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
DevotionGeo
There are 3 main formatters for Erlang which you can use from the command-line, rebar3_format, Steamroller elmfmt. Visual Studio Cod...
New
First poster: bot
In this post we’re going to be looking at a more advanced use of Gleam’s type system, known as phantom types. Hopefully by the end of thi...
New
First poster: bot
Django 3.2 is just around the corner and it’s packed with new features. Django versions are usually not that exciting (it’s a good thing!...
New
First poster: bot
Creation vs. Evolution Consider the history of Elixir: first you take Erlang, which was invented by Joe Armstrong and team to solve the ...
New
First poster: bot
I’ve been more serious about learning Rust recently, after dragging on with passive learning for a while. My first real programming langu...
New
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
penelopa
Learn how set up an RTMP server for free using the open-source Red5 software. This tutorial covers all steps from downloading the code fr...
New

Other popular topics Top

Devtalk
Reading something? Working on something? Planning something? Changing jobs even!? If you’re up for sharing, please let us know what you’...
1052 21915 398
New
dasdom
No chair. I have a standing desk. This post was split into a dedicated thread from our thread about chairs :slight_smile:
New
PragmaticBookshelf
Design and develop sophisticated 2D games that are as much fun to make as they are to play. From particle effects and pathfinding to soci...
New
AstonJ
I have seen the keycaps I want - they are due for a group-buy this week but won’t be delivered until October next year!!! :rofl: The Ser...
New
PragmaticBookshelf
Learn different ways of writing concurrent code in Elixir and increase your application's performance, without sacrificing scalability or...
New
rustkas
Intensively researching Erlang books and additional resources on it, I have found that the topic of using Regular Expressions is either c...
New
PragmaticBookshelf
Use WebRTC to build web applications that stream media and data in real time directly from one user to another, all in the browser. ...
New
AstonJ
Was just curious to see if any were around, found this one: I got 51/100: Not sure if it was meant to buy I am sure at times the b...
New
PragmaticBookshelf
Author Spotlight Rebecca Skinner @RebeccaSkinner Welcome to our latest author spotlight, where we sit down with Rebecca Skinner, auth...
New
New