*****************
Intro and Example
*****************

.. image:: _static/pyro-large.png
  :align: center

This chapter contains a little overview of Pyro's features and a simple example to show how it looks like.

.. index:: features

About Pyro: feature overview
============================

Pyro is a library that enables you to build applications in which
objects can talk to each other over the network, with minimal programming effort.
You can just use normal Python method calls, with almost every possible parameter
and return value type, and Pyro takes care of locating the right object on the right
computer to execute the method. It is designed to be very easy to use, and to
generally stay out of your way. But it also provides a set of powerful features that
enables you to build distributed applications rapidly and effortlessly.
Pyro is a pure Python library and runs on many different platforms and Python versions.

Here's a quick overview of Pyro's features:

- written in 100% Python so extremely portable, runs on Python 2.7, Python 3.5 and newer, IronPython, Pypy 2 and 3.
- works between different system architectures and operating systems.
- able to communicate between different Python versions transparently.
- defaults to a safe serializer (`serpent <https://pypi.python.org/pypi/serpent>`_) that supports many Python data types.
- supports different serializers (serpent, json, marshal, msgpack, pickle, cloudpickle, dill).
- support for all Python data types that are serializable when using the 'pickle', 'cloudpickle' or 'dill' serializers [1]_.
- can use IPv4, IPv6 and Unix domain sockets.
- optional secure connections via SSL/TLS (encryption, authentication and integrity), including certificate validation on both ends (2-way ssl).
- lightweight client library available for .NET and Java native code ('Pyrolite', provided separately).
- designed to be very easy to use and get out of your way as much as possible, but still provide a lot of flexibility when you do need it.
- name server that keeps track of your object's actual locations so you can move them around transparently.
- yellow-pages type lookups possible, based on metadata tags on registrations in the name server.
- support for automatic reconnection to servers in case of interruptions.
- automatic proxy-ing of Pyro objects which means you can return references to remote objects just as if it were normal objects.
- one-way invocations for enhanced performance.
- batched invocations for greatly enhanced performance of many calls on the same object.
- remote iterator on-demand item streaming avoids having to create large collections upfront and transfer them as a whole.
- you can define timeouts on network communications to prevent a call blocking forever if there's something wrong.
- asynchronous invocations if you want to get the results 'at some later moment in time'. Pyro will take care of gathering the result values in the background.
- remote exceptions will be raised in the caller, as if they were local. You can extract detailed remote traceback information.
- http gateway available for clients wanting to use http+json (such as browser scripts).
- stable network communication code that works reliably on many platforms.
- can hook onto existing sockets created for instance with socketpair() to communicate efficiently between threads or sub-processes.
- possibility to use Pyro's own event loop, or integrate it into your own (or third party) event loop.
- three different possible instance modes for your remote objects (singleton, one per session, one per call).
- many simple examples included to show various features and techniques.
- large amount of unit tests and high test coverage.
- reliable and established: built upon more than 15 years of existing Pyro history, with ongoing support and development.


.. index:: history

Pyro's history
^^^^^^^^^^^^^^
I started working on the first Pyro version in 1998, when remote method invocation technology such as Java's RMI
and CORBA were quite popular. I wanted something like that in Python and there was nothing available, so I decided
to write my own. Over the years it slowly gained features till it reached version 3.10 or so.
At that point it was clear that the code base had become quite ancient and couldn't reliably support any new features,
so Pyro4 was born in early 2010, written from scratch.

``Pyro`` is the package name of the old and no longer supported 3.x version of Pyro.
``Pyro4`` is the package name of the current version. Its concepts are similar to Pyro 3.x but it is not
backwards compatible. To avoid conflicts, this version has a different package name.

If you're somehow still interested in the old version, here is `its git repo <https://github.com/irmen/Pyro3>`_
and it is also still `available on PyPi <http://pypi.python.org/pypi/Pyro/>`_ -- but don't use it unless
you really have to.


.. index:: usage

What can you use Pyro for?
==========================

Essentially, Pyro can be used to distribute and integrate various kinds of resources or responsibilities:
computational (hardware) resources (cpu, storage, printers),
informational resources (data, privileged information)
and business logic (departments, domains).

An example would be a high performance compute cluster with a large storage system attached to it.
Usually this is not accessible directly, rather, smaller systems connect to it and
feed it with jobs that need to run on the big cluster. Later, they collect the results.
Pyro could be used to expose the available resources on the cluster to other computers.
Their client software connects to the cluster and calls the Python program there to perform its
heavy duty work, and collect the results (either directly from a method call return value,
or perhaps via asynchronous callbacks).

Remote controlling resources or other programs is a nice application as well.
For instance, you could write a simple
remote controller for your media server that is running on a machine somewhere in a closet.
A simple remote control client program could be used to instruct the media server
to play music, switch playlists, etc.

Another example is the use of Pyro to implement a form of `privilege separation <http://en.wikipedia.org/wiki/Privilege_separation>`_.
There is a small component running with higher privileges, but just able to execute the few tasks (and nothing else)
that require those higher privileges. That component could expose one or more Pyro objects
that represent the privileged information or logic.
Other programs running with normal privileges can talk to those Pyro objects to
perform those specific tasks with higher privileges in a controlled manner.

Finally, Pyro can be a communication glue library to easily integrate various pars of a heterogeneous system,
consisting of many different parts and pieces. As long as you have a working (and supported) Python version
running on it, you should be able to talk to it using Pyro from any other part of the system.

Have a look at the :file:`examples` directory in the source archive, perhaps one of the many example
programs in there gives even more inspiration of possibilities.


.. index:: example

Simple Example
==============

This example will show you in a nutshell what it's like to use Pyro in your programs.
A much more extensive introduction is found in the :doc:`tutorials`.
Here, we're making a simple greeting service that will return a personalized greeting message to its callers.
First let's see the server code::

    # saved as greeting-server.py
    import Pyro4

    @Pyro4.expose
    class GreetingMaker(object):
        def get_fortune(self, name):
            return "Hello, {0}. Here is your fortune message:\n" \
                   "Behold the warranty -- the bold print giveth and the fine print taketh away.".format(name)

    daemon = Pyro4.Daemon()                # make a Pyro daemon
    uri = daemon.register(GreetingMaker)   # register the greeting maker as a Pyro object

    print("Ready. Object uri =", uri)      # print the uri so we can use it in the client later
    daemon.requestLoop()                   # start the event loop of the server to wait for calls

Open a console window and start the greeting server::

    $ python greeting-server.py
    Ready. Object uri = PYRO:obj_edb9e53007ce4713b371d0dc6a177955@localhost:51681

Great, our server is running. Let's see the client code that invokes the server::

    # saved as greeting-client.py
    import Pyro4

    uri = input("What is the Pyro uri of the greeting object? ").strip()
    name = input("What is your name? ").strip()

    greeting_maker = Pyro4.Proxy(uri)         # get a Pyro proxy to the greeting object
    print(greeting_maker.get_fortune(name))   # call method normally

Start this client program (from a different console window)::

    $ python greeting-client.py
    What is the Pyro uri of the greeting object?  <<paste the uri that the server printed earlier>>
    What is your name?  <<type your name; in my case: Irmen>>
    Hello, Irmen. Here is your fortune message:
    Behold the warranty -- the bold print giveth and the fine print taketh away.

As you can see the client code called the greeting maker that was running in the server elsewhere,
and printed the resulting greeting string.

With a name server
^^^^^^^^^^^^^^^^^^
While the example above works, it could become tiresome to work with object uris like that.
There's already a big issue, *how is the client supposed to get the uri, if we're not copy-pasting it?*
Thankfully Pyro provides a *name server* that works like an automatic phone book.
You can name your objects using logical names and use the name server to search for the
corresponding uri.

We'll have to modify a few lines in :file:`greeting-server.py` to make it register the object in the name server::

    # saved as greeting-server.py
    import Pyro4

    @Pyro4.expose
    class GreetingMaker(object):
        def get_fortune(self, name):
            return "Hello, {0}. Here is your fortune message:\n" \
                   "Tomorrow's lucky number is 12345678.".format(name)

    daemon = Pyro4.Daemon()                # make a Pyro daemon
    ns = Pyro4.locateNS()                  # find the name server
    uri = daemon.register(GreetingMaker)   # register the greeting maker as a Pyro object
    ns.register("example.greeting", uri)   # register the object with a name in the name server

    print("Ready.")
    daemon.requestLoop()                   # start the event loop of the server to wait for calls

The :file:`greeting-client.py` is actually simpler now because we can use the name server to find the object::

    # saved as greeting-client.py
    import Pyro4

    name = input("What is your name? ").strip()

    greeting_maker = Pyro4.Proxy("PYRONAME:example.greeting")    # use name server object lookup uri shortcut
    print(greeting_maker.get_fortune(name))

The program now needs a Pyro name server that is running. You can start one by typing the
following command: :command:`python -m Pyro4.naming` (or simply: :command:`pyro4-ns`) in a separate console window
(usually there is just *one* name server running in your network).
After that, start the server and client as before.
There's no need to copy-paste the object uri in the client any longer, it will 'discover'
the server automatically, based on the object name (:kbd:`example.greeting`).
If you want you can check that this name is indeed known in the name server, by typing
the command :command:`python -m Pyro4.nsc list` (or simply: :command:`pyro4-nsc list`), which will produce::

    $ pyro4-nsc list
    --------START LIST
    Pyro.NameServer --> PYRO:Pyro.NameServer@localhost:9090
    example.greeting --> PYRO:obj_663a31d2dde54b00bfe52ec2557d4f4f@localhost:51707
    --------END LIST

(Once again the uri for our object will be random)
This concludes this simple Pyro example.

.. note::
 In the source archive there is a directory :file:`examples` that contains a truckload
 of example programs that show the various features of Pyro. If you're interested in them
 (it is highly recommended to be so!) you will have to download the Pyro distribution archive.
 Installing Pyro only provides the library modules. For more information, see :doc:`config`.

Other means of creating connections
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The example above showed two of the basic ways to set up connections between your client and server code.
There are various other options, have a look at the client code details: :ref:`object-discovery`
and the server code details: :ref:`publish-objects`. The use of the name server is optional, see
:ref:`name-server` for details.


.. index:: performance, benchmark

Performance
===========
Pyro4 is pretty fast. On a typical networked system you can expect:

- a few hundred new proxy connections per second to one server
- similarly, a few hundred initial remote calls per second to one server
- a few thousand remote method calls per second on a single proxy
- tens of thousands batched or oneway remote calls per second
- 10-100 Mb/sec data transfer

Results do vary depending on many factors such as:

- network speed
- machine and operating system
- I/O or CPU bound workload
- contents and size of the pyro call request and response messages
- the serializer being used
- python version being used

Experiment with the ``benchmark``, ``batchedcalls`` and ``hugetransfer`` examples to see what results you get on your own setup.


.. rubric:: Footnotes

.. [1] When configured to use the :py:mod:`pickle`, :py:mod:`cloudpickle` or :py:mod:`dill` serializer,
    your system may be vulnerable
    because of the security risks of these serialization protocols (possibility of arbitrary
    code execution).
    Pyro does have some security measures in place to mitigate this risk somewhat.
    They are described in the :doc:`security` chapter. It is strongly advised to read it.
    By default, Pyro is configured to use the safe `serpent` serializer, so you won't have
    to deal with these issues unless you configure it explicitly to use one of the other serializers.
