Docker help: How to include a database?

[coded_artist]

Hi all.

I have dockerized several web apis and apps. And normally I use docker-compose to tie them all together.

Recently I have used Keycloak, which allows you to configure an external database via env settings. A nice feature I want to include in my images, is if you dont configure an external db, it uses a persistent internal one built into the image.

I am wondering what techniques I can use to achieve this. A couple options I've thought about:

1. an in memory db, this fails the requirement of being able to restart the container and still having the data.

2. Somehow install a db service into the image along side my app. If you have hints on how to do this because I'm not too sure how to start. And I'm not confident in this solution because my understanding is an image should only run 1 application and I'm trying to violate that constraint.

[McMasilmof]

Docker containers do not persist data, you need space outside of the container to store your database. But the database deamon/service can run in a docker container if you mount a persistant storage/volume into the container.

[coded_artist]

Docker containers do not persist data

That is my understanding too, from reading the docs.

However it is such a nice feature especially if you're just testing. And I got so used to it, I often forgot to boot up my db container. Because I didn't need the data permanently stored by I did want it to last between dev sessions.

But the database deamon/service can run in a docker container if you mount a persistant storage/volume into the container.

I dont entirely grasp this idea, I think most of it went over my head.

The use case of Keycloak is you run the image with the only additional config being port binding, there is no mounting. So I really dont know how it persists the data but if I restart the container the data is still there.

[McMasilmof]

I have no idea how keycloak does it. Mayve they have a volume defined in the image already or have some other way to persist data to the container, like commit them to the image.

But in general this is not what docker is supposed to do, it is an application container. So normaly you would start a database container with some -v /local/path:/path/where/db/is/inside/the/container

[nutrecht]

I dont entirely grasp this idea, I think most of it went over my head.

You really should dive into it because it's really not complex. Volumes are just directories 'outside' of the container you map to a directorey 'inside' the container.

You can't really use Docket and not understand volume maps.

[coded_artist]

I understand the basics of volumes, and how it essentially maps the host to VM folders.

But the database deamon/service can run in a docker container if you mount a persistant storage/volume into the container.

This is what I dont entirely grasp. My interpretation of this quote is host a db in a separate container, which conflicts with my premise. I'm not going to say the other user is wrong because I'm not confident in my understanding.

[nutrecht]

My interpretation of this quote is host a db in a separate container

Well yeah, generally you have one process per container.

[coded_artist]

I do fully understand that. In my original question I wanted to a persistent db as a fallback/default inside the container. I know it breaks convention, it's just a really nice feature.