#############
Prerequisites
#############

This page lists software needed to install and use the LSST Science Pipelines.

.. _prereq-platforms:

Platform compatibility
======================

The LSST Data Management reference platform is CentOS 7.
This is the platform we officially develop, test, and operate with.

Besides the reference platform, our developers and automatic tests regularly compile and run the Pipelines under CentOS 8, a variety of other Linux distributions, and various versions of macOS.
See `LSST Stack Testing Status <https://ls.st/faq>`_ reports of building LSST software on various platforms.

.. _system-prereqs:

System prerequisites
====================

The Science Pipelines are developed and built using a standard `Conda`_ environment.
This provides (almost; see below) all necessary tools and libraries for building and running the Pipelines.
On installation, you will be given the option of automatically installing that environment.
If you decline, your system must have all of the prerequisites listed below available:

.. jinja:: default

   - `macOS <https://github.com/lsst/scipipe_conda_env/blob/{{ scipipe_conda_ref }}/etc/conda3_packages-osx-64.yml>`_.
   - `Linux <https://github.com/lsst/scipipe_conda_env/blob/{{ scipipe_conda_ref }}/etc/conda3_packages-linux-64.yml>`_.

In addition to the Conda environment, the following packages must be installed on the host system.

.. _Conda: https://conda.io

CentOS
------

On CentOS 8, patch and diffutils are required.
On CentOS 7, diffutils is included with the operating system; only patch must be installed.
They may be installed as follows:

.. code-block:: bash

   sudo yum install patch diffutils

If you wish to follow the instructions for :doc:`lsstsw` (recommended for some developers, but not necessary for most users), you will also need to install git:

.. code-block:: bash

   sudo yum install git

Debian/Ubuntu
-------------

On Debian and Ubuntu systems, curl and patch are required.
These may be installed as follows:

.. code-block:: bash

   sudo apt-get update && sudo apt-get install curl patch

If you wish to follow the instructions for :doc:`lsstsw` (recommended for some developers, but not necessary for most users), you will also need to install git:

.. code-block:: bash

   sudo apt-get update && sudo apt-get install git

macOS
-----

On macOS systems, please install the Xcode Command Line Tools:

.. code-block:: bash

   xcode-select --install
   
The ``scipipe_conda_env`` environment used for the Science Pipelines through Release 21.0.0 requires the software development kit (SDK) from Xcode 11.x (11.7 recommended); it is **not** compatible with the Xcode 12 SDK.

.. _filesystem-prereqs:

Filesystem prerequisites
========================

Filesystems used for compiling the Stack and hosting output data repositories must support the ``flock`` system call for file locking.
Local filesystems virtually always have this support.
Network filesystems are sometimes mounted without such support to improve performance; the output of the :command:`mount` command may show the ``nolock`` or ``noflock`` option in those cases.

.. _optional-deps:

Optional dependencies
=====================

Some pipeline components use `SAOImage DS9 <http://ds9.si.edu/site/Home.html>`_, if available, for image display purposes.