base-alpine

Base-alpine provides an image suitable for running Alpine Linux in Tutum/Kubernetes style hosted distributed environments. It comes with S6 process manager by default, if you don't use a process manager things can get a bit messy.

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Note

Please make sure you use a tagged version of base-alpine, such as:

FROM sillelien/base-alpine:0.10

Introduction

This is a simple but powerful base image, based on Alpine Linux with S6 as a process supervisor and dnsmasq for DNS management, both of which have extremely small footprints adding virtually no runtime overhead and a minimal filesystem overhead.

Why a supervisor process? Firstly because it solves the PID 1 Zombie Problem but most importantly because many containers need to run multiple processes.

Running multiple 'applications' in a single container is of course not The Docker Way (tm) - however running multiple processes is often required. S6 provides a very simple, low resource and elegant processor supervisor which fits in well with the Alpine Linux minimalism.

Also this image supports syslog logging, all syslog messages will be sent to stderr - no more losing syslog logging!

Read this first (Gotchas)

• Use Fully Qualified Domain Names (FQDN) always, Alpine Linux does not support the 'search' value in resolv.conf. So you must use myserver.local instead of just myserver.

Usage Notes

Shell

Alpine Linux uses BusyBox to provide a lot of the core Unix/Linux utilities. As part of that we get the Ash shell, which is very similar to the Bourne (BASH) shell. Just make sure you realise there are differences, it is almost POSIX compliant, so if in doubt use the POSIX complaint syntax rather than BASH extensions.

You can of course install bash - and why not?. Doing so will add a few more meg to your tiny image.

S6

S6 is a supervisor or process management system, similar to using runit or even supervisord in nature.It's a very powerful system so I recommend reading the docs - however the quick and dirty way to get started is:

1. Just use CMD as usual in your Dockerfile, the ENTRYPOINT is set to a script that will run the CMD under S6 and shutdown the entire image on CMD failure.

2. Add additional scripts using this format

COPY myservice.sh /etc/services.d/myservice/run
RUN chmod 755 /etc/services.d/myservice/run

Note: If you want to get access to environment variables passed in to your container start your scripts with:

#!/usr/bin/with-contenv sh

Syslog

The base image contains a running syslog daemon, which is set to send all output to stderr - this ensures you don't lose any messages sent by Linux applications.

DNS and the Alpine resolv.conf problem.

The authors of musl-libc decided for their own reasons not to support the search or domain options in resolv.conf. This means that systems that rely on that behaviour (include Tutum.co and Kubernetes) cannot use Alpine Linux properly. This base image does some magic for you to make sure that all linked containers resolve to their shortnames correctly. This magic works hand in hand with dnsmasq which is a tiny (uses about 17K of memory) DNS cache and forwarder.

You can add additional flags using the environment variable DNSMASQ_ARGS

Understanding the DNS Startup/Boot Sequence

The entire boot sequence related to DNS and related fixes is timelimited by the env var DNS_INIT_TIMEOUT which defaults to 45 seconds. If the timeout is exceeded the entire container is shutdown.

Makes sure Dnsmasq is the current nameserver

If it isn't it copies the current /etc/resolv.conf into /etc/dnsmasq-resolv.conf.

Checks whether the container is on Tutum

If the container is running on Tutum all linked containers will be added to the hosts file, not just ones with exposed ports.

Adds linked containers to /etc/hosts

If on Tutum this is all containers, otherwise only those who expose ports.

Pings each host

The script will pause while it pings each linked container. The script won't finish (and therefore the container won't start) until all can be reached.

Starts Dnsmasq

Dnsmasq is the local caching nameserver that is used to resolve all DNS queries from within the container.

Starts monitoring loop

The monitoring loop checks for changes to /etc/resolv.conf and when found updates the DNS information.

Good Practises

Don't Run as Root

During the build we run:

RUN addgroup -g 999 app && adduser -D  -G app -s /bin/false -u 999 app

This creates a non root user for you to use. Then in your S6 scripts you can run your commands using:

#!/usr/bin/env sh
exec s6-applyuidgid -u 999 -g 999 mycommand.sh 

The exec will write over the shell's process space reducing the memory overhead and s6-applyuidgid -u 999 -g 999 will run it as app the non root user.

Keep it Small

Don't put RUN instructions in your Dockerfile, instead create a build.sh script and run that:

COPY build.sh /build.sh
RUN chmod 755 /build.sh
RUN /build.sh

Of course you can save doing this until it's a last minute optimization when you've got everything running.

In your build.sh file start with:

#!/usr/bin/env sh
set -ex
cd /tmp
apk upgrade
apk update

And end with

apk del <applications that were used only for building, like gcc, make etc.>
rm -rf /tmp/*
rm -rf /var/cache/apk/*

This will clean up any mess you created while building. set -e causes the script to fail on any single commands failure and set -x lists all commands executed to stderr

Consider logging using logger

The logger command is a command-line tool to send the output of another command to syslog simply by doing

 mycommand 2>&1 | logger

I would advise using it where possible instead of just sending output directly to stderr - this means that if you decide to collect your log entries via syslog at a later time you won't need to change your app.

Differences to Ubuntu

APK not APT

Instead of apt-get install -y you have apk add

Find packages

You can search for packages by name or by file contents here: http://pkgs.alpinelinux.org/packages

Curl needs to be added

apk add curl ca-certificates

The standard build tools

apk add make gcc build-base

Python

apk add python python-dev py-pip

curl https://bootstrap.pypa.io/ez_setup.py | python

Java

Use our vizzbuzz/base-java image which adds Java to this image.

##Credits

Originally taken from https://github.com/just-containers/base-alpine credit to John Regan [email protected] which itself is taken from https://github.com/gliderlabs/docker-alpine credit to Gliderlabs for that.

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#Copyright and License

(c) 2015 Sillelien all rights reserved. Please see LICENSE for license details of this project. Please visit http://sillelien.com for help and commercial support or raise issues on GitHub.