brentjanderson

brentjanderson

Zero downtime Postgres upgrades | Knock

We recently performed a zero downtime Postgres upgrade at Knock. This blog post goes into details about the Postgres considerations, although the BEAM & Elixir ecosystem were invaluable in managing the cutover process.

Some Elixir-related bits:

  1. We used DynamicSupervisor to start/stop Oban during the cutover. Without going into too much detail, Oban needs a Notifier to coordinate global state, and we rely on the default Postgres notifier (Although there is a new PubSub notifier that we have our eyes on). This Notifier establishes a persistent Postgres connection that is managed outside of your Ecto Repo, and the easiest way to move Oban over to the new instance was to stop and restart it with a new configuration using DynamicSupervisor
  2. Metaprogramming made it easier to build facade interfaces to our Ecto Repos so that database requests could be routed to the old or new database instance, based on the state of a launch darkly flag. Internally, LD uses ETS and a persistent websocket connection, which helps lookups be fast and up to date. ETS is still not quite fast enough for the volume of DB queries in our system, and so we also used :persistent_term to set the flag that determined what state we were in during the cutover process. Benchee helped us verify that we weren’t going to have a performance regression.
  3. We looked at Ecto Repo’s get_dynamic_repo function, but determined that using a facade was a better fit for our use case. It’s worth knowing what’s available, though, since Ecto comes with a lot of extensibility out of the box.
  4. The concurrency model of the BEAM makes it so much easier to reason about all of the stuff involved in a migration like this. Although you can take a similar approach with most languages, the BEAM didn’t get in the way of reasoning through the specifics of the cutover, especially around how we maintain uptime and data consistency during a cutover like this.

We may do a full post on the BEAM-related details of this upgrade in the future.

Where Next?

Popular Backend topics Top

New
First poster: bot
Rust vs Go — Bitfield Consulting. Which is better, Rust or Go? Which language should you choose for your next project, and why? How do t...
New
First poster: AstonJ
Ten years without Elixir. I never got into Elixir, largely because it looked like Ruby. I was a Rubyist for a good while, spent time and...
New
paulanthonywilson
So you’re enjoying using WebSockets with Elixir’s Phoenix Framework, and you want to send some binary messages. Maybe it’s an audio clip,...
New
First poster: Exadra37
Summary: I describe a simple interview problem (counting frequencies of unique words), solve it in various languages, and compare perform...
New
First poster: brennan
The perspective of an ignorant computer science undergrad It’s likely that you read the title of this post and thought “what is this guy ...
New
New
brainlid
We take a deeper dive with Nathan Long into IOLists in Elixir. We cover what they are, how they work, the power they have when concatenat...
New
brainlid
In episode 81 of Thinking Elixir, we talk with Digit and Quinn Wilton about the Burrito project. It wraps up Elixir to a single binary, e...
New
StuntProgrammer
In building lofi.limo, media storage and distribution naturally came up. I have songs, announcements, and background image loops which I ...
New

Other popular topics Top

PragmaticBookshelf
Ruby, Io, Prolog, Scala, Erlang, Clojure, Haskell. With Seven Languages in Seven Weeks, by Bruce A. Tate, you’ll go beyond the syntax—and...
New
AstonJ
What chair do you have while working… and why? Is there a ‘best’ type of chair or working position for developers?
New
wolf4earth
@AstonJ prompted me to open this topic after I mentioned in the lockdown thread how I started to do a lot more for my fitness. https://f...
New
Exadra37
Please tell us what is your preferred monitor setup for programming(not gaming) and why you have chosen it. Does your monitor have eye p...
New
Rainer
My first contact with Erlang was about 2 years ago when I used RabbitMQ, which is written in Erlang, for my job. This made me curious and...
New
AstonJ
Just done a fresh install of macOS Big Sur and on installing Erlang I am getting: asdf install erlang 23.1.2 Configure failed. checking ...
New
Exadra37
Oh just spent so much time on this to discover now that RancherOS is in end of life but Rancher is refusing to mark the Github repo as su...
New
New
AstonJ
This is a very quick guide, you just need to: Download LM Studio: https://lmstudio.ai/ Click on search Type DeepSeek, then select the o...
New
AstonJ
Curious what kind of results others are getting, I think actually prefer the 7B model to the 32B model, not only is it faster but the qua...
New