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Workers, the Scheduler, and Clients communicate by sending each other Python objects (such as :ref:`protocol` messages or user data). The communication layer handles appropriate encoding and shipping of those Python objects between the distributed endpoints. The communication layer is able to select between different transport implementations, depending on user choice or (possibly) internal optimizations.

The communication layer lives in the :mod:`distributed.comm` package.


Communication addresses are canonically represented as URIs, such as tcp:// For compatibility with existing code, if the URI scheme is omitted, a default scheme of tcp is assumed (so is really the same as tcp:// The default scheme may change in the future.

The following schemes are currently implemented in the distributed source tree:

  • tcp is the main transport; it uses TCP sockets and allows for IPv4 and IPv6 addresses.
  • tls is a secure transport using the well-known TLS protocol over TCP sockets. Using it requires specifying keys and certificates as outlined in :ref:`tls`.
  • inproc is an in-process transport using simple object queues; it eliminates serialization and I/O overhead, providing almost zero-cost communication between endpoints as long as they are situated in the same process.

Some URIs may be valid for listening but not for connecting. For example, the URI tcp:// will listen on all IPv4 and IPv6 addresses and on an arbitrary port, but you cannot connect to that address.

Higher-level APIs in distributed may accept other address formats for convenience or compatibility, for example a (host, port) pair. However, the abstract communications layer always deals with URIs.


There are a number of top-level functions in :mod:`distributed.comm` to help deal with addresses:

.. autofunction:: distributed.comm.parse_address

.. autofunction:: distributed.comm.unparse_address

.. autofunction:: distributed.comm.normalize_address

.. autofunction:: distributed.comm.resolve_address

.. autofunction:: distributed.comm.get_address_host

Communications API

The basic unit for dealing with established communications is the Comm object:

.. autoclass:: distributed.comm.Comm

You don't create Comm objects directly: you either listen for incoming communications, or connect to a peer listening for connections:

.. autofunction:: distributed.comm.connect

.. autofunction:: distributed.comm.listen

Listener objects expose the following interface:

.. autoclass:: distributed.comm.core.Listener

Extending the Communication Layer

Each transport is represented by a URI scheme (such as tcp) and backed by a dedicated :class:`Backend` implementation, which provides entry points into all transport-specific routines.

.. autoclass:: distributed.comm.registry.Backend