Run multiple services in a container

A container’s main running process is the ENTRYPOINT and/or CMD at the end of the Dockerfile. It is generally recommended that you separate areas of concern by using one service per container. That service may fork into multiple processes (for example, Apache web server starts multiple worker processes). It’s ok to have multiple processes, but to get the most benefit out of Docker, avoid one container being responsible for multiple aspects of your overall application. You can connect multiple containers using user-defined networks and shared volumes.

The container’s main process is responsible for managing all processes that it starts. In some cases, the main process isn’t well-designed, and doesn’t handle “reaping” (stopping) child processes gracefully when the container exits. If your process falls into this category, you can use the --init option when you run the container. The --init flag inserts a tiny init-process into the container as the main process, and handles reaping of all processes when the container exits. Handling such processes this way is superior to using a full-fledged init process such as sysvinit, upstart, or systemd to handle process lifecycle within your container.

If you need to run more than one service within a container, you can accomplish this in a few different ways.

• Put all of your commands in a wrapper script, complete with testing and debugging information. Run the wrapper script as your CMD. This is a very naive example. First, the wrapper script:

  #!/bin/bash
  
  # Start the first process
  ./my_first_process -D
  status=$?
  if [ $status -ne 0 ]; then
    echo "Failed to start my_first_process: $status"
    exit $status
  fi
  
  # Start the second process
  ./my_second_process -D
  status=$?
  if [ $status -ne 0 ]; then
    echo "Failed to start my_second_process: $status"
    exit $status
  fi
  
  # Naive check runs checks once a minute to see if either of the processes exited.
  # This illustrates part of the heavy lifting you need to do if you want to run
  # more than one service in a container. The container exits with an error
  # if it detects that either of the processes has exited.
  # Otherwise it loops forever, waking up every 60 seconds
  
  while sleep 60; do
    ps aux |grep my_first_process |grep -q -v grep
    PROCESS_1_STATUS=$?
    ps aux |grep my_second_process |grep -q -v grep
    PROCESS_2_STATUS=$?
    # If the greps above find anything, they exit with 0 status
    # If they are not both 0, then something is wrong
    if [ $PROCESS_1_STATUS -ne 0 -o $PROCESS_2_STATUS -ne 0 ]; then
      echo "One of the processes has already exited."
      exit 1
    fi
  done
  

  Next, the Dockerfile:

  FROM ubuntu:latest
  COPY my_first_process my_first_process
  COPY my_second_process my_second_process
  COPY my_wrapper_script.sh my_wrapper_script.sh
  CMD ./my_wrapper_script.sh
  

• If you have one main process that needs to start first and stay running but you temporarily need to run some other processes (perhaps to interact with the main process) then you can use bash’s job control to facilitate that. First, the wrapper script:

  #!/bin/bash
  
  # turn on bash's job control
  set -m
  
  # Start the primary process and put it in the background
  ./my_main_process &
  
  # Start the helper process
  ./my_helper_process
  
  # the my_helper_process might need to know how to wait on the
  # primary process to start before it does its work and returns
  
  
  # now we bring the primary process back into the foreground
  # and leave it there
  fg %1
  

  FROM ubuntu:latest
  COPY my_main_process my_main_process
  COPY my_helper_process my_helper_process
  COPY my_wrapper_script.sh my_wrapper_script.sh
  CMD ./my_wrapper_script.sh
  

• Use a process manager like supervisord. This is a moderately heavy-weight approach that requires you to package supervisord and its configuration in your image (or base your image on one that includes supervisord), along with the different applications it manages. Then you start supervisord, which manages your processes for you. Here is an example Dockerfile using this approach, that assumes the pre-written supervisord.conf, my_first_process, and my_second_process files all exist in the same directory as your Dockerfile.

  FROM ubuntu:latest
  RUN apt-get update && apt-get install -y supervisor
  RUN mkdir -p /var/log/supervisor
  COPY supervisord.conf /etc/supervisor/conf.d/supervisord.conf
  COPY my_first_process my_first_process
  COPY my_second_process my_second_process
  CMD ["/usr/bin/supervisord"]