Update installation notes to warn against common problems.

We have recently seen a number of issues created by users who
tried to install airflow with poetry or pip-tools or who had
successes with using the latest pip 20.3.3. This change aims
to update the 'note' content and make sure installation
instructions are consistent everywhere, so that new users
are warned against using anything else than PIP and that they
are aware about potential problems with 'pip 20.3' and ways
to mitigate the problems.

This responds to the needs of confused users such as
one in apache#13711 (comment)

CONTRIBUTING.rst

@@ -671,10 +671,22 @@ Airflow dependencies
 .. note::
 
    On November 2020, new version of PIP (20.3) has been released with a new, 2020 resolver. This resolver
-   does not yet work with Apache Airflow and might lead to errors in installation - depends on your choice
-   of extras. In order to install Airflow you need to either downgrade pip to version 20.2.4
-   ``pip install --upgrade pip==20.2.4`` or, in case you use Pip 20.3, you need to add option
-   ``--use-deprecated legacy-resolver`` to your pip install command.
+   might work with Apache Airflow as of 20.3.3, but it might lead to errors in installation. It might
+   depend on your choice of extras. In order to install Airflow you might need to either downgrade
+   pip to version 20.2.4 ``pip install --upgrade pip==20.2.4`` or, in case you use Pip 20.3,
+   you need to add option ``--use-deprecated legacy-resolver`` to your pip install command.
+
+   While ``pip 20.3.3`` solved most of the ``teething`` problems of 20.3, this note will remain here until we
+   set ``pip 20.3`` as official version in our CI pipeline where we are testing the installation as well.
+   Due to those constraints, only ``pip`` installation is currently officially supported.
+
+   While they are some successes with using other tools like `poetry <https://python-poetry.org/>`_ or
+   `pip-tools <https://pypi.org/project/pip-tools/>`_, they do not share the same workflow as
+   ``pip`` - especially when it comes to constraint vs. requirements management.
+   Installing via ``Poetry`` or ``pip-tools`` is not currently supported.
+
+   If you wish to install airflow using those tools you should use the constraint files and convert
+   them to appropriate format and workflow that your tool requires.
 
 
 Extras
@@ -904,10 +916,22 @@ Pinned constraint files
 .. note::
 
    On November 2020, new version of PIP (20.3) has been released with a new, 2020 resolver. This resolver
-   does not yet work with Apache Airflow and might lead to errors in installation - depends on your choice
-   of extras. In order to install Airflow you need to either downgrade pip to version 20.2.4
-   ``pip install --upgrade pip==20.2.4`` or, in case you use Pip 20.3, you need to add option
-   ``--use-deprecated legacy-resolver`` to your pip install command.
+   might work with Apache Airflow as of 20.3.3, but it might lead to errors in installation. It might
+   depend on your choice of extras. In order to install Airflow you might need to either downgrade
+   pip to version 20.2.4 ``pip install --upgrade pip==20.2.4`` or, in case you use Pip 20.3,
+   you need to add option ``--use-deprecated legacy-resolver`` to your pip install command.
+
+   While ``pip 20.3.3`` solved most of the ``teething`` problems of 20.3, this note will remain here until we
+   set ``pip 20.3`` as official version in our CI pipeline where we are testing the installation as well.
+   Due to those constraints, only ``pip`` installation is currently officially supported.
+
+   While they are some successes with using other tools like `poetry <https://python-poetry.org/>`_ or
+   `pip-tools <https://pypi.org/project/pip-tools/>`_, they do not share the same workflow as
+   ``pip`` - especially when it comes to constraint vs. requirements management.
+   Installing via ``Poetry`` or ``pip-tools`` is not currently supported.
+
+   If you wish to install airflow using those tools you should use the constraint files and convert
+   them to appropriate format and workflow that your tool requires.
 
 
 By default when you install ``apache-airflow`` package - the dependencies are as open as possible while

CONTRIBUTORS_QUICK_START.rst

@@ -31,11 +31,11 @@ Prerequisites
 
 
 Installing Prerequisites on Ubuntu
-###################################
+##################################
 
 
 Docker Community Edition
------------------------------------
+------------------------
 
 
 1. Installing required packages for Docker and setting up docker repo
@@ -84,7 +84,7 @@ Note : After adding user to docker group Logout and Login again for group member
 
 
 Docker Compose
---------------------------
+--------------
 
 1. Installing latest version of Docker Compose
 
@@ -108,8 +108,8 @@ Docker Compose
 
 
 
-pyenv and setting up virtual-env
---------------------------------------------
+Pyenv and setting up virtual-env
+--------------------------------
 
 1. Checking required packages
 
@@ -162,7 +162,28 @@ pyenv and setting up virtual-env
 
 
 Setup Airflow with Breeze and PyCharm
-#####################################################
+#####################################
+
+
+.. note::
+
+   On November 2020, new version of PIP (20.3) has been released with a new, 2020 resolver. This resolver
+   might work with Apache Airflow as of 20.3.3, but it might lead to errors in installation. It might
+   depend on your choice of extras. In order to install Airflow you might need to either downgrade
+   pip to version 20.2.4 ``pip install --upgrade pip==20.2.4`` or, in case you use Pip 20.3,
+   you need to add option ``--use-deprecated legacy-resolver`` to your pip install command.
+
+   While ``pip 20.3.3`` solved most of the ``teething`` problems of 20.3, this note will remain here until we
+   set ``pip 20.3`` as official version in our CI pipeline where we are testing the installation as well.
+   Due to those constraints, only ``pip`` installation is currently officially supported.
+
+   While they are some successes with using other tools like `poetry <https://python-poetry.org/>`_ or
+   `pip-tools <https://pypi.org/project/pip-tools/>`_, they do not share the same workflow as
+   ``pip`` - especially when it comes to constraint vs. requirements management.
+   Installing via ``Poetry`` or ``pip-tools`` is not currently supported.
+
+   If you wish to install airflow using those tools you should use the constraint files and convert
+   them to appropriate format and workflow that your tool requires.
 
 
 Forking and cloning Project
@@ -274,7 +295,7 @@ Setting up Breeze
   source ~/.bashrc
 
 Using Breeze
------------------------------------------
+------------
 
 1. Starting breeze environment using ``breeze start-airflow`` starts Breeze environment with last configuration run(
    In this case python and backend will be picked up from last execution ``./breeze --python 3.8 --backend mysql``)
@@ -457,7 +478,7 @@ Following are some of important topics of Breeze documentation:
 
 
 Setting up Debug
------------------------------------------
+----------------
 
 1. Configuring Airflow database connection
 
@@ -528,11 +549,11 @@ Setting up Debug
 
 
 Starting development
-###################################
+####################
 
 
 Creating a branch
------------------------------------
+-----------------
 
 1. Click on branch symbol in the bottom right corner of Pycharm
 
@@ -555,7 +576,7 @@ Creating a branch
 
 
 Testing
------------------------------------
+-------
 
 All Tests are inside ./tests directory.
 
@@ -720,7 +741,7 @@ All Tests are inside ./tests directory.
 
 
 Pre-commit
------------------------------------
+----------
 
 Before committing changes to github or raising a pull request, code needs to be checked for certain quality standards
 such as spell check, code syntax, code formatting, compatibility with Apache License requirements etc. This set of
@@ -881,7 +902,7 @@ To avoid burden on CI infrastructure and to save time, Pre-commit hooks can be r
 
 
 Contribution guide
------------------------------------
+------------------
 
 - To know how to contribute to the project visit |CONTRIBUTING.rst|
 
@@ -917,7 +938,7 @@ Contribution guide
 
 
 Raising Pull Request
------------------------------------
+--------------------
 
 1. Go to your Github account and open your fork project and click on Branches
 

IMAGES.rst

@@ -123,10 +123,22 @@ parameter to Breeze:
 .. note::
 
    On November 2020, new version of PIP (20.3) has been released with a new, 2020 resolver. This resolver
-   does not yet work with Apache Airflow and might lead to errors in installation - depends on your choice
-   of extras. In order to install Airflow you need to either downgrade pip to version 20.2.4
-   ``pip install --upgrade pip==20.2.4`` or, in case you use Pip 20.3, you need to add option
-   ``--use-deprecated legacy-resolver`` to your pip install command.
+   might work with Apache Airflow as of 20.3.3, but it might lead to errors in installation. It might
+   depend on your choice of extras. In order to install Airflow you might need to either downgrade
+   pip to version 20.2.4 ``pip install --upgrade pip==20.2.4`` or, in case you use Pip 20.3,
+   you need to add option ``--use-deprecated legacy-resolver`` to your pip install command.
+
+   While ``pip 20.3.3`` solved most of the ``teething`` problems of 20.3, this note will remain here until we
+   set ``pip 20.3`` as official version in our CI pipeline where we are testing the installation as well.
+   Due to those constraints, only ``pip`` installation is currently officially supported.
+
+   While they are some successes with using other tools like `poetry <https://python-poetry.org/>`_ or
+   `pip-tools <https://pypi.org/project/pip-tools/>`_, they do not share the same workflow as
+   ``pip`` - especially when it comes to constraint vs. requirements management.
+   Installing via ``Poetry`` or ``pip-tools`` is not currently supported.
+
+   If you wish to install airflow using those tools you should use the constraint files and convert
+   them to appropriate format and workflow that your tool requires.
 
 
 This will build the image using command similar to:

LOCAL_VIRTUALENV.rst

@@ -64,10 +64,22 @@ Extra Packages
 .. note::
 
    On November 2020, new version of PIP (20.3) has been released with a new, 2020 resolver. This resolver
-   does not yet work with Apache Airflow and might lead to errors in installation - depends on your choice
-   of extras. In order to install Airflow you need to either downgrade pip to version 20.2.4
-   ``pip install --upgrade pip==20.2.4`` or, in case you use Pip 20.3, you need to add option
-   ``--use-deprecated legacy-resolver`` to your pip install command.
+   might work with Apache Airflow as of 20.3.3, but it might lead to errors in installation. It might
+   depend on your choice of extras. In order to install Airflow you might need to either downgrade
+   pip to version 20.2.4 ``pip install --upgrade pip==20.2.4`` or, in case you use Pip 20.3,
+   you need to add option ``--use-deprecated legacy-resolver`` to your pip install command.
+
+   While ``pip 20.3.3`` solved most of the ``teething`` problems of 20.3, this note will remain here until we
+   set ``pip 20.3`` as official version in our CI pipeline where we are testing the installation as well.
+   Due to those constraints, only ``pip`` installation is currently officially supported.
+
+   While they are some successes with using other tools like `poetry <https://python-poetry.org/>`_ or
+   `pip-tools <https://pypi.org/project/pip-tools/>`_, they do not share the same workflow as
+   ``pip`` - especially when it comes to constraint vs. requirements management.
+   Installing via ``Poetry`` or ``pip-tools`` is not currently supported.
+
+   If you wish to install airflow using those tools you should use the constraint files and convert
+   them to appropriate format and workflow that your tool requires.
 
 
 You can also install extra packages (like ``[ssh]``, etc) via
@@ -126,10 +138,22 @@ To create and initialize the local virtualenv:
 .. note::
 
    On November 2020, new version of PIP (20.3) has been released with a new, 2020 resolver. This resolver
-   does not yet work with Apache Airflow and might lead to errors in installation - depends on your choice
-   of extras. In order to install Airflow you need to either downgrade pip to version 20.2.4
-   ``pip install --upgrade pip==20.2.4`` or, in case you use Pip 20.3, you need to add option
-   ``--use-deprecated legacy-resolver`` to your pip install command.
+   might work with Apache Airflow as of 20.3.3, but it might lead to errors in installation. It might
+   depend on your choice of extras. In order to install Airflow you might need to either downgrade
+   pip to version 20.2.4 ``pip install --upgrade pip==20.2.4`` or, in case you use Pip 20.3,
+   you need to add option ``--use-deprecated legacy-resolver`` to your pip install command.
+
+   While ``pip 20.3.3`` solved most of the ``teething`` problems of 20.3, this note will remain here until we
+   set ``pip 20.3`` as official version in our CI pipeline where we are testing the installation as well.
+   Due to those constraints, only ``pip`` installation is currently officially supported.
+
+   While they are some successes with using other tools like `poetry <https://python-poetry.org/>`_ or
+   `pip-tools <https://pypi.org/project/pip-tools/>`_, they do not share the same workflow as
+   ``pip`` - especially when it comes to constraint vs. requirements management.
+   Installing via ``Poetry`` or ``pip-tools`` is not currently supported.
+
+   If you wish to install airflow using those tools you should use the constraint files and convert
+   them to appropriate format and workflow that your tool requires.
 
 
    .. code-block:: bash

README.md

@@ -139,15 +139,27 @@ constraints files separately per major/minor python version.
 You can use them as constraint files when installing Airflow from PyPI. Note that you have to specify
 correct Airflow tag/version/branch and python versions in the URL.
 
+
 1. Installing just Airflow:
 
 NOTE!!!
 
 On November 2020, new version of PIP (20.3) has been released with a new, 2020 resolver. This resolver
-does not yet work with Apache Airflow and might lead to errors in installation - depends on your choice
-of extras. In order to install Airflow you need to either downgrade pip to version 20.2.4
-`pip install --upgrade pip==20.2.4` or, in case you use Pip 20.3, you need to add option
-`--use-deprecated legacy-resolver` to your pip install command.
+might work with Apache Airflow as of 20.3.3, but it might lead to errors in installation. It might
+depend on your choice of extras. In order to install Airflow reliably, you might need to either downgrade
+pip to version 20.2.4 `pip install --upgrade pip==20.2.4` or, in case you use Pip 20.3,
+you might need to add option] `--use-deprecated legacy-resolver` to your pip install command.
+While `pip 20.3.3` solved most of the `teething` problems of 20.3, this note will remain here until we
+set `pip 20.3` as official version in our CI pipeline where we are testing the installation as well.
+Due to those constraints, only `pip` installation is currently officially supported.
+
+While they are some successes with using other tools like [poetry](https://python-poetry.org) or
+[pip-tools](https://pypi.org/project/pip-tools), they do not share the same workflow as
+`pip` - especially when it comes to constraint vs. requirements management.
+Installing via `Poetry` or `pip-tools` is not currently supported.
+
+If you wish to install airflow using those tools you should use the constraint files and convert
+them to appropriate format and workflow that your tool requires.
 
 
 ```bash

docs/apache-airflow/installation.rst

@@ -64,7 +64,7 @@ either downgrade pip to version 20.2.4 ``pip install --upgrade pip==20.2.4`` or,
 ``--use-deprecated legacy-resolver`` to your pip install command.
 
 While they are some successes with using other tools like `poetry <https://python-poetry.org/>`_ or
-`pip-tools <https://pypi.org/project/pip-tools/>`_, but they do not share the same workflow as
+`pip-tools <https://pypi.org/project/pip-tools/>`_, they do not share the same workflow as
 ``pip`` - especially when it comes to constraint vs. requirements management.
 Installing via ``Poetry`` or ``pip-tools`` is not currently supported. If you wish to install airflow
 using those tools you should use the :ref:`constraint files <installation:constraints>`  and convert them to appropriate

docs/apache-airflow/start.rst

@@ -23,13 +23,26 @@ Quick Start
 .. note::
 
    On November 2020, new version of PIP (20.3) has been released with a new, 2020 resolver. This resolver
-   does not yet work with Apache Airflow and might lead to errors in installation - depends on your choice
-   of extras. In order to install Airflow you need to either downgrade pip to version 20.2.4
-   ``pip install --upgrade pip==20.2.4`` or, in case you use Pip 20.3, you need to add option
-   ``--use-deprecated legacy-resolver`` to your pip install command.
+   might work with Apache Airflow as of 20.3.3, but it might lead to errors in installation. It might
+   depend on your choice of extras. In order to install Airflow you might need to either downgrade
+   pip to version 20.2.4 ``pip install --upgrade pip==20.2.4`` or, in case you use Pip 20.3,
+   you need to add option ``--use-deprecated legacy-resolver`` to your pip install command.
 
+   While ``pip 20.3.3`` solved most of the ``teething`` problems of 20.3, this note will remain here until we
+   set ``pip 20.3`` as official version in our CI pipeline where we are testing the installation as well.
+   Due to those constraints, only ``pip`` installation is currently officially supported.
 
-The installation is quick and straightforward.
+   While they are some successes with using other tools like `poetry <https://python-poetry.org/>`_ or
+   `pip-tools <https://pypi.org/project/pip-tools/>`_, they do not share the same workflow as
+   ``pip`` - especially when it comes to constraint vs. requirements management.
+   Installing via ``Poetry`` or ``pip-tools`` is not currently supported.
+
+   If you wish to install airflow using those tools you should use the constraint files and convert
+   them to appropriate format and workflow that your tool requires.
+
+
+The installation of Airflow is painless if you are following the instructions below. Airflow uses
+constraint files to enable reproducible installation, so using ``pip`` and constraint files is recommended.
 
 .. code-block:: bash
 
@@ -38,8 +51,12 @@ The installation is quick and straightforward.
     # (optional)
     export AIRFLOW_HOME=~/airflow
 
-    # install from pypi using pip
-    pip install apache-airflow
+    AIRFLOW_VERSION=2.0.0
+    PYTHON_VERSION="$(python --version | cut -d " " -f 2 | cut -d "." -f 1-2)"
+    # For example: 3.6
+    CONSTRAINT_URL="https://raw.githubusercontent.com/apache/airflow/constraints-${AIRFLOW_VERSION}/constraints-${PYTHON_VERSION}.txt"
+    # For example: https://raw.githubusercontent.com/apache/airflow/constraints-2.0.0/constraints-3.6.txt
+    pip install "apache-airflow==${AIRFLOW_VERSION}" --constraint "${CONSTRAINT_URL}"
 
     # initialize the database
     airflow db init