How Quasa.rs utilizes Blue/Green Deployments

Artist’s rendering of a quasar (ESO/M. Kornmesser CC BY 4.0)

I am the creator and administrator of quasa.rs, a social link-sharing web application (like reddit or hackernews) for astrophysics. It’s a fun side project that keeps me from getting rusty with Ruby on Rails because, sadly, I don’t use Ruby for my day job anymore. Plus I get to talk and think about space! What’s more fun than that?

Because quasa.rs is a side project, it was important to keep its server costs low. However, just having a single box wasn’t an acceptable solution either: Quasa.rs does have some users, so I need to have some way to prevent downtime during deployments. With that in mind, I thought that Blue/Green Deployments would be the best deployment strategy for quasa.rs.

Martin Fowler explains Blue/Green Deployments better than I could ever do here, but i’ll try to explain it briefly here for the unaware. Essentially, you have two production environments—one “green” and one “blue.” A floating IP or load balancer would route production traffic to whichever environment was “active,” therefore making the other environment the “idle” environment. Whenever you start a deployment, you deploy to whichever environment is currently the “idle” one, verify that the deployment was successful, and then toggle your floating IP or load balancer to start routing production traffic to it and thereby making it the “active” environment. When you make the switch, of course, the old “active” environment becomes “idle.”

Let me show you how quasa.rs specifically does this and hopefully that’ll make things clearer.

As you can see, requests to quasa.rs get routed to a floating IP that currently points to the quasars-app-green box. If we were to start a new deployment, we would deploy to quasars-app-blue, run any verifications we deem necessary, and then toggle the floating IP to point to quasars-app-blue.

This has several advantages.

  • If the deploy goes badly, we can always flip the floating IP back to the original box while keeping the bad deployment in an unaltered state to investigate.
  • Both our “production” and “staging” environments take turns receiving real production traffic, assuring that our staging environment will never be stale or deviate from production.
  • In our case, we can run migrations and verifications on the box we deployed to without interrupting user traffic.
  • If I suddenly need scale, I can always swap out the floating IP with a load balancer and serve requests to both environments (does require manually asserting that both environments have the same deployment version which is less-than-ideal).
  • For roughly $15/month I can have an active-active setup.
Martin Fowler’s diagram of blue/green deployments

You may have noticed that both quasars-app-green and quasars-app-blue connect to the same database quasars-db. While Martin Fowler mentions this strategy as a provision, it is not the standard way he diagrams it. Instead, he has two separate databases, one for each environment. However, I decided against this for a couple reasons.

For starters, adding another database would increase my monthly costs by another $5, which was not ideal for a hobby project. If I felt like it was necessary to have it anyways, though, I would also have to figure out how to reconcile the two databases in an eventually-consistent manner. Something I felt was out of scope for a small web application. For my purposes, just having one database has been perfect.

I still have a few corners to button up—for example, I still manually toggle the floating IP in DigitalOcean’s dashboard instead of having that step be automated—but otherwise this deployment strategy has worked great for a small project. In fact, I suspect that this would be a great way to get a startup’s application off the ground in a relatively inexpensive manner. It scales well too, as you can have each environment be a self-contained cluster of boxes behind a load balancer instead of a solitary box.

Thanks for reading and feel free to sign up at https://quasa.rs if astrophysics and space interest you!


Update: I recently made changes so that floating IP assignment is completely automated during deployments. Check out the PR here!