Tips and Tricks for containerizing services

Tips and Tricks for containerizing services

This document contains a list of tips and tricks that are useful when containerizing an OpenStack service.

Monitoring containers

It’s often useful to monitor the running containers and see what has been executed and what not. The puppet containers are created and removed automatically unless they fail. For all the other containers, it’s enough to monitor the output of the command below:

$ watch -n 0.5 docker ps -a --filter label=managed_by=docker-cmd

Viewing container logs

You can view the output of the main process running in a container by running:

$ docker logs $CONTAINER_ID_OR_NAME

Ideally all containerized processes would log everything to stdout/stderr and the above command would suffice. Not all services are quite there yet, so we export traditional logs from containers into the /var/log/containers directory on the host, where you can look at them.

Debugging container failures

The following commands are useful for debugging containers.

  • inspect: This command allows for inspecting the container’s structure and metadata. It provides info about the bind mounts on the container, the container’s labels, the container’s command, etc:

    $ docker inspect $CONTAINER_ID_OR_NAME

    There’s no shortcut for rebuilding the command that was used to run the container but, it’s possible to do so by using the docker inspect command and the format parameter:

    $ docker inspect --format='{{range .Config.Env}} -e "{{.}}" {{end}} {{range .Mounts}} -v {{.Source}}:{{.Destination}}{{if .Mode}}:{{.Mode}}{{end}}{{end}} -ti {{.Config.Image}}' $CONTAINER_ID_OR_NAME

    Copy the output from the command above and append it to the one below, which will run the same container with a random name and remove it as soon as the execution exits:

    $ docker run --rm $OUTPUT_FROM_PREVIOUS_COMMAND /bin/bash
  • exec: Running commands on or attaching to a running container is extremely useful to get a better understanding of what’s happening in the container. It’s possible to do so by running the following command:

    $ docker exec -ti $CONTAINER_ID_OR_NAME /bin/bash

    Replace the /bin/bash above with other commands to run oneshot commands. For example:

    $ docker exec -ti mysql mysql -u root -p $PASSWORD

    The above will start a mysql shell on the mysql container.

  • export When the container fails, it’s basically impossible to know what happened. It’s possible to get the logs from docker but those will contain things that were printed on the stdout by the entrypoint. Exporting the filesystem structure from the container will allow for checking other logs files that may not be in the mounted volumes:

    $ docker export $CONTAINER_ID_OR_NAME | tar -C /tmp/$CONTAINER_ID_OR_NAME -xvf -

Using docker-toool

In addition to the above, there is also now a json file that is generated that contains all the information for all the containers and how they are run. This file is /var/lib/docker-container-startup-configs.json.

docker-toool was written to read from this file and start containers for debugging purposes based on those commands. For now this utility is in the tripleo-heat-templates repo in the docker/ directory.

By default this tool lists all the containers that are started and their start order.

If you wish to see the command line used to start a given container, specify it by name using the –container (or -c) argument. –run (or -r) can then be used with this to actually execute docker to run the container. To see all available options use:

./docker-toool --help

Other options listed allow you to modify this command line for debugging purposes. For example:

./docker-toool -c swift-proxy -r -e /bin/bash -u root -i -n test

The above command will run the swift proxy container with all the volumes, permissions etc as used at runtime but as the root user, executing /bin/bash, named ‘test’, and will run interactively (eg -ti). This allows you to enter the container and run commands to see what is failing, perhaps install strace and strace the command etc. You can also verify configurations or any other debugging task you may have.


The script manages the config file generation and puppet tasks for each service. This also exists in the docker directory of tripleo-heat-templates. When writing these tasks, it’s useful to be able to run them manually instead of running them as part of the entire stack. To do so, one can run the script as shown below:

CONFIG=/path/to/task.json /path/to/

The json file must follow the following form:

        "config_image": ...,
        "config_volume": ...,
        "puppet_tags": ...,
        "step_config": ...

Using a more realistic example. Given a puppet_config section like this:

  config_volume: glance_api
  puppet_tags: glance_api_config,glance_api_paste_ini,glance_swift_config,glance_cache_config
  step_config: {get_attr: [GlanceApiPuppetBase, role_data, step_config]}
      - '/'
      - [ {get_param: DockerNamespace}, {get_param: DockerGlanceApiImage} ]

Would generated a json file called /var/lib/docker-puppet-tasks2.json that looks like:

        "config_image": "tripleoupstream/centos-binary-glance-api:latest",
        "config_volume": "glance_api",
        "puppet_tags": "glance_api_config,glance_api_paste_ini,glance_swift_config,glance_cache_config",
        "step_config": "include ::tripleo::profile::base::glance::api\n"

Setting the path to the above json file as value to the CONFIG var passed to will create a container using the centos-binary-glance-api:latest image and it and run puppet on a catalog restricted to the given puppet puppet_tags.

As mentioned above, it’s possible to create custom json files and call manually, which makes developing and debugging puppet steps easier. also supports the environment variable SHOW_DIFF, which causes it to print out a docker diff of the container before and after the configuration step has occurred.

By default runs things in parallel. This can make it hard to see the debug output of a given container so there is a PROCESS_COUNT variable that lets you override this. A typical debug run for docker-puppet might look like:


Testing in CI

When new service containers are added, ensure to update the image names in container-images/overcloud_containers.yaml tripleo-common repo. These service images are pulled in and available in the local docker registry that the containers ci job uses:

    - imagename: tripleoupstream/centos-binary-example:latest
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