# Plugins¶

The simplest way to bootstrap your Girder Worker task plugin is to use our cookiecutter plugin to fill in the boilerplate. See Creating a Task Plugin for instructions.

This is an example plugin that demonstrates how to extend girder_worker by allowing it to run additional tasks. Plugins are implemented as separate pip installable packages. To install this example plugin you can checkout this code base, change directories to examples/plugin_example/ and run pip install . This will add the gwexample plugin to girder_worker. If you then run girder_worker with a log level of ‘info’ (e.g. girder-worker -l info) you should see the following output:

(girder)\$ girder-worker -l info

-------------- celery@minastirith v3.1.23 (Cipater)
---- **** -----
--- * ***  * -- Linux-4.8.6-1-ARCH-x86_64-with-glibc2.2.5
-- * - **** ---
- ** ---------- [config]
- ** ---------- .> app:         girder_worker:0x7f69bfff1050
- ** ---------- .> transport:   amqp://guest:**@localhost:5672//
- ** ---------- .> results:     amqp://
- *** --- * --- .> concurrency: 32 (prefork)
-- ******* ----
--- ***** ----- [queues]
-------------- .> celery           exchange=celery(direct) key=celery

. girder_worker.convert
. girder_worker.run
. girder_worker.validators

[2016-11-08 12:22:56,163: INFO/MainProcess] Connected to amqp://guest:**@127.0.0.1:5672//
[2016-11-08 12:22:56,184: INFO/MainProcess] mingle: searching for neighbors
[2016-11-08 12:22:57,198: INFO/MainProcess] mingle: all alone


Notice that the task gwexample.analyses.tasks.fibonacci is now available. With the girder-worker processes running, you should be able to execute python example_client.py in the current working directory. After a brief delay, this should print out 121393 - the Fibonacci number for 26.

Adding additional tasks to the girder_worker infrastructure is easy and takes three steps. (1) Creating tasks, (2) creating a plugin class and (3) adding a girder_worker_plugins entry point to your setup.py.

Creating tasks follows the standard celery conventions. The only difference is the celery application that decorates the function should be imported from girder_worker.app. E.g.:

from girder_worker.app import app

def fibonacci(n):
if n == 1 or n == 2:
return 1
return fibonacci(n-1) + fibonacci(n-2)


#### Plugin Class¶

Each plugin must define a plugin class the inherits from girder_worker.GirderWorkerPluginABC. GirderWorkerPluginABC’s interface is simple. The class must define an __init__ function and a task_imports function. __init__ takes the girder_worker’s celery application as its first argument. This allows the plugin to store a reference to the application, or change configurations of the application as necessary. The task_imports function takes no arguments and must return a list of the package paths (e.g. importable strings) that contain the plugin’s tasks. As an example:

from girder_worker import GirderWorkerPluginABC

class GWExamplePlugin(GirderWorkerPluginABC):
def __init__(self, app, *args, **kwargs):
self.app = app

# Update the celery application's configuration
# it is not necessary to change the application configuration
# this is simply included to illustrate that it is possible.
self.app.config.update({
})



#### Entry Point¶

Finally, in order to make the plugin class discoverable, each plugin must define a custom entry point in its setup.py. For our example, this entry point looks like this:

from setuptools import setup

setup(name='gwexample',
# ....
entry_points={
'girder_worker_plugins': [
'gwexample = gwexample:GWExamplePlugin',
]
},
# ....
)


Python Entry Points are a way for python packages to advertise classes and objects to other installed packages. Entry points are defined in the following way:

entry_points={
'entry_point_group_id': [
'entry_point_name = importable.package.name:class_or_object',
]
}


The girder_worker package introduces a new entry point group girder_worker_plugins. This is followed by a list of strings which are parsed by setuptools. The strings must be in the form name = module:plugin_class Where name is an arbitrary string (by convention the name of the plugin), module is the importable path to the module containing the plugin class, and plugin_class is a class that inherits from GirderWorkerPluginABC.

### Final notes¶

With these three components (Tasks, Plugin Class, Entry Point) you should be able to add arbitrary tasks to the girder_worker client.

girder_worker provides support for signaling that a task should be canceled using Celery’s revoke mechanism. In order for a task to be able to be canceled cleanly it must periodically check if it has been canceled, if it has then is can do any necessary cleanup and return. girder_worker provides a task base class (girder_worker.utils.Task) that provides a property that can be used to check if the task has been canceled. An example of its use is shown below:
from girder_worker.app import app

The Girder job model associated with the canceled task will be moved into the JobStatus.CANCELED state.