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Zephyr is an open-source Java-based plugin framework with smart dependency management, a modular design, and a small footprint.
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build.gradle Adding some tests Nov 24, 2019
gradlew Moving gradlew files Nov 20, 2019


Sunshower Kernel

The Sunshower kernel is a flexible microkernel allowing developers to quickly develop, test, and deploy individual applications.


User Interface

File System

This is an overview of the Kernel File System


To build Zephyr, install Maven 3+ and Gradle 5+ (we use 6). Then run:

mvn clean install -f bom && gradle clean assemble build sA--this will install all the dependencies and execute the tests


The Kernel filesystem is structured as follows:


When a kernel module is installed, a new "directory" is created at: droplet://// where <droplet-group>, <droplet-name> and <droplet-version> correspond to the following META-INF/MANIFEST.MF entries of the installed assembly

version (must be lower-case) name (lower-case) group (lower-case)


the directory droplet:/// corresponds to the root of the file-system and may only be accessed by kernel modules. There are several notable files and directories in this filesystem:

  1. kernel.idx--an index of the installed plugins and kernel-modules, as well as information about location, digests, and state
  2. modules/ a directory containing the list (symlinked) of installed modules
  3. plugins/ a directory containing the list (symlinked) of installed plugins

Module URI structure

Given a module with group=sunshower:artifact=stuff:version=1.0.0, the module's structure can be located at droplet:///sunshower/artifact/1.0.0. Everything within this URI corresponds to the physical directory structure of the assembly installed. The Sunshower kernel creates several additional files at this scheme:

  1. plugin.idx: droplet:///sunshower/artifact/1.0.0/plugin.idx -- this file contains kernel-specific information and must not be modified
  2. droplet:///sunshower/artifact/1.0.0/ -- this file contains information about this plugin and its state 1 paths.idx : etc. --this file contains kernel-specific information about the plugin


Some components in the Kernel manage operations in phases, which allows for extensibility, traceability, and simplicity.

For instance, consider the DefaultModuleManager. A user or process provides an installation request, at which the following phases are executed:

  1. The ModuleDownloadPhase downloads the module to the Kernel temporary storage folder and dispatches the following events
    1. DownloadStarted
    2. DownloadProgressed
    3. DownloadComplete
  2. The ModuleUnpackPhase

Modules vs Plugins

Modules and plugins share many similarities, but differ in important ways.
For instance, any kernel-module's classpath is visible to the kernel. Second, kernel-modules are loaded before any plugins are.

Kernel Module Registry

when a kernel module is installed, the kernel saves the entry to droplet://kernel/registry.modlist upon starting the kernel, modules are read from this list

Kernel Module Lifecycle

The kernel module lifecycle is designed so that kernel module functionality and classpaths become available at the very earliest opportunity. The lifecycle is as follows:

  1. Kernel Start
    1. Kernel Filesystem is created
    2. droplet://kernel/modules.list is read, producing list of existing kernel modules
    3. Kernel classloader is created as the combination of all kernel modules
    4. Kernel loads all existing modules (initially none), sorted by order
    5. Plugin set is loaded


The modules.list file contains the minimum amount of information required to load the kernel modules, and has the following format

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