Development Guidelines

This repository is part of the Dask projects. General development guidelines including where to ask for help, a layout of repositories, testing practices, and documentation and style standards are available at the Dask developer guidelines in the main documentation.


After setting up an environment as described in the Dask developer guidelines you can clone this repository with git:

git clone

and install it from source:

cd distributed
python install


Test using py.test:

py.test distributed --verbose


Dask.distributed is a Tornado TCP application. Tornado provides us with both a communication layer on top of sockets, as well as a syntax for writing asynchronous coroutines, similar to asyncio. You can make modest changes to the policies within this library without understanding much about Tornado, however moderate changes will probably require you to understand Tornado IOLoops, coroutines, and a little about non-blocking communication.. The Tornado API documentation is quite good and we recommend that you read the following resources:

Additionally, if you want to interact at a low level with the communication between workers and scheduler then you should understand the Tornado TCPServer and IOStream available here:

Dask.distributed wraps a bit of logic around Tornado. See Foundations for more information.

Writing Tests

Testing distributed systems is normally quite difficult because it is difficult to inspect the state of all components when something goes wrong. Fortunately, the non-blocking asynchronous model within Tornado allows us to run a scheduler, multiple workers, and multiple clients all within a single thread. This gives us predictable performance, clean shutdowns, and the ability to drop into any point of the code during execution. At the same time, sometimes we want everything to run in different processes in order to simulate a more realistic setting.

The test suite contains three kinds of tests

  1. @gen_cluster: Fully asynchronous tests where all components live in the same event loop in the main thread. These are good for testing complex logic and inspecting the state of the system directly. They are also easier to debug and cause the fewest problems with shutdowns.
  2. with cluster(): Tests with multiple processes forked from the master process. These are good for testing the synchronous (normal user) API and when triggering hard failures for resilience tests.
  3. popen: Tests that call out to the command line to start the system. These are rare and mostly for testing the command line interface.

If you are comfortable with the Tornado interface then you will be happiest using the @gen_cluster style of test

def test_submit(c, s, a, b):
    assert isinstance(c, Client)
    assert isinstance(c, Scheduler)
    assert isinstance(a, Worker)
    assert isinstance(b, Worker)

    future = c.submit(inc, 1)
    assert future.key in c.futures

    # result = future.result()  # This synchronous API call would block
    result = yield future._result()
    assert result == 2

    assert future.key in s.tasks
    assert future.key in or future.key in

The @gen_cluster decorator sets up a scheduler, client, and workers for you and cleans them up after the test. It also allows you to directly inspect the state of every element of the cluster directly. However, you can not use the normal synchronous API (doing so will cause the test to wait forever) and instead you need to use the coroutine API, where all blocking functions are prepended with an underscore (_). Beware, it is a common mistake to use the blocking interface within these tests.

If you want to test the normal synchronous API you can use a with cluster style test, which sets up a scheduler and workers for you in different forked processes:

def test_submit_sync(loop):
    with cluster() as (s, [a, b]):
        with Client(('', s['port']), loop=loop) as c:
            future = c.submit(inc, 1)
            assert future.key in c.futures

            result = future.result()  # use the synchronous/blocking API here
            assert result == 2

            a['proc'].terminate()  # kill one of the workers

            result = future.result()  # test that future remains valid
            assert result == 2

In this style of test you do not have access to the scheduler or workers. The variables s, a, b are now dictionaries holding a multiprocessing.Process object and a port integer. However, you can now use the normal synchronous API (never use yield in this style of test) and you can close processes easily by terminating them.

Typically for most user-facing functions you will find both kinds of tests. The @gen_cluster tests test particular logic while the with cluster tests test basic interface and resilience.

You should avoid popen style tests unless absolutely necessary, such as if you need to test the command line interface.