119 lines
3.7 KiB
Markdown
119 lines
3.7 KiB
Markdown
title: Hosting Django Apps in Kubernetes [WIP]
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---
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pub_date: 2022-06-03
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---
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author: ungleich django team
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---
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twitter_handle: ungleich
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---
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_hidden: no
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---
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_discoverable: yes
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---
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abstract:
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Django running on top of k8s
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---
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body:
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## Situation
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At ungleich we are hosting quite a lot of Django applications. As of
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2022-06-03, most of them are still deployed on a traditional VM based
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setup.
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We are using this blog entry to document a possible blueprint and the
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progress of migration at ungleich.
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## General design
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Our Kubernetes clusters usually use
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[ArgoCD](https://argo-cd.readthedocs.io/en/stable/) for deployments,
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so Django applications should potentially be defined the same way.
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Most of our kubernetes applications are defined in helm charts and
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thus the "general django application" should probably also be defined
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in a helm chart.
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## Freedom of choice
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While as a hoster it might be tempting to define a specific image that
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Django applications should be using (like
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[python](https://hub.docker.com/_/python)), but we want to give us and
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our customers the freedom to choose the image they use themselves. It
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might potentially even come from a private registry.
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## Interface definition
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All of our Django applications are using Postgresql for storing
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data. Postgresql is used by quite some other applications that we
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deployed in k8s, so this is a no-brainer. Django hosting at ungleich,
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even in k8s, will be based on Postgresql.
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Static data of Django applications can easily be stored on a PVC. This
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has the drawback that filesystem PVCs based on ceph block devices are
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usually RWO and thus in case of restart, there will be a short downtime.
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This is, generally speaking probably accepted, like a deploy would
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have caused a short downtime on a VM as well.
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However alternatives would be a shared filesystem (such as
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NFS/CephFS), but they are usually slower than dedicated block
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devices - so reliability can be traded against speed. Maybe we offer
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both options or add an NFS server as an option to our Django Hosting.
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## Django startup / processes
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On startup, Django will need to ensure the database schema
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has been upgraded to the latest version. so something like `python
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manage.py migrate` should probably be called in an InitContainer for
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most apps. We could specify that the customer provided container
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supports multiple commands:
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- /init - anything that needs to be done *once* on startup
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- /run - something that runs the actual site
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Some django apps however utilise Celery
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or [Django Q](https://django-q.readthedocs.io/en/latest/)
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for async tasks. We don't know which system is used, but it would be easy to
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add a flag to the hosting whether or not a third container
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should be utilised. Thus we could define:
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- If async container is defined, enable it and run /async (or similar)
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## Secrets
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Django applications usually have some kind of secrets and most Django
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applications have DIFFERENT types of secrets. Thus defining a specific
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environment variable does not seem to be a smart idea.
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Instead, we should probably offer to store secrets in something like
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[SealedSecrets](https://github.com/bitnami-labs/sealed-secrets).
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Database connection information is provided by default and is
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cluster/app specific.
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## Deployment
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We are likely going with a git-ops style deployment in which
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everything is defined for the Django app. This repository is read
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write for each client/customer.
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This probably includes:
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- (Possible encrypted) Secrets
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- Image definitions
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- Maybe even (part of) a pod definition?
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Some parameters are likely to be stored in a different, ungleich only
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writable repository, such as:
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- Size of RAM
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- Number of CPUs
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- Storage size (Postgresql, Static files, ...)
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## Status
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This document is still WIP and will be used as a basis for
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deploying our own Django apps first. |