TaskRunner

Execute a certain sequence of tasks and later their cleanups. It is useful for running tasks with many varying configurations. It doesn't have any dependencies, just the standard library.

# file examples/simple.py
import taskrunner
class ExampleTask(taskrunner.Task):
    def __init__(self, msg, clean_msg, **kwargs):
        super(ExampleTask, self).__init__(**kwargs)
        self.msg = msg
        self.clean_msg = clean_msg

    def run(self, context):
        print self.msg

    def cleanup(self, context):
        print self.clean_msg


task1 = {'task': ExampleTask,
         'name': 'task1',
         'msg': 'hello world',
         'clean_msg': 'goodbye'}
task2 = {'task': ExampleTask,
         'name': 'task2',
         'msg': 'hello again',
         'clean_msg': ':)'}

pipeline = [task1, task2]

$ bin/taskrunner -f examples/simple.py pipeline
2013-12-02 19:46:37,952 - taskrunner - INFO - =========== run task1 ===========
hello world
2013-12-02 19:46:37,953 - taskrunner - INFO - =========== run task2 ===========
hello again
2013-12-02 19:46:37,953 - taskrunner - INFO - --------- cleanup task2 ---------
:)
2013-12-02 19:46:37,953 - taskrunner - INFO - --------- cleanup task1 ---------
goodbye

How it works

The pipeline is a list of task configurations, which are normal Python dictionaries with the special item 'task':ExampleTask. It goes trough the list and for each task, it instantiates ExampleTask with the rest of the dictionary content as parameters. Then it executes ExampleTask.run() for all of the tasks. After it passes trough the whole list, it goes trough it in reverse order and executes ExampleTask.cleanup() for each item. The tasks can write into context and the content of it will be passed to the next task.

Usage

You can specify the pipeline directly as arguments:

$ bin/taskrunner -f examples/simple.py task1 task2

Or you can combine multiple pipelines, which will run all the tasks from each pipeline:

$ bin/taskrunner -f examples/simple.py pipeline another_task_pipeline

Or even combine pipelines and tasks (this will run task2 twice):

$ bin/taskrunner -f examples/simple.py pipeline task2

To use the tool as a library, you can directly use execute:

taskrunner.execute([task1, task2])

Taking control of the cleanup execution

Sometimes you want to only execute the run() part of the tasks, debug something and only run the cleanups after you are done. To skip the cleanups, you can do:

$ bin/taskrunner -f examples/simple.py pipeline --cleanup=never

To run the cleanups only:

$ bin/taskrunner -f examples/simple.py pipeline --cleanup=pronto

You can also use the options --cleanup=on_success or --cleanup=on_failure, which will get executed based on how the run turned out.

Don't forget to make the cleanups independent of the runs, otherwise this won't work.

Exception and signal handling

When an exception occurs in the task run, its traceback is logged and it jumps right into the cleanups. However, it doesn't clean up the tasks that didn't run (but it does clean up the task which failed and got only partially executed).

If you get an exception during some cleanup, the traceback is logged but execution continues with the next task's cleanup.

The list of errors gets logged again after everything else finishes, in the order they happened.

If you terminate the run using ctrl-c (also known as SIGINT), it will go straight to the cleanups. Sending the termination signal again will stop it completely. This works for the SIGTERM signal too.

The name of a task

By default, the name of a task is the class name. To have more readable logs, you can specify the keyword name in the task configuration. The task names can be important for configuration redefinition from the command line.

Redefining the task configuration trough CLI arguments

Sometimes you want to run a sequence of tasks with some changes in their configuration, but don't want to change the files. You can redefine it using the parameter -D.

$ bin/taskrunner -f examples/simple.py pipeline -D task1.msg=ping

It can't contain any spaces, has to be in the exact format of taskname.key1.key2.key3...=newvalue, where taskname is either the name of the task specified in the configuration dictionary or the class name. If more tasks have the same name, it will get rewritten for all of them. For example,

$ bin/taskrunner -f examples/simple.py pipeline -D ExampleTask.msg=ping

will change the message for both task1 and task2, because they have the same class name.

Using multiple files for the task configurations

You probably don't want to have everything in a single file. You can load multiple modules and reference the tasks normally.

$ bin/taskrunner -f examples/advanced.py -f examples/simple.py \
    mytask task1 task2

In case you have any name conflicts, you can specify the name of the module.

$ bin/taskrunner -f examples/advanced.py -f examples/simple.py \
    advanced.pipeline

Best practices for writing tasks and their configurations

• don't make the cleanup method dependent on run, because with the option --cleanup=pronto, the run method won't get executed
• don't assume that the run got executed completely
• put the tasks into a separate file, which will be imported in the file with the task configurations
• use the minimum of Python features in the task configuration files (which are just .py files), variable definitions and if conditions are usually sufficient. You will be later able to switch to some other configuration format.