Dokku is an open-source PAAS (Platform as a Service) alternative to Heroku. It is characterized as "The smallest PaaS implementation you've ever seen".

Here are its primary features and functionalities:

1. Architecture and Core Functionality

• PAAS Implementation: Dokku helps users build and manage the lifecycle of applications from the initial building phase through scaling.

• Powered by Docker: The platform is built and powered by Docker. This allows users to install it on any hardware and utilize it on inexpensive cloud providers.

• Single-Host Heroku: The end result is the user's own, single-host version of Heroku.

• Isolated Containers: Deployed applications run in isolated containers.

• No Vendor Lock-in: By owning their PaaS, users leverage tools they already know and avoid vendor lock-in.

2. Deployment Workflow

• Heroku Compatibility: Once Dokku is set up on a host, users can push Heroku-compatible applications to it via Git.

• Buildpacks: Applications automatically build using Heroku buildpacks.

• Quick App Creation: Users can quickly create a new app using the command dokku apps:create test-app.

3. Extensibility and Customization

• Plugins: Dokku is designed to be extensible and customizable through a robust plugin system.

• Plugin Development: Users can write Dokku plugins in any language, share them, and extend existing ones.

• Core Structure: Dokku itself is built out of plugins. Plugins are essential for adding additional features beyond the default installation.

4. Setup and Support

• Installation: Installation is simple using a bootstrap.sh script that can be downloaded and run. Setup requires configuring the server domain and adding an SSH key to the dokku user.

• Support: The community offers support via Discord, Slack, IRC, and GitHub Discussions.

• Pro Version: A Pro version is available for users needing features not covered in the open-source release.