Containers and Application Modernization: Extend, Refactor, or Rebuild?


Technology is a
constantly changing field, and as a result, any application can feel out
of date in a matter of months. With this constant feeling of impending
obsolescence, how can we work to maintain and modernize legacy
applications? While rebuilding a legacy application from the ground up
is an engineer’s dream, business goals and product timelines often make
this impractical. It’s difficult to justify spending six months
rewriting an application when the current one is working just fine, code
debt be damned. Unfortunately, we all know that product development is
never that black and white. Compromises must be made on both sides of
the table, meaning that while a complete rewrite might not be possible,
the long-term benefits of application modernization efforts must still
be valued. While many organizations don’t have the luxury of building
brand new, cloud-native applications, there are still techniques that
can be used to modernize existing applications using container
technology like Docker. These modernization techniques ultimately fall
into three different categories: extend, refactor, and rebuild. But
before we get into them, let’s first touch on some Dockerfile basics.

Dockerfile Basics

For the uninitiated, Docker is a containerization platform that “wraps
a piece of software in a complete filesystem that contains everything
needed to run: code, runtime, system tools, system libraries” and
basically everything that can be installed on a server, without the
overhead of a virtualization platform. While the pros and cons of
containers are out of the scope of this article, one of the biggest
benefits of Docker is the ability to quickly and easily spin up
lightweight, repeatable server environments with only a few lines of
code. This configuration is accomplished through a file called the
Dockerfile, which is essentially a blueprint that Docker uses to build
container images. For reference, here’s a Dockerfile that spins up a
simple Python-based web server (special thanks to Baohua
for the awesome example):

# Use the python:2.7 base image
 FROM python:2.7

# Expose port 80 internally to Docker process

# Set /code to the working directory for the following commands
 WORKDIR /code

# Copy all files in current directory to the /code directory
 ADD . /code

# Create the index.html file in the /code directory
 RUN touch index.html

# Start the python web server
 CMD python

This is a simplistic example, but it does a good job of illustrating
some Dockerfile basics, namely extending pre-existing images, exposing
ports, and running commands and services. Even these few instructions
can be used to spin up extremely powerful microservices, as long as the
base source code is architected properly.

Application Modernization

At a high level, containerizing an existing application is a relatively
straightforward process, but unfortunately not every application is
built with containerization in mind. Docker has an ephemeral filesystem,
which means that storage within a container is not persistent. Any file
that is saved within a Docker container will be lost unless specific
steps are taken to avoid this. Additionally, parallelization is another
big concern with containerized applications. Because one of the big
benefits of Docker is the ability to quickly adapt to increasing traffic
requirements, these applications need to be able to run in parallel with
multiple instances. As mentioned above, in order to prepare a legacy
application for containerization, there are a few options available:
extend, refactor, or rebuild. But which solution is the best depends
entirely on the needs and resources of an organization.


Extending the existing functionality of a non-containerized application
often requires the least amount of commitment and effort on this list,
but if it isn’t done right, the changes that are made can lead to
significantly more technical debt. The most effective way to extend an
existing application with container technology is through microservices
and APIs. While the legacy application itself isn’t being
containerized, isolating new features into Docker-based microservices
allows for the modernization of a product, and at the same time tees the
legacy code up for easier refactoring or rebuilding in the future.

At a high level, extension is a great choice for applications that are
likely to be rebuilt or sunset at some point in the not-too-distant
future—but the older the codebase, the more it might be necessary to
completely refactor certain parts of it to accommodate a Docker


Sometimes, extending an application through microservices or APIs isn’t
practical or possible. Whether there is no new functionality to be
added, or the effort to add new features through extension is too high
to justify, refactoring parts of a legacy codebase might be necessary.
This can be easily accomplished by isolating individual pieces of
existing functionality from the current application into containerized
microservices. For example, refactoring an entire social network into a
Docker-ready application might be impractical, but pulling out the piece
of functionality that runs the user search engine is a great way to
isolate individual components as separate Docker containers.

Another great place to refactor a legacy application is the storage
mechanism used for writing things like logs, user files, etc. One of the
biggest roadblocks to running an application within Docker is the
ephemeral filesystem. Dealing with this can be handled in one of a few
ways, the most popular of which is through the use of a cloud-based
storage method like Amazon S3 or Google Cloud Storage. By refactoring
the file storage method to utilize one of these platforms, an
application can be easily run in a Docker container without losing any


When a legacy application is unable to support multiple running
instances, it might be impossible to add Docker support without
rebuilding it from the ground up. Legacy applications can have a long
shelf life, but there comes a point when poor architecture and design
decisions made in the early stages of an application can prevent
efficient refactoring of an application in the future. Being aware of
impending development brick walls is crucial to identifying risks to

Ultimately, there is no hard rule when it comes to modernizing legacy
applications with container technology. The best decision is often the
one that is dictated by both the needs of the product and the needs of
the business, but understanding how this decision affects the
organization in the long run is crucial to ensuring a stable application
without losing productivity.

To learn more about using containers, join our February Online
Meetup: More Tips and Tricks for Running Containers Like a
happening Tuesday, Feb 28.

Zachary Flower (@zachflower) is a
freelance web developer, writer, and polymath. He’s built projects for
the NSA and created features for companies like and Buffer.

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