Heroku-20 Stack
Last updated 19 November 2020
Table of Contents
This article describes the Heroku-20 stack, based on Ubuntu 20.04. What is a stack?
What’s new
This stack is now based on Ubuntu 20.04, compared to Ubuntu 18.04 used in the Heroku-18 stack.
The python program will now run Python version 3, as Python 2 has reached end-of-life status. Python 2 is still available under the python2 program. Users may also use Python 3 explicitly by invoking python3.
The availability of a python program is a change from a standard installation of Ubuntu 20.04, which will only offer python2 and python3 programs. The python-is-python3 package ensures that existing software that executes python will be able to do so, but this requires the called Python code to be compatible with Python version 3.
This change of default for python only affects customers who use Python for ancillary purposes such as scripting during app startup or at build time. Python apps using the official Heroku Python support always receive the exact Python version specified by the app’s codebase, which takes precedence over the python program provided at the operating system level.
Available software
Every stack on Heroku supports different operating system packages and language runtime versions. This support is typically confined to software that was still actively developed by the respective maintainers at the time the stack was first released.
Language runtimes
For the most accurate information on supported language runtime versions, please check the individual language pages:
| Buildpack | Shorthand | Runtime versions |
|---|---|---|
| Ruby | heroku/ruby |
Runtime versions |
| Node.js | heroku/nodejs |
Runtime versions |
| Python | heroku/python |
Runtime versions |
| Java | heroku/java |
Runtime versions |
| PHP | heroku/php |
Runtime versions |
| Go | heroku/go |
Runtime versions |
Operating system packages
For a full list of operating system packages available on Heroku-20, please refer to article Ubuntu Packages on Heroku Stacks.
Support period
Heroku-20 is based on Ubuntu 20.04. It will be supported through April 2025. Learn more about Heroku’s stack update policy.
Using Heroku-20
Heroku-18 is currently the default stack for newly created apps.
To use Heroku-20 instead, it must be specified explicitly when creating an app:
$ heroku create --stack heroku-20
You may change the stack on an existing app; the next build performed will then use the new stack:
$ heroku stack:set heroku-20
If you are using app.json, you should also specify the stack there to ensure that your Review Apps and Heroku CI runs use the same stack:
{
"stack": "heroku-20"
}
An existing app’s stack cannot be changed using app.json. The stack specified is only applied to newly created apps that are a Review App, a Heroku CI test run app, or an app created using a Heroku Button.
Upgrading to Heroku-20
Please refer to the stack upgrading guide to understand the procedures to follow when upgrading to a new stack.
We recommend that you monitor your application closely after migrating an app to the new stack to ensure it’s performing correctly.
Upgrade notes
The python program now points to Python 3
If your application’s code invokes the python program e.g. for scripting purposes during build or at runtime, this will now execute Python 3, so the code you’re running this way must be compatible with Python 3. You may alternatively use the python2 program to temporarily use Python 2, but keep in mind that this version of Python is end-of-life and no longer maintained.
This change of default for python only affects customers who use Python for ancillary purposes such as scripting during app startup or at build time. Python apps using the official Heroku Python support always receive the exact Python version specified by the app’s codebase, which takes precedence over the python program provided at the operating system level.
Heroku-20 Docker image
Heroku-20 is available as two Docker images:
- The runtime image (
heroku/heroku:20) is recommended for most workloads; - The build image (
heroku/heroku:20-build) is larger and includes development headers. It is only recommended for customers that need to compile source code.
Use the following command in your Dockerfile to use Heroku-20 as your base image:
FROM heroku/heroku:20
To learn more about deploying Docker images, please refer to the Heroku Container Registry and Runtime documentation.