# Custom Graphs

There may be times when you want to do parallel computing but your application doesn’t fit neatly into something like Dask Array or Dask Bag. In these cases, you can interact directly with the Dask schedulers. These schedulers operate well as standalone modules.

This separation provides a release valve for complex situations and allows advanced projects to have additional opportunities for parallel execution, even if those projects have an internal representation for their computations. As Dask schedulers improve or expand to distributed memory, code written to use Dask schedulers will advance as well.

## Example

As discussed in the motivation and specification sections, the schedulers take a task graph (which is a dict of tuples of functions) and a list of desired keys from that graph.

Here is a mocked out example building a graph for a traditional clean and analyze pipeline:

    def load(filename):
        ...

    def clean(data):
        ...

    def analyze(sequence_of_data):
        ...

    def store(result):
        with open(..., 'w') as f:
            f.write(result)

    dsk = {'load-1': (load, 'myfile.a.data'),
           'load-2': (load, 'myfile.b.data'),
           'load-3': (load, 'myfile.c.data'),
           'clean-1': (clean, 'load-1'),
           'clean-2': (clean, 'load-2'),
           'clean-3': (clean, 'load-3'),
           'analyze': (analyze, ['clean-%d' % i for i in [1, 2, 3]]),
           'store': (store, 'analyze')}

    from dask.threaded import get
    get(dsk, 'store')  # executes in parallel

## Keyword arguments in custom Dask graphs

Sometimes, you may want to pass keyword arguments to a function in a custom Dask graph. You can do that using the `dask.utils.apply()` function, like this:

    from dask.utils import apply

    task = (apply, func, args, kwargs)  # equivalent to func(*args, **kwargs)

    dsk = {'task-name': task,
            ...
           }

In the example above:

- `args` should be a tuple (eg: `(arg_1, arg_2, arg_3)`), and

- `kwargs` should be a dictionary (eg: `{"kwarg_1": value, "kwarg_2": value}`

## Related Projects

The following excellent projects also provide parallel execution:

- Joblib

- Multiprocessing

- IPython Parallel

- Concurrent.futures

- Luigi

Each library lets you dictate how your tasks relate to each other with various levels of sophistication. Each library executes those tasks with some internal logic.

Dask schedulers differ in the following ways:

1. You specify the entire graph as a Python dict rather than using a specialized API.

2. You get a variety of schedulers, ranging from a single-machine, single-core scheduler to threaded, multi-process, and distributed options.

3. You benefit from logic to execute the graph in a way that minimizes memory footprint with the Dask single-machine schedulers.

But the other projects offer different advantages and different programming paradigms. One should inspect all such projects before selecting one.