Skip to content
This repository has been archived by the owner on Aug 26, 2023. It is now read-only.

Latest commit

 

History

History
192 lines (146 loc) · 10.4 KB

README.md

File metadata and controls

192 lines (146 loc) · 10.4 KB
Raw

Introduction

This repository provides a wrapper which improves the handling of Docker containers run as systemd services.

If a Docker container is started as a systemd service using the "usual" docker run ... instruction, f.ex. ExecStart=docker run ..., systemd interacts with the Docker client process instead of the container process, which can lead to situations where systemd's capacity to monitor process health is affected:

  • the client can detach or crash while the container is doing fine, yet systemd would trigger failure handling
  • worse, the container crashes and should be taken care of, but the client stalled - systemd is blind and won't do
    anything
  • when a container is stopped with docker stop ..., attached client processes exit with error code 143, not 0/success, which triggers systemd's failure handling unless it's explicitely configured to ignore this using SuccessExitStatus=143, but that's a workaround. The problem is well explained in this issue description

The key thing that this wrapper does is that it moves the container process from the cgroups set up by Docker to the service unit's cgroup to give systemd the supervision of the actual Docker container process.
It's written in Golang and allows to leverage all the cgroup functionality of systemd and systemd-notify.

Repository history and credits

Installation

Supposing that a Go environment is available, the build instruction is go get github.com/DonTseTse/systemd-docker. The executable can then be found in the Go binary directory (usually something like $GO_ROOT/bin) and it's called systemd-docker.

It can also be build using a stand-alone docker image, see here. Just be sure to specify this fork with the branch name: github.com/embtom/systemd-docker@master

Use

Both

  • systemctl to manage systemd services, and
  • the docker CLI

can be used and everything should stay in sync.

In the systemd unit files, the instruction to launch the Docker container takes the form

ExecStart=/path/to/systemd-docker [<systemd-docker_options>] run <docker-run_parameters>

where

The example below shows a typical systemd unit file using systemd-docker (supposed to be in /usr/bin), running a Nginx container:

[Unit]
Description=Nginx
After=docker.service
Requires=docker.service

[Service]
#--- if systemd-notify is used
Type=notify
NotifyAccess=all
#------------------------
ExecStart=/usr/bin/systemd-docker run --rm --name %n nginx
Restart=always
RestartSec=10s
TimeoutStartSec=120
TimeoutStopSec=15

[Install]
WantedBy=multi-user.target

The use of %n is a systemd feature explained here. Supposing that the unit file example given above is stored under the likely path /etc/systemd/system/nginx.service, the container is named nginx.

For the details about Type=notify and NotifyAccess=all and systemd-notify, see systemd notifications.

For a general documentation of all systemd unit file configurations options, see this documentation.

Container names

Container names are compulsory to make sure that each systemd service always relates to/acts upon the same container(s). While it may seem as if that could be omitted as long as the --rm flag is used to make Docker remove any stopped container, that's misleading: the deletion process triggered by this flag is actually part of the Docker client logic and if the client detaches for whatever reason from the running container, the information is lost (even if another client is re-attached later) and the container will not be deleted upon termination. systemd-docker adds an additional check and looks for the named container when systemd-docker ... run ... is called - if a stopped container exists, it's removed.

Systemd integration details

Automatic container naming

While it processes unit files, systemd populates a range of variables among which %n stands for the name of service, derived from it's filename. This allows to write a self-configuring ExecStart instruction using the parameters

ExecStart=/path/to/systemd-docker ... run ... --name %n --rm ...

Use of systemd environment variables

systemd handles environment variables with the instructions Environment=... and EnvironmentFile=.... To inject variables into other instructions, the pattern is ${variable_name}. With the docker run flag -e they can be passed from systemd to the Docker container

Example: ExecStart=/path/to/systemd-docker ... run -e ABC=${ABC} -e XYZ=${XYZ} ...

systemd-docker has an option to pass on all defined environment variables using the --env flag, explained here

Systemd notifications (systemd-notify)

systemd-notify can be used to schedule and sequence the launch of different services. The systemd documentation explains the configuration optionss available in unit files:

  • Type=notify: "... it is expected that the daemon sends a notification message via sd_notify(3) or an equivalent call when it has finished starting up. systemd will proceed with starting follow-up units after this notification message has been sent."
  • NotifyAccess=all: "Controls access to the service status notification socket, as accessible via the sd_notify(3) call. ... If all, all services updates from all members of the service's control group are accepted."

By default systemd-docker will send READY=1 to the systemd notification socket but it can also be configured to delegate this to the container as explained here.

Please be aware that systemd-notify comes with its own quirks - more info can be found in this mailing list thread. In short, systemd-notify is not reliable because often the child dies before systemd has time to determine which cgroup it is a member of.

Systemd-docker options

Cgroups

By default all application cgroups are moved to systemd. It's also possible to control individually which cgroups are transfered using a --cgroups flags for each cgroup to transfer. -cgroups name=systemd is the strict minimum to have
systemd supervise the container. This implies that the docker run flags --cpuset and/or -m are incompatible.

Example: ExecStart=/path/to/systemd-docker ... --cgroups name=systemd --cgroups=cpu ... run ...

The above command will use the name=systemd and cpu cgroups of systemd but then use Docker's cgroups for all the others, like the freezer cgroup.

Logging

By default the container's stdout/stderr is written to the system journal. This may be disabled with --logs=false.

Example: ExecStart=/path/to/systemd-docker ... --logs=false ... run ...

Environment Variables

The systemd environment variables are automatically passed through to the Docker container if the --env flag is set.
It will essentially read all the current environment variables and add the appropriate -e ... flags to the docker run command.

EnvironmentFile=/etc/environment
ExecStart=systemd-docker ... --env ... run ...

In the example above, all environment variables defined in /etc/environment will be passed to the docker run command.

PID File

To create a PID file for the container, use the flag --pid-file=</path/to/pid_file>.

Example: ExecStart=/path/to/systemd-docker ... --pid-file=/var/run/%n.pid ... run ...

systemd-notify support

The systemd-docker flag --notify makes systemd-docker delegate the systemd-notify READY=1 call to the container itself. To allow the container to achieve this, systemd-docker bind mounts the systemd notification socket into the container and sets the NOTIFY_SOCKET environment variable.

Example: ExecStart=/path/to/systemd-docker ... --notify ... run ...

Container removal behavior

To disable systemd-docker's "remove stopped container" procedure, the flag ... --rm=false ... can be used.

Example: ExecStart=/path/to/systemd-docker ... --rm=false ... run ...

Docker restrictions

--cpuset and/or -m

These flags can't be used because they are incompatible with the cgroup migration(s) inherent to systemd-docker.

-d (detaching the Docker client)

The -d flag provided to docker run has no effect under systemd-docker. To cause the Docker client to detach after the container is running, use the systemd-docker options --logs=false --rm=false. If either --logs or --rm is true, the Docker client instance used by systemd-docker is kept alive until the systemd service is stopped or the container exits.

Known issues

Inconsistent cgroup

CentOS 7 is inconsistent in the way it handles some cgroups. It has 3:cpuacct,cpu:/user.slice in /proc/[pid]/cgroups but the corresponding path /sys/fs/cgroup/cpu,cpuacct/ doesn't exist. This causes systemd-docker to fail when it tries to move the PIDs there. To solve this the name=systemd cgroup must be explicitely mentioned:

/path/to/systemd-docker ... --cgroups name=systemd ... run ...

See ibuildthecloud#15 for details.

License

See repository history and credits for acknowledgments. The work on this repository was done in 2018 by DonTseTse.

Licensed under the Apache License, Version 2.0