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
It’s not legacy code — it’s PHP. Vimeo has been using PHP in production for over 15 years. Find out how we keep a million lines of PHP i...
New
First poster: bot
When I need to configure something in a complicated way, I find myself reviewing the embedded language that provided the server to create...
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
paulanthonywilson
Following up on the previous post on using UDP multicasting to broadcast and detect peers on a network, I create a registry of those peer...
New
First poster: bot
This post is a spiritual successor to Loris Cro’s Go cross-compilation. The encounter During a recent stage 2 meeting Jakub Konka wanted...
New
axelson
I describe how we use Hot Reloading with Webpack to develop faster and show how to integrate Webpack 5, webpack-dev-server, and Phoenix f...
New
brainlid
There is a new community resource available on writing “Safe Ecto Migrations”. When we get a migration wrong, it can lock up your product...
New
brainlid
In episode 78 of Thinking Elixir, we talk with Chase Granberry about Logflare. We learn why Chase started the company, what Logflare does...
New
Johannes-Werbrouck
In this blog post over on Fly.io I take a look at PHP 8.1’s new Enum class, and show you how it can be used to set up a level structure f...
New

Other popular topics Top

PragmaticBookshelf
Free and open source software is the default choice for the technologies that run our world, and it’s built and maintained by people like...
New
PragmaticBookshelf
Learn from the award-winning programming series that inspired the Elixir language, and go on a step-by-step journey through the most impo...
New
DevotionGeo
I know that these benchmarks might not be the exact picture of real-world scenario, but still I expect a Rust web framework performing a ...
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
AstonJ
poll poll Be sure to check out @Dusty’s article posted here: An Introduction to Alternative Keyboard Layouts It’s one of the best write-...
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
AstonJ
Continuing the discussion from Thinking about learning Crystal, let’s discuss - I was wondering which languages don’t GC - maybe we can c...
New
New
husaindevelop
Inside our android webview app, we are trying to paste the copied content from another app eg (notes) using navigator.clipboard.readtext ...
New
NewsBot
Node.js v22.14.0 has been released. Link: Release 2025-02-11, Version 22.14.0 'Jod' (LTS), @aduh95 · nodejs/node · GitHub
New