# Plugins¶

Parsl has several places where code can be plugged in. Parsl usually provides several implementations that use each plugin point.

This page gives a brief summary of those places and why you might want to use them, with links to the API guide.

## Executors¶

When the parsl dataflow kernel is ready for a task to run, it passes that task to an ParslExecutor. The executor is then responsible for running the task’s Python code and returning the result. This is the abstraction that allows one executor to run code on the local submitting host, while another executor can run the same code on a large supercomputer.

## Providers, Launchers and Channels¶

Some executors are based on blocks of workers (for example the HighThroughputExecutor: the submit side requires a batch system (eg slurm, kubernetes) to start worker processes, which then execute tasks.

The particular way in which a system makes those workers start is implemented by providers and launchers.

An ExecutionProvider allows a command line to be submitted as a request to the underlying batch system to be run inside an allocation of nodes.

A Launcher modifies that command line when run inside the allocation to add on any wrappers that are needed to launch the command (eg srun inside slurm). Providers and launchers are usually paired together for a particular system type.

A Channel allows the commands used to interact with an ExecutionProvider to be executed on a remote system. The default channel executes commands on the local system, but a few variants of an SSHChannel are provided.

## File staging¶

Parsl can copy input files from an arbitrary URL into a task’s working environment, and copy output files from a task’s working environment to an arbitrary URL. A small set of data staging providers is installed by default, for file:// http:// and ftp:// URLs. More data staging providers can be added in the workflow configuration, in the storage parameter of the relevant ParslExecutor. Each provider should subclass the Staging class.

## Memoization/checkpointing¶

When parsl memoizes/checkpoints an app parameter, it does so by computing a hash of that parameter that should be the same if that parameter is the same on subsequent invocations. This isn’t straightforward to do for arbitrary objects, so parsl implements a checkpointing hash function for a few common types, and raises an exception on unknown types:

ValueError("unknown type for memoization ...")


You can plug in your own type-specific hash code for additional types that you need and understand using id_for_memo.