AWS SAM – My Exciting First Open Source Experience

10. December 2018 2018 0

My first acquaintance with AWS Cloud happened through a wonderful tool – SAM CLI. It was a lot of fun playing around with it –  writing straightforward YAML based resource templates, deploying them to AWS Cloud or simply invoking Lambda functions locally. But after trying to debug my .NET Core Lambda function, I realized that it was not yet supported by the CLI. At this point, I started looking through the open issues and found this one that described exactly what I was experiencing it turned out to be a hot topic, so it got me interested instantly. As I reviewed the thread to get additional context about the issue, I learned that the project was looking for help.

The ability to debug Lambda on .NET locally was critical for me to fully understand how exactly my function was going to be invoked in the Cloud, and troubleshoot future problems. I decided to answer the call to help and investigate the problem further. Feeling like Superman that came to save the day, I posted a comment on the thread to say that, “I’m looking into this too.” It took no more than 10 minutes to receive a welcoming reply from members of the core team. When you receive a reply in a matter of minutes, you definitely understand that the project is truly alive and the maintainers are open to collaboration –  especially since being hosted on GitHub does not necessarily mean that a project is intended for contribution and community support. Feeling inspired, I began my investigation.

The Problem

Unfortunately the .NET Core host program doesn’t have any “debug-mode” switch (you can track the issue here), meaning that there is no way to start .NET program so it will immediately pause after launching, wait for the debugger to attach to it, and only then proceed (for example Java (using JDWP and its suspend flag) and Node.js are capable of doing this), and SAM needed some way to work around this. After several days of investigation and prototyping, I came up with a prototype of a working solution that used a remote debugger attachment to the .NET Lambda runner inside of the running Docker container (to execute AWS Lambda function locally, SAM internally spins up a Docker container with a Lambda-like environment).


I joined the AWS Serverless Application Slack organization to collaborate with the maintainers and understand more about the current CLI design. Thanks to the very welcoming community on the Slack channel, I almost immediately started to feel like I was part of the SAM team! . At this point, I was ready to write up a POC of .NET Core debugging with SAM. I have a you can look through which explains my solution in greater detail.


My POC was reviewed by the maintainers of SAM CLI. Together we discussed and resolved all open questions, shaped up the design to perfection and agreed upon the approach. To streamline the process and break it down, one of the members of the core team suggested that I propose a solution and identify the set of tasks through a design document. I had no problems doing so because the SAM CLI project has a well defined and structured template for writing this kind of document. The core team reviewed and approved. Everything was set up and ready to go, and I started implementing the feature.


Docker Lambda

My first target was the Docker Lambda repo. More specifically, it was the implementation of a “start in break mode” flag for .NET Core 2.0 and 2.1 runner (implementation details here).

I must admit, that this was my very first open source pull request, and it turned out to be overkill, so my apologies to the maintainer. PR included many more changes than required to solve the problem. The unnecessary changes were related to code style refactoring and minor improvements. And don’t get me wrong – best practices and refactoring changes are not undesired nor unwelcome, but they should be implemented in a separate PR to be a good citizen in the open source community. Okay, with that lesson learned, I’ve opened a slim version of my original PR with only the required code changes and with detailed explanations of them. SAM CLI is up next on our list.




Thanks to the awesome development guide at SAM CLI repo, it was super easy to set up the environment and get going. Even though I am not a Python guy, the development felt smooth and straightforward. The CLI codebase has a fine-grained modular architecture, and everything was clearly named and documented, so I faced no problems getting around the project. Configured Flake8 and Pylint linters kept an eye on my changes, so following project code-style guidelines was just a matter of fixing warnings if they appear. And of course, decent unit tests code coverage (97% at the time of writing) not only helped me to rapidly understand how each component worked within the system, but also made me feel rock solid confident as I was introducing changes.


The core team and contributors all deserve great thumbs up ? for keeping the project in a such a wonderful and contribution-friendly state!


However, I did encounter some troubles running unit and integration tests locally on my beloved Windows machine. I got plenty of unexpected Python bugs, “windows-not-supported” limitations and stuff like that along the way, at one point I was in complete despair. But help comes if you seek it. After asking the community for guidance, we collectively came up with a Docker solution by running tests inside Ubuntu container. And finally, I was able to setup my local environment and run all required tests before submitting the pull request. Ensuring that you hadn’t broken anything is a critical process while collaborating on any kind of project, and especially open source!

According to my initial approach to .NET Core debugging, I implemented --container-name feature, which should’ve allowed users of SAM CLI to specify a name for the running Lambda container, to identify it later to perform attaching. During review, the core team found some corner cases when this approach introduces some limitations and diversity for the whole debugging experience compared to the other runtimes, so I started to look for possible workarounds.

I looked at the problem from a different perspective and came up with a solution, that enabled .NET Core debugging for AWS SAM CLI with almost no changes made ? (more about this approach here and here). The team immediately loved this approach, because it closely aligned with the way other runtimes deal with debugging currently, providing a consistent experience for the users. I am happy, that I got valuable feedback from the community because exactly that got me thinking about the way to improve my solution. Constructive criticism, when handled properly, makes perfection. Now, the feature is merged into develop and is waiting for the next release to come into play! Thanks to everyone, who’d taken part into this amazing open source journey to better software!


Bottom line, I can say that I had a smooth and pleasant first open source experience. I’m happy that it was with SAM, as I had a super interesting time collaborating with other passionate SAM developers and contributors across the globe through GitHub threads, emails and Slack. I liked how rapid and reactive all of those interactions were – it was a great experience writing up design docs for various purposes, and being part of a strong team collaborating to reach the common goal. It was both challenging and fun to quickly explore SAM codebase and then follow its fully automated code-style rules. And, lastly, it was inspiring to experience how easy it is to contribute to a big and very well-known open source project like SAM framework.


Based on my experience, I came up with this battle-tested and bulletproof checklist:

  1. Before getting your hands dirty with code, take your time and engage in discussion with the core team to make sure you understand the issue and try to get as much context on it as possible. ?
  2. I suggest writing up a descriptive design doc, which explains the problem (or feature) and proposed solution (or implementation) in great detail, especially if it’s a common practice on the project you’re contributing to
  3. Don’t be afraid or embarrassed to ask for help from the community or the maintainers if you’re stuck. Everyone wants you to succeed ?
  4. Unit tests are a great source of truth in decent projects, so look through them to get a better understanding of all the moving parts. Try to cover your changes with required tests, as these tests also help future contributors (which could be you)!
  5. Always try your best to match project code-style – thanks to modern linters this one is a piece of ?
  6. Keep your code changes small and concise. Don’t try to pack every single code style and best practices change into a tiny PR intended to fix a minor bug
  7. The PR review process is a conversation between you and other collaborators about the changes you’ve made. Reading and reviewing every comment you receive carefully, helps to build a common understanding of the problem and avoid confusion. Never take those comments personally and try to be polite and respectful while working on them. Don’t be afraid to disagree or have a constructive discussion with reviewers. Try to come to a mutual agreement in the end and, if required, introduce specified changes
  8. Try not to force your pull request as the team may have some important tasks at hand or approaching release deadlines. Remember that good pull requests get pulled and not pushed.


It’s good to know that anyone from the community can have an impact just by leaving a comment or fixing a small bug. It’s cool that new features can be brought in by the community with the help of the core team. Together we can build better open source software!


Based on my experience, SAM is a good and welcoming project to contribute to thanks to the well-done design and awesome community. If you have any doubt, just give it a try. SAM could be a perfect start for your open source career ?


When you start it is hard to stop: #795, #828, #829, #843

Contribute with caution

Thank You!

I want to give special thanks to @sanathkr, @jfuss, @TheSriram, @mikemorain and @mhart for supporting me continuously along this path.


About the Author

My name is Nikita Dobriansky, from sunny Odessa, Ukraine. Throughout my career in software development, I’ve been working with C# desktop applications. Currently, I’m a C# engineer at Lohika, building bleeding-edge UWP apps. I am super excited about .NET Core and container-based applications. For me, AWS is a key to better serverless ☁️ future.

In my free time, I enjoy listening to the great music and making some myself ?


About the Editor

Jennifer Davis is a Senior Cloud Advocate at Microsoft. Jennifer is the coauthor of Effective DevOps. Previously, she was a principal site reliability engineer at RealSelf, developed cookbooks to simplify building and managing infrastructure at Chef, and built reliable service platforms at Yahoo. She is a core organizer of devopsdays and organizes the Silicon Valley event. She is the founder of CoffeeOps. She has spoken and written about DevOps, Operations, Monitoring, and Automation.