Zero downtime on Heroku

Question

Is it possible to do something like the Github zero downtime deploy on Heroku using Unicorn on the Cedar stack?

I'm not entirely sure how the restart works on Heroku and what control we have over restarting processes, but I like the possibility of zero downtime deploys and up until now, from what I've read, it's not possible

There are a few things that would be required for this to work.

1. First off, we'd need backwards compatible migrations. I leave that up to our team to figure out.
2. Secondly, we'd want to migrate the db right after a push, but before the restart (assuming our migrations are fully backwards compatible, this should not affect anything)
3. Thirdly, we'd want to instruct Unicorn to launch a new master process and fork some workers, then swap the PIDs and gracefully shut down the old process/workers

I've scoured the docs but I can't find anything that would indicate this is possible on Heroku. Any thoughts?

Answer 1

I can't address migrations, but the part about restarting processes and avoiding wait time:

There is an beta feature for heroku called preboot. After a deploy, it boots your new dynos first and waits a while before switching traffic and killing the old ones:

https://devcenter.heroku.com/articles/labs-preboot/

I also wrote a blog post that has some measurements on my app's performance improvements using this feature:

http://ylan.segal-family.com/blog/2012/08/27/deploy-to-heroku-with-near-zero-downtime/