API_KEY: heroku api key (get from dashboard or `heroku auth:token`
# APP_NAME: set this to your app_name (this could be hardcoded in the profile.d script but
# would make it harder to manage apps with multiple environments
res=$(curl -s -H "Accept: application/vnd.heroku+json; version=3"\
-H "Authorization: Bearer $API_KEY"\
-H "Range: version ..; order=desc, max=1"\
-X GET https://api.heroku.com/apps/$APP_NAME/releases)
release_id=$(ruby -rjson -e "j = JSON.parse('$res'); puts j[0]['id']")
export RAILS_CACHE_ID=$release_id
In the rails app, ENV['RAILS_CACHE_ID'] should now be set to the most recent release id. You could also use this same strategy in a rails initializer.