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Plugins

Overview

A {Singularity} plugin is a package that can be dynamically loaded by the {Singularity} runtime, augmenting {Singularity} with experimental, non-standard and/or vendor-specific functionality.

Plugins can influence the behaviour of {Singularity} in specific ways:

  • A cli plugin can use the Command callback to add or modify CLI subcommands and/or flags.
  • A cli plugin can use the SingularityEngineConfig callback to change the container configuration before it is passed to the runtime, e.g. adding bind mounts etc.
  • A runtime plugin can use the MonitorContainer callback to watch the container process as it is executing.
  • A runtime plugin can use the PostStartProcess callback to carry out a task after the container has been started.
  • A runtime plugin can use the RegisterImageDriver callback to implement an alternative way of providing a container image to execute.

Limitations / Requirements

The way that plugin functionality is implemented in the Go language, which {Singularity} is written with, is quite restrictive.

Go plugins must be built with the same Go version, and set of dependencies, as the main program they will be loaded into. This means it is generally impractical to develop and build plugins except in lock-step with the main {Singularity} source tree.

Functionality that can be implemented with plugins is limited to the scope of the exposed plugin callbacks. Container runtimes such as {Singularity} execute using multiple processes, with distinct boundaries that limit the influence a plugin can have.

If you are considering writing a plugin for {Singularity} you may wish to investigate whether the feature can be contributed to the main source tree directly via a PR. This simplifies future maintenance, and avoids the limitations of Go plugins.

Using Plugins

The list command prints the currently installed plugins.

$ singularity plugin list
There are no plugins installed.

Plugins are packaged and distributed as binaries encoded with the versatile Singularity Image Format (SIF). However, plugin authors may also distribute the source code of their plugins. A plugin can be compiled from its source code with the compile command. A number of example plugins are included in the examples/plugins directory of the {Singularity} source.

$ singularity plugin compile examples/plugins/cli-plugin/
INFO:    Plugin built to: /home/dtrudg/Git/singularity/examples/plugins/cli-plugin/cli-plugin.sif

Upon successful compilation, a SIF file will appear in the directory of the plugin's source code.

$ ls examples/plugins/cli-plugin/ | grep sif
cli-plugin.sif

Note

Due to the structure of the {Singularity} project, and the strict requirements of Go plugin compilation, all plugins must be compiled from within the {Singularity} source code tree.

The ability to compile plugins outside of the {Singularity} tree, that previously existed, has been removed due to incompatible changes in Go 1.18.

Every plugin encapsulates various information such as the plugin's author, the plugin's version, etc. To view this information about a plugin, use the inspect command.

$ singularity plugin inspect examples/plugins/cli-plugin/cli-plugin.sif
Name: github.com/sylabs/singularity/cli-example-plugin
Description: This is a short example CLI plugin for Singularity
Author: Sylabs Team
Version: 0.1.0

To compile a plugin, use the compile command.

$ singularity plugin compile examples/plugins/log-plugin/
INFO:    Plugin built to: /home/myuser/singularity/examples/plugins/log-plugin/log-plugin.sif

Note

Before using the plugin compile subcommand, make sure that you trust the origin of the plugin, and that you are certain it does not contain any malicious code.

To install a plugin, use the install command. This operation requires root privilege.

$ sudo singularity plugin install examples/plugins/cli-plugin/cli-plugin.sif
$ singularity plugin list
ENABLED  NAME
    yes  sylabs.io/cli-plugin

Note

Before using the plugin install subcommand, make sure that you trust the origin of the plugin, and that you are certain it does not contain any malicious code.

For further information on verifying the contents of SIF files using cryptographic signatures, see the Sign and Verify section <signNverify>.

After successful installation, the plugin will automatically be enabled. Any plugin can be disabled with the disable command and re-enabled with the enable command. Both of these operations require root privilege.

$ sudo singularity plugin disable sylabs.io/cli-plugin
$ singularity plugin list
ENABLED  NAME
     no  sylabs.io/cli-plugin

$ sudo singularity plugin enable sylabs.io/cli-plugin
$ singularity plugin list
ENABLED  NAME
    yes  sylabs.io/cli-plugin

Finally, to uninstall a plugin, use the uninstall command. This operation requires root privilege.

$ sudo singularity plugin uninstall sylabs.io/cli-plugin
Uninstalled plugin "sylabs.io/cli-plugin".

$ singularity plugin list
There are no plugins installed.

Writing a Plugin

Developers interested in writing {Singularity} plugins can get started by reading the Go documentation for the plugin package.

Example plugins can be found in the {Singularity} source code.