Raspberry Pi Cluster Emulation With Docker Compose

Introduction

The Raspberry Pi is no longer just a low-cost platform for students to learn computing, it's now a legitimate research and development platform that's being used for IoT, networking, distributed systems, and software development. It's even being used administratively in production environments.

Not long after the first Raspberry Pi was released in 2012, several set out to build them into low-cost clusters, often for their research and testing purposes. Interns at DataStax built a multi-datacenter, 32 nodes Cassenda fault-tolerance demo, complete with a big red button to simulate the failure of an entire datacenter. David Guill built a 40-node Raspberry Pi Cluster that was intended to be part of his MSCE thesis. Balena, built "The Beast", a 120 node Raspberry Pi cluster, for scaled testing of their online platform. And on the extreme end of the spectrum, Oracle built a 1060 node Raspberry Pi Cluster, which they introduced at Oracle OpenWorld 2019.

Innovation with the Raspberry Pi continues as they are turned into everything from wi-fi extenders, security cameras, and even bigger clusters. While the main value of these clusters is inherent in their size and low cost, their popularity makes them an increasingly common development platform. Since the Raspberry Pi uses an ARM processor, this can make development problematic for those of us who work exclusively in the cloud. While commercial solutions exist, we will be building our own emulated cluster using a fully open source stack hosted on Google Compute Engine.

Use Cases

Other than learning from the experience, Dockerizing an emulated Raspberry Pi enables us to do three things. One, it turns into software that would otherwise be a hardware-only device that nobody has to remember to carry around (I'm always losing the peripheral cables). Two, it enables Docker to do for the Pi what Docker does best for everything else: it makes software portable, easy to manage, and easy to replicate. And three, it takes up no physical space. If we can build one Raspberry Pi with Docker, we can build many. If we can build many, we can network them all together. While we may encounter some limitations, this build will emulate a cluster of Raspberry Pi 1s that's logically equivalent to a simple, multi-node physical cluster.

Emulated Hardware Architecture

While technically not identical, the emulation software we will be using, QEMU, provides an ARM-Versatile architecture that's roughly compatible with what is found on a Raspberry Pi 1. Some modifications to the kernel are necessary in order for it to work properly with Raspbian, but for our purposes, it's one of the more stable open source solutions available.

pi@raspberrypi:~$ cat /proc/cpuinfo 

processor       : 0 

model name      : ARMv6-compatible processor rev 7 (v6l) 

BogoMIPS        : 577.53 

Features        : half thumb fastmult vfp edsp java tls 

CPU implementer : 0x41 

CPU architecture: 7 

CPU variant     : 0x0 

CPU part        : 0xb76 CPU revision    : 7 

Hardware        : ARM-Versatile (Device Tree Support) 

Revision        : 0000 

Serial          : 0000000000000000

Compared to a Physical Raspberry Pi 1, they are nearly identical:

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pi@raspberrypi:~ $ cat /proc/cpuinfo 

processor    : 0 

model name    : ARMv6-compatible processor rev 7 (v6l) 

BogoMIPS    : 697.95 

Features    : half thumb fastmult vfp edsp java tls 

CPU implementer    : 0x41 

CPU architecture: 7 

CPU variant    : 0x0 

CPU part    : 0xb76 CPU revision    : 7 

Hardware    : BCM2835 

Revision    : 000d 

Serial        : 000000003d9a54c5

Background

What Is QEMU?

QEMU is a processor emulator. It supports a number of different processors, but the only thing that we're interested in is something that can run Raspberry Pi images natively without lot of difficulties. In this case, we're going to be using QEMU 4.2.0, which supports an ARM11 instruction set that's compatible with the Broadcom BCM2835 (ARM1176JZFS) chip found on the Raspberry Pi 1 and Zero. We will use ARM1176 support on QEMU, which will allow us to more or less emulate a Raspberry Pi 1. I say more or less because we will still need to use a customized Raspbian kernel in order to boot on the emulated hardware. QEMU support for the Pi is still in development, so our approach to getting it to work here is just a clever hack that will by no means be optimal or efficient in terms of CPU utilization.

QEMU Features

QEMU supports many of the same features found in Docker, however, it can run full software emulation without a host kernel driver. This means that it can run inside Docker, or any other virtual machine, without host virtualization support. The QEMU feature list is extensive, and the learning curve is steep. However, the primary feature that we will be focused on for this build is host port forwarding so that data can be passed to the host.

Dockerized QEMU

One of Docker's strengths is that it doesn't handle full-fledged virtualization, but instead relies on the architecture of the host system. Since our host system will be running an Intel processor, we can't expect Docker to handle ARM operations on its own. So, we will be placing QEMU inside a Docker container. Since Docker is designed to run software at near-native performance, the operational efficiency challenge will be with QEMU itself. QEMU, on the other hand, supports emulations of a machine's architecture completely with software. The advantage this has is that it can run inside any virtualized system or container, independent of its system architecture. If patient, we could even run a Dockerized Raspberry Pi container inside another Dockerized Raspberry Pi container. The drawback to QEMU is that it has comparatively poor performance compared to other types of virtualization. But we can benefit from the best of both worlds by leveraging QEMU's ARM emulation while depending on Docker for everything else.

Raspbian

Based on Debian, Raspbian is a popular and well supported operating system for the Raspberry Pi, and is one of the most often recommended for the platform. The community is very active and well managed.

Physical Raspberry Pi Speed Comparison

The following tests are intended as a baseline for comparing our virtualized systems. Since we will be emulating a single-core, these tests are only single-core, single thread, regardless of how many physical cores are incorporated into the architecture.

Raspberry Pi 1 2011,12

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Test execution summary:    total time:                          330.5514s    total number of events:              10000    total time taken by event execution: 330.5002    per-request statistics:         min:                                 32.92ms         avg:                                 33.05ms         max:                                 40.94ms         approx.  95 percentile:              33.24ms 

Threads fairness:    events (avg/stddev):           10000.0000/0.00    execution time (avg/stddev):   330.5002/0.00

Raspberry Pi 1 A+ V1.1 2014

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Test execution summary:

    total time:                          328.7505s

    total number of events:              10000

    total time taken by event execution: 328.6931

    per-request statistics:

         min:                                 32.71ms

         avg:                                 32.87ms

         max:                                 78.93ms

         approx.  95 percentile:              33.03ms 

Threads fairness:

    events (avg/stddev):           10000.0000/0.00

    execution time (avg/stddev):   328.6931/0.00

Raspberry Pi Zero W v1.1 2017

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Test execution summary:

    total time:                          228.2025s

    total number of events:              10000

    total time taken by event execution: 228.1688

    per-request statistics:

         min:                                 22.76ms

         avg:                                 22.82ms

         max:                                 35.29ms

         approx.  95 percentile:              22.94ms 

Threads fairness:

    events (avg/stddev):           10000.0000/0.00

    execution time (avg/stddev):   228.1688/0.00

Raspberry Pi 2 Model B v1.1 2014

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Test execution summary:

    total time:                          224.9052s

    total number of events:              10000

    total time taken by event execution: 224.8738

    per-request statistics:

         min:                                 22.20ms

         avg:                                 22.49ms

         max:                                 32.85ms

         approx.  95 percentile:              22.81ms 

Threads fairness:

    events (avg/stddev):           10000.0000/0.00

    execution time (avg/stddev):   224.8738/0.00

Raspberry Pi 3 Model B v1.2 2015

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Test execution summary:

    total time:                          139.6140s

    total number of events:              10000

    total time taken by event execution: 139.6087

    per-request statistics:

         min:                                 13.94ms

         avg:                                 13.96ms

         max:                                 34.06ms

         approx.  95 percentile:              13.96ms 

Threads fairness:

    events (avg/stddev):           10000.0000/0.00

    execution time (avg/stddev):   139.6087/0.00

Raspberry Pi 4 B 2018

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Test execution summary:

    total time:                          92.6405s

    total number of events:              10000

    total time taken by event execution: 92.6338

    per-request statistics:

         min:                                  9.22ms

         avg:                                  9.26ms

         max:                                 23.50ms

         approx.  95 percentile:               9.27ms 

Threads fairness:

    events (avg/stddev):           10000.0000/0.00

    execution time (avg/stddev):   92.6338/0.00

Project Requirements

Single Host Specifications

Historically, QEMU has been single-threaded, emulating all cores of a system's architecture on a single CPU. While that's no longer the case, we are still going to be emulating a single core Raspberry Pi. We will do some benchmarks later to compare how different CPU limits on each node impacts performance. But for now, we will use one CPU per single-core node. Since QEMU has the potential to use a lot of CPU resources due to its inherent inefficiency, our initial three-node cluster will start with a baseline of at least one CPU per node, leaving one CPU dedicated to the host to avoid performance problems. The VM specs selected for this task are as follows.

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Cloud Provider: Google Cloud Platform 

Instance Type: n1-standard-4 

CPUs: 4 

Memory: 15GB 

Disk: 100GB 

Operating System: Ubuntu 18.04 LTS

Docker

Installed on the host, we're also using the default version of Docker that is available on the default apt repository for Ubuntu 18.04 LTS.

# docker -v 
Docker version 18.09.7, build 2d0083d


Docker Compose

# docker-compose -v 
docker-compose version 1.25.0, build 0a186604


Docker Hub Ubuntu Image

18.04, bionic-20200112, bionic, latest


QEMU

Installed inside the Docker container, we will be using the following version of QEMU for ARM:

# qemu-system-arm --version 
QEMU emulator version 4.2.0 
Copyright (c) 2003-2019 Fabrice Bellard and the QEMU Project developers


QEMU Customized Kernel for Raspbian

Loaded from QEMU inside Docker, we will use Dhruv Vyas's compiled kernel for Raspbian, which has been modified to be usable with QEMU.

Raspbian Lite Image

Also booted from QEMU, we will use an unmodified version of Raspbian Lite from 9/30/2019.

Expect (Tcl/Tk)

Installed on the Docker container is the following version of Expect:

# expect -v expect version 5.45.4


ssh/sshd

sshd will need to be enable on each Raspbian node, and ssh should be enabled on the host.

Ansible

The following version of Ansible is also being used, along with its other dependencies:

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# ansible --version 

ansible 2.5.1

  config file = /etc/ansible/ansible.cfg

  configured module search path = [u'/root/.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']  ansible python module location = /usr/lib/python2.7/dist-packages/ansible

  executable location = /usr/bin/ansible  python version = 2.7.17 (default, Nov  7 2019, 10:07:09) [GCC 7.4.0]

Building the Docker Images

QEMU Build Container

We will compile QEMU 4.2.0 from the source. It will need all the supporting build tools, so to keep our app container as small as possible, we will create a separate build container for the QEMU build using a minimal version of Ubuntu 18.04 from Docker Hub.

QEMU App Container

Once the QEMU is compiled from the source, we will transfer it to the app container. Also from Docker Hub, we will use the same minimal version of Ubuntu 18.04 to host the QEMU binary.

Docker Configuration

The Dockerfile

We will be using the following Dockerfile, which may also be found updated in this guide's accompanying pidoc repository on Github. Each code snippet below makes up a segment of the Dockerfile. Thanks goes to Luke Child's for his work on dockerpi.

Build stage for qemu-system-arm:

FROM ubuntu AS qemu-system-arm-builder 

ARG QEMU_VERSION=4.2.0 

ENV QEMU_TARBALL="qemu-${QEMU_VERSION}.tar.xz" 

WORKDIR /qemu 

RUN apt-get update && \

    apt-get -y install \

                       wget \

                       gpg \

                       pkg-config \

                       python \

                       build-essential \

                       libglib2.0-dev \

                       libpixman-1-dev \

                       libfdt-dev \

                       zlib1g-dev \

                       flex \

                       bison

Download source.

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RUN wget "https://download.qemu.org/${QEMU_TARBALL}" 

RUN # Verify signatures... 

RUN wget "https://download.qemu.org/${QEMU_TARBALL}.sig" 

RUN gpg --keyserver keyserver.ubuntu.com --recv-keys CEACC9E15534EBABB82D3FA03353C9CEF108B584 

RUN gpg --verify "${QEMU_TARBALL}.sig" "${QEMU_TARBALL}"

Extract tarball.

RUN tar xvf "${QEMU_TARBALL}"


Build source.

RUN "qemu-${QEMU_VERSION}/configure" --static --target-list=arm-softmmu 
RUN make -j$(nproc) 
RUN strip "arm-softmmu/qemu-system-arm"


Build the intermediary pidoc VM app image.

FROM ubuntu as pidoc-vm 
ARG RPI_KERNEL_URL="https://github.com/dhruvvyas90/qemu-rpi-kernel

/archive/afe411f2c9b04730bcc6b2168cdc9adca224227c.zip" 
ARG RPI_KERNEL_CHECKSUM="295a22f1cd49ab51b9e7192103ee7c917624b063cc5ca

2e11434164638aad5f4"


Transfer binary from build container to app container.

COPY --from=qemu-system-arm-builder /qemu/arm-softmmu/qemu-system-arm 

/usr/local/bin/qemu-system-arm


Download modified kernel and install.

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ADD $RPI_KERNEL_URL /tmp/qemu-rpi-kernel.zip 

RUN apt-get update && \

    apt-get -y install \

                        unzip \

                        expect RUN cd /tmp && \

    echo "$RPI_KERNEL_CHECKSUM  qemu-rpi-kernel.zip" | sha256sum -c && \

    unzip qemu-rpi-kernel.zip && \

    mkdir -p /root/qemu-rpi-kernel && \

    cp qemu-rpi-kernel-*/kernel-qemu-4.19.50-buster /root/qemu-rpi-kernel/ && \

    cp qemu-rpi-kernel-*/versatile-pb.dtb /root/qemu-rpi-kernel/ && \

    rm -rf /tmp/* 

VOLUME /sdcard

Then we copy the entry point script from the host's main directory.

ADD ./entrypoint.sh /entrypoint.sh 
ENTRYPOINT ["./entrypoint.sh"]


Build the final app pidoc image with the Raspbian Lite filesystem loaded.

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FROM pidoc-vm as pidoc 

ARG FILESYSTEM_IMAGE_URL="http://downloads.raspberrypi.org/raspbian_lite/images/raspbian_lite-2019-09-30/2019-09-26-raspbian-buster-lite.zip" 

ARG FILESYSTEM_IMAGE_CHECKSUM="a50237c2f718bd8d806b96df5b9d2174ce8b789eda1f03434ed2213bbca6c6ff" 

ADD $FILESYSTEM_IMAGE_URL /filesystem.zip 

ADD pi_ssh_enable.exp /pi_ssh_enable.exp 

RUN echo "$FILESYSTEM_IMAGE_CHECKSUM  /filesystem.zip" | sha256sum -c

The entrypoint.sh File

First, the script determines if the filesystem has been downloaded or not, and if not, it downloads and decompresses it.

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#!/bin/sh 

raspi_fs_init() {

   image_path="/sdcard/filesystem.img"

  zip_path="/filesystem.zip"

    if [ ! -e $image_path ]; then

    echo "No filesystem detected at ${image_path}!"

    if [ -e $zip_path ]; then

        echo "Extracting fresh filesystem..."

        unzip $zip_path

        mv *.img $image_path

        rm $zip_path

    else

      exit 1

    fi

  fi

 }

The script then checks for an empty raspi-init file, which serves as a marker to determine if Expect has been launched previously to enable ssh on Raspbian.

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if [ ! -e /raspi-init ]; then

  touch /raspi-init

  raspi_fs_init

  echo "Initiating Expect..."

  /usr/bin/expect /pi_ssh_enable.exp `hostname -I`

  echo "Expect Ended..."

If Expect has already been previously enabled, then we only need to launch QEMU, without Expect. Note that we are forwarding port 22 on Raspbian to port 2222 inside the Docker container.

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else

  /usr/local/bin/qemu-system-arm \

        --machine versatilepb \

        --cpu arm1176 \

        --m 256M \

        --hda /sdcard/filesystem.img \

        --net nic \

        --net user,hostfwd=tcp:`hostname -I`:2222-:22 \

        --dtb /root/qemu-rpi-kernel/versatile-pb.dtb \

        --kernel /root/qemu-rpi-kernel/kernel-qemu-4.19.50-buster \

        --append "root=/dev/sda2 panic=1" \

        --no-reboot \

        --display none \

        --serial mon:stdio 

fi

Enable SSHD on Raspbian (Expect Tcl/Tk Method)

QEMU doesn't have a straightforward method for running configuration scripts on boot. And because Raspbian doesn't come with SSH enabled by default, we will have to turn it on ourselves. Our options are to do it manually or to use some sort of scripting tool that can interact with stdio. Another option is to customize the Raspbian image before installation. This would have to be done on the host, however, as Docker restricts the mounting of new filesystems. In any case, to make this build the most portable and host independent, the most straightforward for our purposes will be to use an Expect script, and have it copied into our Docker image on build.

The pi_ssh_enable.exp File

Since an unmodified Raspbian image has no accessible ports by default, we will use Expect to interface with stdio in QEMU, log in with a default username and password, and enable the sshd listener.

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#!/usr/bin/expect -f 

set ipaddr [lindex $argv 0] 

set timeout -1 

spawn /usr/local/bin/qemu-system-arm \

  --machine versatilepb \

  --cpu arm1176 \

  --m 256M \

  --hda /sdcard/filesystem.img \

  --net nic \

  --net user,hostfwd=tcp:$ipaddr:2222-:22 \

  --dtb /root/qemu-rpi-kernel/versatile-pb.dtb \

  --kernel /root/qemu-rpi-kernel/kernel-qemu-4.19.50-buster \

  --append "root=/dev/sda2 panic=1" \

  --no-reboot \

  --display none \

  --serial mon:stdio 

expect "raspberrypi login:" 

send -- "pi\r" expect "Password:" 

send -- "raspberry\r" expect "pi@raspberrypi:" 

send -- "sudo systemctl enable ssh\r" 

expect "pi@raspberrypi:" 

send -- "sudo systemctl 

start ssh\r" 

expect "pi@raspberrypi:" 

expect eof

Build Image

In the folder with the Dockerfile, we will be building our two containers. The first will be our build container that includes all the dependencies for compiling QEMU, and the other will be our app container for running QEMU.

docker build -t pidoc.


Network Forwarding and Troubleshooting

Once the build is complete, bring it up detached and follow the logs.

docker run -itd --name testnode pidoc 
docker logs testnode -f


Raspbian will download and decompressed automatically, and QEMU should begin booting from the image.

Once Raspbian is fully booted, Expect should automatically enable sshd. Log into the docker container and test that SSH is reachable from inside the container on port 2222.

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# docker exec -it testnode bash 

root@d4abc2f655e6:/# hostname -I 

172.17.0.3 

root@d4abc2f655e6:/# cat < /dev/tcp/172.17.0.3/2222 

SSH-2.0-OpenSSH_7.9p1 Raspbian-10

Cancel out and kill the container and remove the volume.

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root@d4abc2f655e6:/# exit 

exit 

# docker 

kill testnode 

testnode 

# docker container rm 

testnode testnode

Testing the Docker Container

Start/Test Container

We will need to start the container for testing. This is primarily to gain some intuition about the performance of QEMU so that we can better make design decisions regarding our cluster. The system should come up clean with maybe a few benign warnings related to differences between the somewhat more generalized emulated hardware and the expected physical raspberry Pi hardware. I found it necessary to make sure port forwarding was working properly between QEMU and the Docker image so that I could further verify that port forwarding between the Docker image and host was working properly. Our first goal is to double forward SSH so that QEMU is accessible directly from the host.

docker run -itd -p 127.0.0.1:2222:2222 --name testnode pidoc 
docker logs testnode -f


Once the system again comes online, test for sshd on port 2222 of the host by using ssh to log into Raspbian:

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# ssh pi@localhost -p 2222 

The authenticity of host '[localhost]:2222 ([127.0.0.1]:2222)' can't be established. 

ECDSA key fingerprint is SHA256:N0oRF23lpDOFjlgYAbml+4v2xnYdyrTmBgaNUjpxnFM. 

Are you sure you want to continue connecting (yes/no)? yes 

Warning: Permanently added '[localhost]:2222' (ECDSA) to the list of known hosts. 

pi@localhost's password: 

Linux raspberrypi 4.19.50+ #1 Tue Nov 26 01:49:16 CET 2019 armv6l 

The programs included with the Debian GNU/Linux system are free software; 

the exact distribution terms for each program are described in the 

individual files in /usr/share/doc/*/copyright. 

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent 

permitted by applicable law. 

Last login: Tue Jan 21 12:24:59 2020 

SSH is enabled and the default password for the 'pi' user has not been changed. 

This is a security risk - please login as the 'pi' user and type 'passwd' to set a new password. 

pi@raspberrypi:~ $

Testing Fractional CPU Utilization

To run this cluster, we're using a GCP n1-standard-4 instance (a 4x15) running Ubuntu 18.04 LTS. But we now notice how inefficient QEMU is once Raspbian begins doing anything. Multiple Raspberry Pi instances might stack fine if their idle, but if we want to keep the system viable, we will need to restrict CPU utilization on each instance, or else the system could be rendered unusable once more than a few nodes are put under load. Fortunately, Docker can handle this for us. We have 15GB of ram on this instance, so let's see what happens if we are slightly more ambitious and squeezed 6 Raspberry Pi containers onto our VM. We will have a whole core left for the host to manage other tasks without much risk of a failure. We can scale this at some point later with Docker Compose.

We will run two test containers at 50% and 100% for benchmark testing.

docker run -itd --cpus="0.50" -p 127.0.0.1:2250:2222 

--name pidoc_50_test pidoc 
docker run -itd --cpus="1.00" -p 127.0.0.1:2200:2222 

--name pidoc_00_test pidoc


At this point, we technically already have a cluster. We just don't have a method to manage them, except by hand.

Performance

While a full core allocation performs at near Physical Raspberry Pi speeds, an instance running at 50% runs rightly at half that speed. This might be manageable under certain circumstances, but it's not the most desirable. The overall efficiency of the cluster may increase, depending on the task at hand. But for now, we will continue with our original full core allocation of 3 nodes, and then later it tests with 6 nodes.

Single Thread Benchmarks

Testing can be done by using the following simple benchmark tests.

CPU Prime Test

sysbench --test=cpu --cpu-max-prime=9999 run


CPU Integer Test

time $(i=0; while ((i<9999999)); do ((i++)); done)


HDD Read Test

dd bs=16K count=102400 iflag=direct if=test_data of=/dev/null


HDD Write Test

dd bs=16k count=102400 oflag=direct if=/dev/zero of=test_data


Results (Single Thread)

For this guide, we will only focus on the CPU Prime Test using sysbench

Host

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General statistics:

    total time:                          10.0009s

    total number of events:              9417 

Latency (ms):

         min:                                  1.04

         avg:                                  1.06

         max:                                  1.63

         95th percentile:                      1.10

         sum:                               9992.36 

Threads fairness:

    events (avg/stddev):           9417.0000/0.00

    execution time (avg/stddev):   9.9924/0.00

Virtual Raspberry Pi - Limit: 100%

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Test execution summary:

    total time:                          397.8781s

    total number of events:              10000

    total time taken by event execution: 397.4056

    per-request statistics:

         min:                                 38.61ms

         avg:                                 39.74ms

         max:                                 57.15ms

         approx.  95 percentile:              40.92ms 

Threads fairness:

    events (avg/stddev):           10000.0000/0.00

    execution time (avg/stddev):   397.4056/0.00

Virtual Raspberry Pi - Limit: 50%

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Test execution summary:

    total time:                          823.8272s

    total number of events:              10000

    total time taken by event execution: 822.9329

    per-request statistics:

         min:                                 38.68ms

         avg:                                 82.29ms

         max:                                184.02ms

         approx.  95 percentile:              94.65ms 

Threads fairness:

    events (avg/stddev):           10000.0000/0.00

    execution time (avg/stddev):   822.9329/0.00

Compose the Cluster

Create docker-compose.yml File

We will use Docker Compose for cluster creation. Initially, we will keep this at three nodes to keep it easy to manage. Once we have a proof of concept cluster, we can then scale it out. The most straightforward way to handle this is to map separate ports to localhost for each container. We can specify a range of ports to be used in the docker-compose.yml file, as noted below.

version: '3'

services:  node:
    image: pidoc
    ports:
      - "2201-2203:2222"


Bring Up Cluster

To bring up three nodes with docker-compose, use the --scale option.

docker-compose up --scale node=3


Ansible Configuration

Now that we have all the infrastructure in place for a cluster, we need to manage it. We could use Docker to double attach to the QEMU monitor, but ssh is much more robust. Since we are using ssh, we can use Ansible. A few basic operations are provided here: update, upgrade, reboot, and shutdown. These can be expanded as needed to develop a more robust system.

hosts File

Please note of the ports we specified in the docker-compose.yml file earlier, and edit your hosts inventory accordingly.

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[all:vars] 

ansible_user=pi 

ansible_ssh_pass=raspberry 

ansible_ssh_extra_args='-o StrictHostKeyChecking=no' 

[pidoc-cluster] 

node_1.localhost:2201 

node_2.localhost:2202 

node_3.localhost:2203

For a more comprehensive walkthrough of Ansible, please read How to Install and Configure Ansible on Ubuntu.

update.yml File

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--- 

- name: Apt update Pi...

  hosts: pidoc-cluster

  tasks:

    - name: Update apt cache...

      become: yes

      apt:

        update_cache=yes

Usage: ansible-playbook playbooks/update.yml -i hosts

upgrade.yml File

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--- 

- name: Upgrade Pi...

  hosts: pidoc-cluster

  gather_facts: no

  tasks:

    - name: Update and upgrade apt packages...

      become: true

      apt:

        upgrade: yes

        update_cache: yes

        cache_valid_time: 86400

Usage: ansible-playbook playbooks/upgrade.yml -i hosts

reboot.yml File

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--- 

- name: Reboot Pi...

  hosts: pidoc-cluster

  gather_facts: no

  tasks:

    - name: Reboot Pi...

      shell: shutdown -r now

      async: 0

      poll: 0

      ignore_errors: true

      become: true

     - name: Wait for reboot...

      local_action: wait_for host={{ ansible_host }}state=started delay=10

      become: false

Usage: ansible-playbook playbooks/reboot.yml -i hosts

shutdown.yml File

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--- 

- name: Shutdown Pi...

  hosts: pidoc-cluster

  gather_facts: no

  tasks:

    - name: 'Shutdown Pi'

      shell: shutdown -h now

      async: 0

      poll: 0

      ignore_errors: true

      become: true

     - name: "Wait for shutdown..."

      local_action: wait_for host={{ ansible_host }} state=stopped

      become: false

Usage: ansible-playbook playbooks/shutdown.yml -i hosts

Scaling Up

Docker Compose makes scaling Raspberry Pi containers on the same host near trivial. By using Ansible for cluster management, it also becomes incredibly easy to scale horizontally to other hosts by changing the port binding from localhost to an IP address that's routable. Here is our example with 6 nodes instead of 3.

docker-compose.yml File

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version: '3' 

services:  node:

    image: pidoc

    ports:

      - "2201-2212:2222"

    deploy:

      resources:

        limits:

          cpus: "0.5"

Bring Up Cluster

We should stop containers from our previous cluster, and prune all volumes before scaling up our revised cluster. To bring up all 6 nodes with docker-compose, use the --scale option again.

docker-compose up --scale node=5


Future Work

Raspberry Pi emulation is still under development for QEMU. While the configuration for this project is relatively stable, there's a lot of room for improvement. Attempting a migration to Raspberry Pi 3 emulation would be an ambitious next step. Docker Compose, though designed for single-host builds, is already easy enough to replicate to other hosts manually or through Ansible. But it could just as easily be scaled out with Swarm or k8s, enabling us to build an emulated Raspberry Pi cluster of any size. Additionally, with one or more port redirects, other systems of control can be put into place, including various node endpoints, depending on purpose and application.