Backend Snapshots
=================

Nexus stores a snapshot - a representation of the state of a :doc:`backend
<backends>` - when important events take place, including compilation and
execution.

Unlike a backend, which is a provider of quantum or other computation, a
**backend snapshot** stores data about a provider at a certain moment in time.

Nexus captures and stores backend snapshots automatically and associates them
with relevant resources such as compilation jobs or execution jobs. You can see
all the backend snapshots in a :ref:`project <projects>` on its "Resources" tab,
or for an individual result or compilation, linked from the result or
compilation page itself.

In python code, backend snapshots are often represented as instances of
``pytket``'s ``BackendInfo`` class. For example, an execution result in
``qnexus`` is represented by an ``ExecutionResultRef`` instance, whose
``download_backend_info()`` method returns a ``pytket`` ``BackendInfo``
representing the state of the compute provider at the time of execution.