Twelve-Factor Applications

In the modern era, software is commonly delivered as a service. These services are commonly referred as “web applications”, or “software-as-a-service”.

The Twelve-Factor App is a methodology for building modern web applications that can be deployed at scale following modern developer best practices.

Twelve-factor is a valuable synthesis of years of experience deploying software-as-a-service applications at scale in the wild, particularly on platforms like Heroku, Cloud Foundry, and the now-defunct Deis Workflow.

The maintainers of the Hippo project have been directly involved in the development and deployment of countless web applications, and some of the maintainers are from the original Deis team.

Hippo is designed to run applications that adhere to the Twelve-Factor App methodology and best practices.

HTTP handlers and WebAssembly

An important workload in event-driven environments is represented by HTTP applications, and Hippo has built-in support for creating and running HTTP components.

At the current writing of this document, WebAssembly modules are single-threaded. As a result, WebAssembly modules cannot run as a standalone web server without blocking the main thread.

To work around this limitation, Hippo deploys applications as HTTP handlers using Spin. The HTTP trigger in Spin is a web server. It listens for incoming requests and based on the application configuration, it routes them to an executor which instantiates the appropriate component, executes its entry point function, then returns an HTTP response.

As more capabilities are provided to the WebAssembly runtime, we will re-evaluate this architecture and provide more capabilities to developers.


Bindle is the term for a versioned package that can contain multiple objects of different types, or aggregate object storage. Each item in a bindle is called a parcel.

Parcels can be any arbitrary data such as:

  • WebAssembly modules
  • Templates
  • Web files such as HTML, CSS, JavaScript, and images
  • Machine learning models
  • Any other files that your application relies upon

A bindle for a website could look like this:

my-web-app 1.2.3
  |- index.html
  |- style.css
  |- library.js
  |- pretty-picture.jpg

Bindles are not just an alternative to a zip file. They can also express groups and relationships between its parcels. By associating related parcels into groups, a client can make decisions about which parcels it needs to download, and only retrieve what it needs. A parcel can be associated with more than one group, which is useful for common components.

Take for example a fantasy football prediction application, with three groups defined:

  • Frontend UI
  • Backend that uses a machine learning model
  • Backend that uses a statistical prediction model

The frontend group would contain HTML, images, and JavaScript files and is a required group. The frontend specifies that it requires a backend, which could be the “machine-learning” backend or the “statistical” backend. When the application is run, the frontend group and its parcels are downloaded, and depending on the client’s configuration, one of the backend groups and their parcels are downloaded as well. A client with plenty of time and resources might select the machine learning backend, while a constrained client might pick the faster statistical formulas.

Individual parcels in a bindle are content addressable and can be retrieved independently. Parcels can be cached by the client and reused across bindles. For example, if three bindles all contained jquery v3.6.0, then the parcel is downloaded when the first bindle is run, and it is retrieved from the cache when the other two bindles are run.

Bindle Server

A Bindle server provides storage for bindles. Bindle servers provide tools for uploading, searching, and downloading bindles. Additionally, they provide signature-based verification and provenance information so that you can assess the reliability of a bindle.

A particular bindle is identified by its name and version. For example, hello/1.2.3 is a valid reference for a bindle, while hello is not. Often, bindle names are qualified with additional information, so it is not uncommon to see bindles with references like

Once a bindle is uploaded to a Bindle server, that bindle is immutable. It cannot be changed. For example, if some part of hello/1.2.3 is changed locally, you will need to change the version before pushing it to a Bindle server.

Bindle CLI

Bindle does include a CLI (called bindle) for working directly with Bindle servers. You can use this tool to upload and download bindles, and also to search a Bindle server. Hippo does not require the bindle CLI. Instead, it provides its own CLI for working with Bindle.

Hippo Server

The Hippo server is a Platform as a Service (PaaS) layer for creating WebAssembly-based micro-services and web applications. It provides a browser-based portal, an API for the client CLI, and the back-end management features to work with Bindle servers, load balancers, and Spin.

Hippo CLI

The hippo command-line tool.


Spin is a framework for building and running event-driven micro-service applications with WebAssembly components.

Spin executes the component(s) as a result of events being generated by the trigger(s) defined in the spin.toml file.