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Introduction to plugins🔗

Plugins contain the implementations of vAccel operations. While the vAccel library itself is the only requirement to compile a vAccel application, you need to use a plugin for the application to actually do something useful.

The use of plugins can become clearer by revisiting the image classification example from Running the examples.

As we have already shown, to specify a backend plugin you have to set the VACCEL_PLUGINS environment variable.

Ensure VACCEL_PLUGINS is not set in the environment while keeping debug logging enabled:

unset VACCEL_PLUGINS
export VACCEL_LOG_LEVEL=4

If you run the example, there will be no errors concerning the binaries themselves (ie. undefined symbols) but the operation will fail at runtime:

$ classify /usr/local/share/vaccel/images/example.jpg 1
2026.04.29-14:49:05.41 - <debug> Initializing vAccel
2026.04.29-14:49:05.41 - <info> vAccel 0.7.1-93-ebc23b1f
2026.04.29-14:49:05.41 - <debug> Config:
2026.04.29-14:49:05.41 - <debug>   plugins = (null)
2026.04.29-14:49:05.41 - <debug>   log_level = debug
2026.04.29-14:49:05.41 - <debug>   log_file = (null)
2026.04.29-14:49:05.41 - <debug>   profiling_enabled = false
2026.04.29-14:49:05.41 - <debug>   version_ignore = false
2026.04.29-14:49:05.41 - <debug> Created top-level rundir: /run/user/0/vaccel/wrlzzl
2026.04.29-14:49:05.41 - <error> No plugins registered
Could not initialize session
2026.04.29-14:49:05.41 - <debug> Cleaning up vAccel
2026.04.29-14:49:05.41 - <debug> Cleaning up sessions
2026.04.29-14:49:05.41 - <debug> Cleaning up resources
2026.04.29-14:49:05.41 - <debug> Cleaning up plugins

It is clear from the output that the session cannot be initialized because no plugin has been loaded. By comparing the output with classify from the previous document, you can see that there are no registered plugins.

You can find out more about the available plugins and their usage in the Plugins section.

Using a plugin from a non-standard library path🔗

It is recommended that vAccel and the vAccel plugins are installed in the standard library search paths for ease of use (ie. /usr/lib, /usr/local/lib etc.). If this is not possible or desirable there are two ways to use the plugins:

  • Assuming vAccel is installed in a standard path, use the full path to specify the plugin. For example, if the NoOp plugin is installed in /opt/noop/lib: 
    export VACCEL_PLUGINS=/opt/noop/lib/libvaccel-noop.so

or,

  • Add the vAccel/plugin library paths to the library search paths and specify the plugin as usual. For example, if vAccel and the NoOp plugin are installed in /opt/vaccel/lib: 
    export LD_LIBRARY_PATH=/opt/vaccel/lib:$LD_LIBRARY_PATH
export VACCEL_PLUGINS=libvaccel-noop.so