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.. _platform_examples:
   
Platform Examples
=================

Here is a very simple platform file, containing 3 resources (two hosts
and one link), and explicitly giving the route between the hosts.

.. literalinclude:: ../../examples/platforms/two_hosts.xml
   :language: xml   

The root tag must be ``<platform>``, and its ``version`` attribute
specifies which version of the DTD is use. When an evolution of the DTD introduces
backward-incompatible changes, this number gets updated. Use the
``simgrid_update_xml`` utility to upgrade your platform files on need.

Then, every platform element must be located within a given
**networking zone** introduced with the :ref:`pf_tag_zone` tag.  Zones
are in charge of the routing. If a host wants to communicate with
another host belonging to the same zone, it is the zone's duty to find the list
of links that are involved in the communication. Here, since we use
``routing="Full"``, all routes must be explicitly given using the
:ref:`pf_tag_route` and :ref:`pf_tag_linkctn` tags (this :ref:`routing
model <pf_rm>` is both simple and inefficient :) It is OK to not
specify each and every route between hosts, as long as you do not try to start
a communication on any of the missing routes during your simulation.

Any zone may contain sub-zones, allowing for a hierarchical
decomposition of the platform. Routing can be made more efficient (as the
inter-zone routing gets factored with :ref:`pf_tag_zoneroute`), and
allows you to have more than one routing model in your platform. For
example, you can have a coordinate-based routing for the WAN parts
of your platforms, a full routing within each datacenter, and a highly
optimized routing within each cluster of the datacenter.  In this
case, determining the route between two given hosts gets :ref:`routing_basics`
"somewhat more complex" but SimGrid still computes
these routes for you in a time- and space-efficient manner.
Here is an illustration of these concepts:

.. image:: img/zone_hierarchy.png

The zone "AS2" models the core of a national network interconnecting a
small flat cluster (AS4) and a larger hierarchical cluster (AS5), a
subset of a LAN (AS6), and a set of peers scattered around the world
(AS7).

.. todo:: Add more examples, such as the cloud example described in
          previous paragraph