.. meta::
    :description lang=en:
		A common problem with Windows is that generally only one version of a DLL can be loaded into a process. xlSlim has the ability to run Python code in a background process. This background process is spawned as a sub-process of Excel. Commmunication with the sub-process is efficient. 

Running Python in a background process
======================================


A common problem with Windows is that generally only one version of a DLL can be loaded into a process. This can be a problem if common DLLs are being loaded by both an Excel add-in and Python code. For example, you could have a valuation library that is available as both an Excel add-in and a Python library, in which case xlSlim would encounter problems if the Excel add-in and Python package used different versions of the valuation library's DLLs. Another common problem is that Excel processes can use a lot of system resources, both memory and CPU. 

xlSlim has the ability to run Python code in a background process. This background process is spawned as a sub-process of Excel. Commmunication with the sub-process is efficient. 

.. warning::
	Running Python in a background process requires a premium licence. See :doc:`/reference/licensing`

We will use the :doc:`/samples/sqlite` example to show how the background process works. Please work through the example to get to the stage that the sales by region are shown in Excel.

.. image:: ../_static/sqlite_result.png
  :width: 400
  :alt: SQLite data in Excel

We need to amend the call to :func:`RegisterPyModule` so a background process is used. Edit the function in A1, passing TRUE for the *UseBackgroundProcess* parameter  ::

    =RegisterPyModule("c:\users\russe\documents\sqlite.py",,,TRUE)

.. image:: ../_static/subprocess_amend_registration.png
  :width: 400
  :alt: Amend registration to use background process
  
The results are processed the same as before, the background processing does not affect how the xlSlim functions behave.

.. image:: ../_static/subprocess_result.png
  :width: 400
  :alt: Result from background process  

Using Process Explorer, we can see the background process running as a child of the EXCEL process:

.. image:: ../_static/subprocess_procexplorer_show_subprocess.png
  :width: 400
  :alt: Background process is a child of Excel

Controlling DLL loading
-----------------------

Let's imagine Excel has an add-in which also uses the SQLite database, however a different version to the Python environment. In this case we need to be sure that SQLite libraries are not being loaded by any Python code within the Excel process.

Running the :doc:`/samples/sqlite` example with the :func:`RegisterPyModule` *UseBackgroundProcess* parameter set to FALSE (or left out) we can see the `_sqlite.pyd` DLL loaded in the Excel process: 

.. image:: ../_static/subprocess_sqlite_in_excel.png
  :width: 400
  :alt: _sqlite3.pyd loaded in Excel

We'd like to run the Python code in a background process and only load SQLite libraries in the background process. This will require a small change to the Python code.

.. code-block:: python
	:emphasize-lines: 11

	# Read a csv file into SQLite and analyse
	import os
	import csv


	def calculate_revenue_by_region(filepath):
		"""Calculates revenue by region.
		
		Loads the csv file into SQLite and groups by region.
		"""
		import sqlite3
		...

We need to move the `sqlite3` import to be within the `calculate_revenue_by_region()` function. This ensures the library is not imported when xlSlim analyses the code when registered.

Now running the :doc:`/samples/sqlite` example with the :func:`RegisterPyModule` *UseBackgroundProcess* parameter set to TRUE, we see the background process running as expected and more importantly we can see that `_sqlite.pyd` is not loaded into the Excel process.

.. image:: ../_static/subprocess_sqlite_not_in_excel.png
  :width: 400
  :alt: _sqlite3.pyd not loaded in Excel

`_sqlite.pyd` is loaded into the PySubProcess process:

.. image:: ../_static/subprocess_sqlite_in_subprocess.png
  :width: 400
  :alt: _sqlite3.pyd not loaded in subprocess

With some careful editing of imports Python code can be run so certain libraries are only loaded in the background process.