• The new paradigm that Docker introduces that is most useful is the prioritization of data over software.
  • In other words, due to the amount of available software, a very large trend has been to adhere to certain software packages, like being part of a fandom of brands, when, in fact, any software that can solve a problem and generate, and / or process, some desired data is a great software to use. The adulation of software over data is over with Docker.
• Linux was never designed with tidiness in mind (except for definitions like the FSH, that came later) such that it is fully expected to unpack all the crap in /bin or on top of / and then just deal with the fallout when some file goes astray and starts creating problems. For that reason, Docker is a great solution because it keeps all the requirements for whatever software is run within a container without polluting the operating system. In some ways, it is the only "sandboxing" feature, with less requirements than hypervisor virtualization and more management tools than a common root-jail, that Linux ever had.
• There is a lot of software that has been created across the years by people with varying backgrounds in computer science, a lot of the software being, in fact, created by scientists working in natural sciences, that tend to churn over obscure libraries with esoteric requirements, either obsolete, hard to find, and mostly hard to recompile, such that a lot of the time, you really just want to get the software up and running without delving into the ins and outs of how a package is compiled. Unfortunately, the sound of the other shoe is that by containerizing, the software runs in a black box and a very tangential consequence, yet pertinent, is that the user is unable to assess how bad the software really is in terms of quality. Sometimes, software that is too difficult to compile, should just not be used.
  • There is a corollary to the former where ironically some software is so badly designed that Docker even seems to complicate the matters further (sometimes, people even suspect that is done deliberately).

• Whilst Docker does its best to mask itself as open-source and being deeply involved with the "community", the reality is that docker is a private company whose decisions might not necessarily align with the decisions of the "community", nor with open-source dialectics (remember when Google made the 20-year old free translation service non-free, overnight and started charging for it such that there is probably a large slice of the Internet that still contains third-party translators made to work with Google translate that can now just be scrapped because they will never work again?).
  • The first warning (concerning docker) a user gets is when they start working with docker and find out for the very first time that their pull requests are, in fact, limited and in order to pull containers more often, the users need to sign up and additionally, for some setups, become paying customers,
• Unless the user takes it upon themselves to recompile everything and build containers themselves (there are a few…), pre-made containers are available from a vast array of third-party individuals that may or may not either update the container with the new software, or also not slide backdoors into containers that are made public.
  • The typical mitigation is to only use official containers, yet that is not always possible and the task to containerize every single application seems fairly daunting, even in terms of workload.
• Unless users partition their containers, there is a lot of data, and now with the docker paradigm, service-deduplication, where two containers might contain the same software (ie: the same database engine).
  • The typical mitigation for this is to split containers up, manually, yet this leads back to the same problem of having the extra workload of (even) breaking up official containers.
• On the low-level, docker containers are treated as disposable, and interaction with the software inside is rather brutal. For example, when a container is shut down, the entire environment is just torn away and disposed, thereby potentially leaving static data in an unknown state (one example thereof is terminating a container whilst a program inside the container holds one file open, leading to possibly trashing the file when the handle is removed abruptly), other examples include the corruption of databases such as SQLite that make a point out of being gracefully opened and gracefully closed for the sake of a consistent (deterministic) workflow.
• There are some semantic lacks that are justified with a lot of dark magic; for instance, there is no way to "just restart" a container within a swarm, the only way to "restart" is to just dispose the entire environment and then re-create it from scratch, the justification thereof being a bleak mention on how containers in the swarm "are just sent out there" so they cannot be restarted (compared to containers without a swarm, that can indeed be restarted via "start" and "stop").
• Typically, every single "fix" that is suggested, involves tearing down the setup and then, admittedly using scripts, recreating the environment from scratch thereby potentially occluding setup mistakes and architectural bad choices, even involving the usage of docker.
• Even though the docker swarm has (had?) potential, one architectural problem is that the swarm is designed to have a handful of "managing nodes", and a plethora of "worker nodes" with the "managing nodes" being the only ones capable of pushing containers to a swarm as well as performing other administrative tasks. This is a design flaw, and a missed opportunity, because all the (few) "managing nodes" become single points of failure and then the entire swarm will need a central managing system (even if multiple managing nodes, consisting of the managing nodes themselves) (making all nodes manager nodes does not work as a workaround to obtain a P2P-like topology!).
• Another problem with the docker swam is that there is no way to perform tasks that should be possible (in fact, some of these tasks are "possible", but via various commands that … hack the result):
  • migrating a container onto a given node is not possible,
  • balancing the swarm across the cluster is performed by some crutch-command that is really just meant to refresh the container, not to make the swarm balance itself,
  • (even though this requires extra code) there is no way to let the swarm automatically balance and re-balance itself depending on what machine containers end up on, the only attainable balancing algorithm being just an equal-share spread of containers across machines, but without accounting for the necessary computational requirements of the containers themselves (ie: distribute a workload / CPU bound heavy container to a more powerful machine),
• The networking side of docker seems to be neatly brushed under the carpet where opening up containers in certain ways or deploying to a swarm ends up creating various network bridges, overlay networks and other paraphernalia resulting sometimes even in cutting off the server's connection to the local network, as well as generating "redundant networks" with containers ending up with multiple IP addresses.
• Networks seem to always be created in the class A private address space (10…) such that if any such network already exists, just installing docker and starting a container will interfere with the local networks.
• It is very difficult to customize low-level properties of the networks created by Docker, simply because the design of Docker extends to multiple domains (applications, networking, etc) without fully covering all the usage cases. For example, it is very difficult to configure Docker to set a custom MTU for the various interfaces it creates and typically one would resort to udev to create rules to configure the interfaces.
• On a philosophical level, docker tends to phase out the importance of various software packages, and makes all the talk about "just installing a bunch of applications" - so the official Apache docker does not contain a module you need? Doesn't matter, scrap the container and find some third-party re-packaged Apache container and use that instead. The result thereof is hiding the need to be well-acquainted or experienced with well-established Linux software packages (Apache, Bind, ISC DHCP) and then adding the philosophy of "just scrapping" the current workflow makes docker the ultimate tool of dumbing down IT in general.
• There is a problem with precedence (or clash of interests) when accounting for software that has an update feature because Docker would rather have the user upgrade the container itself rather than the application within updating itself. Depending on who packaged the application, a container update might not appear as soon as it would be desirable compared to the built-in application update feature.

The Watchtower container can be ran along with the other containers, will automatically monitor the various docker containers and will then automatically stop the containers, update the image and restart the containers.

Similarly, for swarms, shepherd is able to be deployed within a swarm in order to update containers.

The following command:

docker service ls -q | xargs -n1 docker service update --detach=false --force

will list all the services within a swarm and then force the services to be re-balanced and distributed between the nodes of the swarm.

After executing the command, all nodes can be checked with:

docker ps

to see which service got redistributed to which node.

One idea is to run the rebalancing command using crontab in order to periodically rebalance the swarm.

Typically, to open a console, the user would write:

docker run -it CONTAINER bash

where:

• CONTAINER is the name or hash of the container to start "bash" within

However, given that containers are distributed in a swarm, one should first locate on which node the container is running by issuing:

docker service ps CONTAINER

where:

• CONTAINER is a container running in the swarm

The output will display in one of the columns the current node that the container is executing on. Knowing the node, the shell of the node has to be accessed and then the command:

docker ps 

can be used to retrieve the container ID (first column).

Finally, the console can be started within the distributed container by issuing:

docker exec -it CONTAINER_ID sh

where:

• CONTAINER_ID is the id of the container running in the swarm on the local node

The syntax is as follows:

docker login <REGISTRY_HOST>:<REGISTRY_PORT>
docker tag <IMAGE_ID> <REGISTRY_HOST>:<REGISTRY_PORT>/<APPNAME>:<APPVERSION>
docker push <REGISTRY_HOST>:<REGISTRY_PORT>/<APPNAME>:<APPVERSION>

If a worker cannot find the swarm manager when it starts up, at the current time of writing, Docker is made to terminate. This is problematic because the manager might go online after a while such that the workers should just wait to connect.

On some Linux distributions, such as Debian, Docker is started via a service file located at /lib/systemd/system/docker.service and it can be copied to /etc/systemd/system with some modifications in order to make SystemD restart Docker if it terminates.

On Debian, the service file is missing the RestartSec configuration line, such that it should be added to /etc/systemd/system/docker.service after being copied. Here is the full service file with the added line:

[Unit]
Description=Docker Application Container Engine
Documentation=https://docs.docker.com
After=network-online.target docker.socket firewalld.service containerd.service
Wants=network-online.target containerd.service
Requires=docker.socket

[Service]
Type=notify
# the default is not to use systemd for cgroups because the delegate issues still
# exists and systemd currently does not support the cgroup feature set required
# for containers run by docker
EnvironmentFile=-/etc/default/docker
ExecStart=/usr/sbin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock $DOCKER_OPTS
ExecReload=/bin/kill -s HUP $MAINPID
LimitNOFILE=1048576
# Having non-zero Limit*s causes performance problems due to accounting overhead
# in the kernel. We recommend using cgroups to do container-local accounting.
LimitNPROC=infinity
LimitCORE=infinity
# Uncomment TasksMax if your systemd version supports it.
# Only systemd 226 and above support this version.
TasksMax=infinity
TimeoutStartSec=0
# set delegate yes so that systemd does not reset the cgroups of docker containers
Delegate=yes
# kill only the docker process, not all processes in the cgroup
KillMode=process
# restart the docker process if it exits prematurely
Restart=on-failure
StartLimitBurst=3
StartLimitInterval=60s
RestartSec=10s

[Install]
WantedBy=multi-user.target

With this change, SystemD will try to bring Docker up every 10s after it has failed. Unfortunately, this fix has to be applied for all nodes in a Docker swarm.

Unfortunately, services do not spread evenly through the swarm such that re-balancing is necessary. In fact, the strategy of distributing services across the swarm is surprisingly bad, with the manager node taking upon itself most of the services and with very few left over to the last node of the swarm. It seems Docker spreads services on a bucket-fill-like strategy where services are only spread out if the current node is deemed somehow full.

Irrespective of the lack of a strategy, here is one constructed command:

docker service ls | \
    awk '{ print $1 }' | \
    tail -n +2 | \
    xargs docker service ps --format "{{.Node}}" --filter "desired-state=running" | \
    awk ' { node[$0]++ } END { for (i in node) print node[i] } ' | \
    awk '{ x += $0; y += $0 ^ 2 } END { print int(sqrt( y/NR - (x/NR) ^ 2)) }' | \
    xargs -I{} test {} -gt 2 && docker service ls -q | xargs -n1 docker service update --detach=false --force

that performs the following operations in order:

• retrieves the services in the swarm,
• gets the number of services running per node,
• computes the standard deviation of the services running per node,
• if the standard deviation is greater than then the rebalancing command is ran in order to distribute services across the swarm.

In other words, the distribution strategy of the cluster is to place an equal share of services per available nodes.

Intuitively, the command can be placed in a cron script and, compared to just calling the swarm re-distribution command, the script should have no effect when the services are distributed evenly across the nodes due to the standard deviation falling well under (with being the theoretical point that the standard deviation should be when the services are evenly spread out).

Some packages have to be compiled manually such that it is beneficial to use a distributed compiler in order to distribute the compilation workload across multiple computers. However, the system should be flexible enough to include the edge case when a distributed compiler is not available.

To that end, here is a Dockerfile that is meant to define some variables such that "distcc" will be used to distribute the compilation across a range of computers:

FROM debian:latest AS builder

# define compilation variables
ARG DISTCC_HOSTS=""
ARG CC=gcc
ARG CXX=g++

# install required packages
RUN apt-get --assume-yes update && apt-get --assume-yes upgrade && \
    apt-get --assume-yes install \
        build-essential \
        gcc \
        g++ \
        automake \
        distcc

# ... compile ...
RUN DISTCC_HOSTS="${DISTCC_HOSTS}" CC=${CC} CXX=${CXX} make

and the invocation will be as follows:

docker build \
    -t TAG \
    --build-arg DISTCC_HOSTS="a:35001 b:35002" \
    --build-arg CC=distcc \
    --build-arg CXX=distcc \
    .

where:

• TAG is a tag to use for the build (can be used to upload to a registry),
• DISTCC_HOSTS, CC and CXX are the environment variables setting the compiler to distcc and the hosts to be used to compile (in this case, computers a and b listening on port 35001 and 35002)

If you would like a ready-made container for distcc, you can use the Wizardry and Steamworks build.