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

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
We all know how to teach recursion. We’ve done it for decades. We pick some honored, time-tested examples—Fibonacci numbers and factorial...
New
AstonJ
Just finished doing a clean install of macOS (which I highly recommend btw!) and have updated my macOS Ruby & Elixir/Erlang dev env s...
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
AstonJ
Just listening to this now… Totally agree with @FrancescoC’s and @thompson_si’s comment “learn to learn” :sunglasses: In our talk we’...
New
First poster: AstonJ
They expect you to make a onepage application (SPA) The polaris design system officially only supports react Integration with the s...
New
First poster: bot
This post explains why Scala projects are difficult to maintain. Scala is a powerful programming language that can make certain small te...
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
CommunityNews
Have you ever wanted to write a structurally typed function in Rust? Do you spend a lot of time and effort getting your Rust struct s jus...
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

Other popular topics Top

AstonJ
If it’s a mechanical keyboard, which switches do you have? Would you recommend it? Why? What will your next keyboard be? Pics always w...
New
AstonJ
What chair do you have while working… and why? Is there a ‘best’ type of chair or working position for developers?
New
ohm
Which, if any, games do you play? On what platform? I just bought (and completed) Minecraft Dungeons for my Nintendo Switch. Other than ...
New
Exadra37
I am asking for any distro that only has the bare-bones to be able to get a shell in the server and then just install the packages as we ...
New
PragmaticBookshelf
Build highly interactive applications without ever leaving Elixir, the way the experts do. Let LiveView take care of performance, scalabi...
New
PragmaticBookshelf
Rails 7 completely redefines what it means to produce fantastic user experiences and provides a way to achieve all the benefits of single...
New
PragmaticBookshelf
Author Spotlight Jamis Buck @jamis This month, we have the pleasure of spotlighting author Jamis Buck, who has written Mazes for Prog...
New
PragmaticBookshelf
Author Spotlight: VM Brasseur @vmbrasseur We have a treat for you today! We turn the spotlight onto Open Source as we sit down with V...
New
CommunityNews
A Brief Review of the Minisforum V3 AMD Tablet. Update: I have created an awesome-minisforum-v3 GitHub repository to list information fo...
New
PragmaticBookshelf
Fight complexity and reclaim the original spirit of agility by learning to simplify how you develop software. The result: a more humane a...
New