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I have a docker-compose file that creates two instances, one for webserver and another for database.

When I run docker ps I get this kind of output:

7cb1fb603a98   apache          "/usr/bin/dumb-init …"   3 minutes ago    Up 3 minutes (healthy)    0.0.0.0:8080->8080/tcp             compose_webserver_1
74f3bbe506eb   postgres:13     "docker-entrypoint.s…"   6 minutes ago   Up 16 minutes (healthy)   5432/tcp                           compose_postgres_1

If I run docker images:

REPOSITORY        TAG               IMAGE ID       CREATED          SIZE
webserver        latest            17dc61fa0139   2 minutes ago    2.05GB
database         latest            17dc61fa0139   2 minutes ago    2.05GB

I see that both services share image_id. In my docker-compose.yml I have setup:

build:
      context: .
      dockerfile: Dockerfile-test

To run some ADD before creating it. Then, I am creating my own image using another image as a base in the FROM statement.

Once I have my new image with the main content, but also with my added requirement.txt, and other stuff I push it to ECR repository.

Then instead of working with my first two docker containers I work with a single container that has been created using these new image in my ECR repository.

It have both services in a single container, despite in my original docker-compose up --build it raised two containers. Why this is happening?

Is this correct, or should I force with the command statement to create two container instances from the new pulled imaged created by me? For example having always two container instances can allow me to then exchange the postgres container to one mysql for example.

mrc
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  • Does this answer your question? [Can I run multiple programs in a Docker container?](https://stackoverflow.com/questions/19948149/can-i-run-multiple-programs-in-a-docker-container) – Noam Yizraeli Aug 25 '21 at 11:03

2 Answers2

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In the answer below, I'll use the terms 'image' and 'container' and it's important to be aware of the difference. The image is what you build and what is stored in the registry. A container is the process that is running when the image is started.

When images have the same id, they're identical and they only take up the space of one copy of the image in your registry. Your docker-compose --build creates two images because you have 2 services defined in your compose file and it creates an image for each.

As for your setup the common approach is to have each service in it's own image and run them as separate containers. There's nothing wrong with having an image that can do multiple things based on the command you pass it, except that the image is larger than needed. On the other hand, I can't really see any advantages, so the best way is to have a database image and a webserver image that only do what they need.

You can have multiple processes running in a single container, but that's not something you see that often. In your case with a database and a webserver, you might want to scale the number of webservers and still have a single database. You can't do that if they're in the same container.

Hans Kilian
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Why docker containers share image id?

Because they can. Images are just "filesystems templates" to be used by containers.

About the image duplication. It is not duplication, it is a single image 17dc61fa0139 with two names webserver:latest and database:latest

pbacterio
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