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09_Prolog_Epilog.rst

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Prolog & Epilog

The @prolog and @epilog decorators are definitions of binaries to be executed before / after `task` execution on the worker. All kind of PyCOMPSs tasks can have a @prolog or an @epilog, or both at the same time. A basic usage is shown in the example below:

Important

Please note that @prolog and @epilog definitions should be on top of @task decorators.

from pycompss.api.epilog import epilog
from pycompss.api.prolog import prolog
from pycompss.api.task import task


@prolog(binary="/my_service/start.bin")
@epilog(binary="/my_service/stop.bin")
@task()
def run_simulation():
    ...

def main():
    run_simulation()

Both decorators have the same syntax and have 3 parameters: `binary is the only mandatory parameter where args and fail_by_exit_value are optional. args` describe the command line arguments of the binary. Users can also pass the task parameters as arguments. In this case, the task parameter should be surrounded by double curly braces ("{{" and "}}") in the 'args' string. These parameters can be results of previous tasks and PyCOMPSs will handle data dependencies between tasks:

Important

Task parameters used in 'args' strings can be type of primitive types such as int, float, string, and boolean.

from pycompss.api.prolog import prolog
from pycompss.api.task import task

@epilog(binary="mkdir", args="/tmp/{{working_dir}}")
@prolog(binary="tar", args="zcvf {{out_tgz}} /tmp/{{working_dir}}")
@task(returns=1)
def run_simulation(working_dir, out_tgz):
    ...

def main():
    # call to the task function
    run_simulation("my_logs", "my_logs_compressed")

`fail_by_exit_value is used to indicate the behaviour when the prolog or epilog returns an exit value different than zero. Users can set the fail_by_exit_value to *True*, if they want to consider the exit value as a task failure. If set to *False*, failure of the prolog will be ignored and task execution will start as usual. The same rule applies for the epilog` as well. Default value of 'fail_by_exit_value' is True for Prolog and False for Epilog:

from pycompss.api.epilog import epilog
from pycompss.api.prolog import prolog
from pycompss.api.task import task

@prolog(binary="mkdir", args="-p {{sandbox_path}}", fail_by_exit_value=True)
@epilog(binary="rm", args="-r {{sandbox_path}}", fail_by_exit_value=False)
@task()
def run_simulation(sandbox_path):
    ...
    return 1

# call to the task function
run_simulation("/tmp/my_task_sandbox")

In the example above, if creation of the 'sandbox_path' fails, the task execution won't start at all and task will be considered as failed. However, if removing the sandbox is not crucial and can be ignored, `fail_by_exit_value` in the Epilog can be set to False.