Running copies of the same multi-container app with Docker Compose

Question

Problem

I want to run a webapp via Docker by running 2 containers as a unit.

1 container runs my web-server (Tomcat 7).

The other container runs my database (Postgres 9.4).

I can run docker-compose up and Docker is able to spin up my two containers as specified in my docker-compose.yml:

web:                                                                                     
  build: .                                                                         
  ports:                                                                           
   - "5000"                                                                        
  links:                                                                           
   - db                                                                         
db:                                                                             
  image: postgres

I'd like to be able to spin up another copy of my webapp by running docker-compose up again, but this results in Docker telling me that there are already containers running:

$ docker-compose up -d
Creating composetest_db_1
Creating composetest_web_1
$ docker-compose up -d
composetest_db_1 is up-to-date
composetest_web_1 is up-to-date

My work around

I've gotten around this issue by using the -p option to give new copies different project names:

$ docker-compose -p project1 up -d
...
Successfully built d3268e345f3d
Creating project1_web_1
$ docker-compose -p project2 up -d
...
Successfully built d3268e345f3d
Creating project2_web_1

Unfortunately, this creating new images for each copy:

$ docker images
project1_web          latest              d3268e345f3d        2 hours ago         682 MB
project2_web          latest              d3268e345f3d        2 hours ago         682 MB

Question

Is there a way to use docker-compose to spin up multiple instances of a multi-container app by using a single image?

Answer 1

You can re-use your docker compose template by specifying the project name (which defaults to the directory name):

$ docker-compose --project-name inst1 up -d
Creating inst1_web_1

$ docker-compose --project-name inst2 up -d
Creating inst2_web_1

You could also scale up the container instances within a project:

$ docker-compose --project-name inst2 scale web=5
Creating and starting 2 ... done
Creating and starting 3 ... done
Creating and starting 4 ... done
Creating and starting 5 ... done

There should now be 6 containers running:

$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS                     NAMES
5e4ab4cebacf        tomcat:8.0          "catalina.sh run"   43 seconds ago      Up 42 seconds       0.0.0.0:32772->8080/tcp   inst2_web_2
ced61f9ac2db        tomcat:8.0          "catalina.sh run"   43 seconds ago      Up 42 seconds       0.0.0.0:32773->8080/tcp   inst2_web_5
efb1ef13147c        tomcat:8.0          "catalina.sh run"   43 seconds ago      Up 42 seconds       0.0.0.0:32771->8080/tcp   inst2_web_4
58e524da3473        tomcat:8.0          "catalina.sh run"   43 seconds ago      Up 42 seconds       0.0.0.0:32770->8080/tcp   inst2_web_3
0f58c3c3b0ed        tomcat:8.0          "catalina.sh run"   2 minutes ago       Up 2 minutes        0.0.0.0:32769->8080/tcp   inst2_web_1
377e3e5b03e4        tomcat:8.0          "catalina.sh run"   2 minutes ago       Up 2 minutes        0.0.0.0:32768->8080/tcp   inst1_web_1

Answer 2

If you want to reuse the image, you should build the image independent of the compose script.

run docker build -t somewebapp/web:latest

Then change your build section of docker-compose.yml to reference an image.