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 -

Debugging with Paunch

The paunch debug command allows you to perform specific actions on a given container. This can be used to:

  • Run a container with a specific configuration.
  • Dump the configuration of a given container in either json or yaml.
  • Output the docker command line used to start the container.
  • Run a container with any configuration additions you wish such that you can run it with a shell as any user etc.

The configuration options you will likely be interested in include:

--file <file>         YAML or JSON file containing configuration data
--action <name>       Action can be one of: "dump-json", "dump-yaml",
                      "print-cmd", or "run"
--container <name>    Name of the container you wish to manipulate
--interactive         Run container in interactive mode - modifies config
                      and execution of container
--shell               Similar to interactive but drops you into a shell
--user <name>         Start container as the specified user
--overrides <name>    JSON configuration information used to override
                      default config values

file is the name of the configuration file to use containing the configuration for the container you wish to use. TripleO creates configuration files for starting containers in /var/lib/tripleo-config/. If you look in this directory you will see a number of files corresponding with the steps in TripleO heat templates. Most of the time, you will likely want to use /var/lib/tripleo-config/hashed-docker-container-startup-config-step_4.json as it contains most of the final startup configurations for the running containers.

shell, user and interactive are available as shortcuts that modify the configuration to easily allow you to run an interactive session in a given container.

To make sure you get the right container you can use the paunch list command to see what containers are running and which config id they are using. This config id corresponds to which file you will find the container configuration in.

Note that if you wish to replace a currently running container you will want to docker rm the running container before starting a new one.

Here is an example of using paunch debug to start a root shell inside the heat api container:

# paunch debug --file /var/lib/tripleo-config/hashed-docker-container-startup-config-step_4.json --interactive --shell --user root --container heat_api --action run

This will drop you into an interactive session inside the heat api container, starting /bin/bash running as root.

To see how this container is started by TripleO:

# paunch debug --file /var/lib/tripleo-config/hashed-docker-container-startup-config-step_4.json --container heat_api --action print-cmd

docker run --name heat_api-t7a00bfz --detach=true --env=KOLLA_CONFIG_STRATEGY=COPY_ALWAYS --env=TRIPLEO_CONFIG_HASH=b3154865d1f722ace643ffbab206bf91 --net=host --privileged=false --restart=always --user=root --volume=/etc/hosts:/etc/hosts:ro --volume=/etc/localtime:/etc/localtime:ro --volume=/etc/puppet:/etc/puppet:ro --volume=/etc/pki/ca-trust/extracted:/etc/pki/ca-trust/extracted:ro --volume=/etc/pki/tls/certs/ca-bundle.crt:/etc/pki/tls/certs/ca-bundle.crt:ro --volume=/etc/pki/tls/certs/ --volume=/etc/pki/tls/cert.pem:/etc/pki/tls/cert.pem:ro --volume=/dev/log:/dev/log --volume=/etc/ssh/ssh_known_hosts:/etc/ssh/ssh_known_hosts:ro --volume=/var/lib/kolla/config_files/heat_api.json:/var/lib/kolla/config_files/config.json:ro --volume=/var/lib/config-data/heat_api/etc/heat/:/etc/heat/:ro --volume=/var/lib/config-data/heat_api/etc/httpd/conf/:/etc/httpd/conf/:ro --volume=/var/lib/config-data/heat_api/etc/httpd/conf.d/:/etc/httpd/conf.d/:ro --volume=/var/lib/config-data/heat_api/etc/httpd/conf.modules.d/:/etc/httpd/conf.modules.d/:ro --volume=/var/lib/config-data/heat_api/var/www/:/var/www/:ro --volume=/var/log/containers/heat:/var/log/heat

You can also dump the configuration of a container to a file so you can edit it and rerun it with different a different configuration:

# paunch debug --file /var/lib/tripleo-config/hashed-docker-container-startup-config-step_4.json --container heat_api --action dump-json > heat_api.json

You can then use heat_api.json as your --file argument after editing it to your liking.

To add configuration elements on the command line you can use the overrides option. In this example I’m adding a health check to the container:

# paunch debug --file /var/lib/tripleo-config/hashed-docker-container-startup-config-step_4.json --overrides '{"health-cmd": "/usr/bin/curl -f http://localhost:8004/v1/", "health-interval": "30s"}' --container heat_api --action run


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 the CONFIG environment variable 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:

SHOW_DIFF=True PROCESS_COUNT=1 CONFIG=glance_api.json ./

Testing a code fix in a container

Let’s assume that we need to test a code patch or an updated package in a container. We will look at a few steps that can be taken to test a fix in a container on an existing deployment.

For example let’s update packages for the mariadb container:

(undercloud) [stack@undercloud ~]$ docker images | grep mariadb    latest     035a8237c376    2 weeks ago    723.5 MB

So docker image 035a8237c376 is the one we need to base our work on. Since docker images are supposed to be immutable we will base our work off of 035a8237c376 and create a new one:

mkdir -p galera-workaround
cat > galera-workaround/Dockerfile <<EOF
USER root
RUN yum-config-manager --add-repo && yum clean all && rm -rf /var/cache/yum
RUN yum update -y pacemaker pacemaker-remote pcs libqb resource-agents && yum clean all && rm -rf /var/cache/yum
USER mysql

To determine which user is the default one being used in a container you can run docker run -it 035a8237c376 whoami. Then we build the new image and tag it with :workaround1:

docker build --rm -t ~/galera-workaround

Then we push it in our docker registry on the undercloud:

docker push

At this stage we can either point THT to use as the container image by tweaking the necessary environment files and we redeploy the overcloud. If we only want to test a tweaked image, the following steps can be used: First, determine if the containers are managed by pacemaker (those will typically have a :pcmklatest tag) or by paunch. For the paunch-managed containers see Debugging with Paunch. For the pacemaker-managed containers you can (best done on your staging env, as it might be an invasive operation) do the following:

1. `pcs cluster cib cib.xml`
2. Edit the cib.xml with the changes around the bundle you are tweaking
3. `pcs cluster cib-push --config cib.xml`

Testing in CI

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

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