Headless Visual Testing For Web Applications

Some time back, A team had come across an interesting challenge. The team, let’s call them Rangers, had embarked on a journey creating a cross platform enterprise application which would allow them to write business logic once and deploy it as an iOS app, an Android app as well as a Web application.

Rangers were doing great. They had precise test suites honoring the test pyramid. A well thought of CI system to execute them and a clear path to production. They also implemented UI Testing framework to run on top of their current infrastructure and capture UI issues as part of the feedback. It worked flawlessly on the mobile apps.

But not the web app. The tests were flaky and would fail for no apparent reasons in some runs while they would pass on others. Rangers spent hours debugging the root cause: the chrome browser version, the chromedriver, imagemagick comparisons and the fuzz factor. They even tried to have exact identical agents, hardware and software images but still couldn’t pin point the root cause.

On verge of giving up, one Ranger asked a very important question: Is it the physical displays on the agents?

Is it physical displays?

Images are made up of pixels. The physical displays (desktop monitors, laptop screens) arrange and render pixels onto the illuminated screen. The number of pixels in an image is always constant. However, it’s representation varies across displays depending on the capability of a display to pack pixels on the screen. this capability is measured as Pixel Per Inch (PPI) and can be deduced using formula below where height and width is the resolution set for the display (1080p, 720p, etc..) and diagonal is the diagonal length of display: $$ \frac{\sqrt { height^2 + width^2 }}{diagonal} $$

This means that for same resolution, displays of different sizes will render same image with a fuzz or slight variance.

The Rangers realised that the key to get repeatable robust results is to control this PPI. However, the PPIs are property of physical display and cannot be altered or mocked. What required was a virtual display which can be configured, looked at and taken screenshot off. and Xvfb does just that. It’s a virtual frame buffer for X server, a full-fledged implementation which mocks the physical display with detailed configuration and even allows one to VNC!

The Plan

Rangers decided that the best robust, repeatable and portable solution would be a containerized setup with test suites mounted and run against.

The plan was to have a Docker image with Xvfb, Chromium and Chrome-driver setup to start at launch, along with x11vnc to provide a VNC just in case it was required to debug. The required modules for test automation viz. Selenium, ImageMagick was also baked in to the image.

Consider below sample Dockerfile which creates a light weight docker image and starts the Xvfb and VNC:

FROM alpine
RUN echo "http://dl-4.alpinelinux.org/alpine/latest-stable/main" >> /etc/apk/repositories && \
    echo "http://dl-4.alpinelinux.org/alpine/latest-stable/community" >> /etc/apk/repositories

RUN apk update && \
    apk add xvfb x11vnc chromium chromium-chromedriver xwininfo imagemagick

ARG UID=1000
VOLUME /images

RUN echo 'echo "starting Xvfb and x11vnc"' >> /app/init.sh && \
echo "`which Xvfb` $DISPLAY -dpi 100 -screen 0 1024x768x24 &" >> /app/init.sh && \
echo "`which x11vnc` -display $DISPLAY -forever >&2 &" >> /app/init.sh && \
echo 'echo "done."' >> /app/init.sh

RUN adduser -D -u $UID testrunner
USER testrunner

CMD ["/bin/sh","-c","sh init.sh && /bin/sh"]

The above docker image, once built, can then be executed as

$ docker run -ti -p 5900:5900 -v ~/test_dir/images:/images <image_name>

The above will start up the docker image, mountes the directories of interest, starts Xvfb and x11vnc that can be vnced @

The Result

The flakiness was gone and it opened up new possibilities of testing in isolation various different device configurations like iPhone, Samsung S8, tablets for responsive web tests using Chromium flags. But that’s a story to be told some other day.

This blog is a transcript of a talk I gave at VodQA Pune. The slides are also available here.

PS: There are tools and frameworks in markets which can be used instead, but cost considerable amount of money. The idea here is to utilize best of open source and existing investments to get fast feedbacks.

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