Lamby puts a Rack application on AWS Lambda without a rewrite

The adapter is the whole product

The README is short because the mechanism is borrowed rather than built. Lamby is an AWS Lambda Web Adapter for Rack applications, which means it does not reimplement anything about how a Rails application handles a request. The Web Adapter is the mechanism AWS provides for running an ordinary HTTP server inside a Lambda function, and Lamby supplies the Rails and Rack specific parts. The consequence is that your controller, model, view and routing code keep working the way they did, and what changes is the process around them. The repository description calls it a simple Rails and AWS Lambda integration, and the project topics name the pieces: aws, aws-lambda, aws-sam, aws-sam-cli, cloudformation, fullstack-serverless, rack, rails and ruby-on-rails. The presence of cloudformation and aws-sam-cli among the topics is the part that tells you this is a deployment project rather than a library, since the interesting part of putting an app on Lambda is the infrastructure around the function, not the request handling inside it. The primary language is Ruby, and the project is MIT licensed under a LICENSE.txt file at the root.

Three front doors, and the choice is yours

What Lamby supports is stated in one sentence and it is a real decision. The project supports Lambda Function URLs, API Gateway with either HTTP or REST APIs and all payload versions, and even Application Load Balancer integrations. These are three different traffic paths with three different operational profiles. A Function URL is the simplest, with AWS giving you a dedicated endpoint for the function and no API component to manage, which makes it the natural starting point for a small application or an internal tool. API Gateway is the conventional choice if you already run a gateway, since it brings request validation, authorisation, throttling and usage plans with it, and the support for all payload versions matters because the HTTP API and the REST API have different request shapes. An Application Load Balancer integration is the third option, and it is the one that keeps your networking closest to what you had before, with a load balancer in front of the function and the path rewriting handled by AWS rather than by you. The presence of ALB support is the one people forget, and it is the option that matters most if your Rails app is behind a proxy today.

The deployment story lives on a separate site

The README is deliberately a pointer rather than a manual, and the two links it gives are the whole of the documentation. The quick start is at lamby.cloud/docs/quick-start and the full documentation, called the anatomy page, is at lamby.cloud/docs/anatomy. That structure tells you something about the project's shape. There is no long configuration reference in the repository, no sample application to copy and no CloudFormation template to inspect in the tree, so a team evaluating Lamby is evaluating the project through its documentation site rather than through its source. The repository itself is small in the way a well factored tool should be. The top level listing shows a Gemfile and Gemfile.lock, a lamby.gemspec, a lib/ directory, a bin/ directory, a test/ directory, a Rakefile, a .ruby-version file, a CHANGELOG.md, a .devcontainer/ directory and a .github/ directory, plus the CODE_OF_CONDUCT.md and the images used in the README. There are no GitHub releases on record, so the changelog and the gem registry are where version history lives, and the project publishes under the lamby gem name with the site at lamby.cloud.

Development happens in a dev container, not on your laptop

The contributing section says something that tells you a lot about how the project expects to be used. This project is built for GitHub Codespaces using the Development Container specification, and once you have the repo cloned and set up with a dev container using either Codespaces or VS Code, you run the setup and test commands. The project links a Codespaces badge so you can open the environment directly from the repository page, and for local work it points at the Visual Studio Code Dev Container extension and the Open Folder in Container command, which gives you the same integrated terminal. Then the two commands that install the packages and run the tests are the same everywhere.

$ ./bin/setup
$ ./bin/test

The reason for the dev container is not stylistic. A Rack application running on Lambda has more moving parts than a plain gem, since the local environment needs to approximate the function execution environment, and a container gives every contributor the same starting point. The bin/ directory in the top level listing is where those two scripts live, and the .devcontainer/ directory holds the definition. For a team that already develops in containers this is a non issue, and for a team that does not it is a small imposition that buys a working local environment on the first attempt.

What running Rails on Lambda actually changes

Nothing in the README describes the runtime constraints, so the judgement here has to come from what the tool is and what it is not. Lamby is an adapter, not a serverless framework, which means it does not give you a FaaS programming model. Your application still boots like a Rails application, it still has a middleware stack, and it still assumes it owns a process. In a Lambda execution environment that assumption has to be paid for on every cold start, and the things a traditional Rails deployment relies on, a warm process, local disk state, a background worker that runs alongside the web process, are either unavailable or have to move somewhere else. The README's own framing is modest, a simple integration using Rack, and that modesty is accurate. What you get is the ability to deploy the application you have to a serverless front door, with the plumbing around it handled by the adapter and by the AWS services in front of it. What you do not get is a set of answers to the questions a serverless deployment raises, and those questions are the real work in the migration.

Where the alternative approaches differ

There are three other routes to the same end state and each makes a different trade. The first is to run Rails on a container service instead, a managed container platform behind a load balancer, which keeps the process model of a traditional deployment and keeps a persistent filesystem, at the cost of a server to size and operate. The second is to use a Rails specific hosting product that handles the deployment for you, where the trade is that you give up control of the infrastructure and of the release process. The third is to rewrite the front end as a separate service and let the Rails application become an API, which is the architecture the serverless world was designed for, and the one that means your serverless runtime and your Rails application stop being the same thing. Lamby sits between the first and the third. It keeps the application whole and moves the execution model, and that is the appeal for an existing Rails codebase that a rewrite would be expensive. The cost is the same one the adapter cannot hide, which is that a Rails app in a function pays a boot cost per cold start and has to live with the limits of the execution environment. The project is not archived and its last push was on 2026-06-06, so the code is being worked on, but there are no tagged releases on record, which is worth weighing before you pin a version.

Editorial conclusion

Lamby suits a team that already has a Rack application, mostly Rails, and has decided the serverless operating model is worth the change in how requests are handled, because the adapter lets the existing application run behind Lambda without being restructured. It is the wrong tool if you are starting a new application with no existing code, since the value is in reusing what you have, and it is the wrong tool if you cannot accept that the deployment unit is now a function and the local development loop changes with it. Before you adopt it, read the anatomy page on lamby.cloud, which the README links as the full documentation, and confirm that your application does not rely on long lived processes, local disk state or anything else that a Lambda execution environment does not provide.

Frequently asked questions

Official sources

1. Issues
2. License: MIT
3. Project website
4. rails-lambda/lamby on GitHub
5. README