configuration section and download cache docs

configuration.rst

@@ -0,0 +1,12 @@
+.. _configuration:
+
+Configuration
+---------------
+
+The Conan client can be configured to behave differently. Most of the configuration can be found in the :ref:`conan.conf reference <conan_conf>`,
+but this section aims to be an introduction to the configuration based on different use cases.
+
+.. toctree::
+   :maxdepth: 1
+
+   configuration/download_cache

configuration/download_cache.rst

@@ -0,0 +1,94 @@
+.. _download_cache:
+
+Download cache
+==============
+
+.. warning::
+
+    This is an **experimental** feature subject to breaking changes in future releases.
+
+.. warning::
+
+    This is an **advanced** feature, use it only if you fully understand this document. Do not use it if you are mixing api V1 (without revisions)
+    and api V2 (with revisions) clients.
+
+
+
+Conan implements a shared download cache that can be used to reduce the time needed to populate the Conan package cache
+with commands like :command:`install`, :command:`create`.
+
+This cache is purely an optimization mechanism. It is completely different to the :ref:`Conan package cache <custom_cache>`, (typically the ``<userhome>/.conan`` folder).
+It is not related to the ``short_paths`` mechanism for long path in Windows, nor to the ``short_paths`` cache folder. The cache will contain a copy
+of the artifacts, it is not a new location of files. Those files will still be copied to the Conan package cache, which will not change anything,
+its behavior, layout or location of any file.
+
+This cache (whose path can be configured in the *conan.conf* file) will store the following items:
+
+- All files that are downloaded from a Conan server (conan_server, Artifactory), both in the api V1 (without revisions) and V2 (with revisions).
+  This includes files like *conanfile.py*, but also the zipped artifacts like *conan_package.tgz* or *conan_sources.tgz*.
+- The downloads done by users with the ``tools.download()`` or ``tools.get()`` helpers, as long as they provide a checksum (md5, sha1, etc.). If
+  a checksum is not provided, even if the download cache is enabled, the download will be always executed and the files will not be cached.
+
+The cache computes a sha256 checksum of the download URL and the file checksum whenever is available.
+
+.. warning::
+
+    As not always the file checksums are available, the download cache will not be able to correctly cache artifacts with revisions enabled 
+    if those artifacts are created and uploaded repeatedly in a client without revisions, because that will keep overwriting the revision "0".
+    Similarly, the download cache will fail if a proxy suddenly and transparently changes a existing server and moves it to a new location,
+    without the clients changing the URL too.
+
+
+Activating/deactivating the download cache
+------------------------------------------
+
+The download cache is activated and configured in the :ref:`conan_conf` like this:
+
+.. code-block:: text
+
+    [storage]
+    download_cache=/path/to/my/cache
+
+It can be defined from the command line:
+
+.. code-block:: bash
+
+    $ conan config set storage.download_cache="/path/to/my/cache"
+    # Display it
+    $ conan config get storage.download_cache
+
+
+And, as the *conan.conf* is part of the configuration, you can also put a common *conan.conf* file in a git repo or zip file and use
+the :ref:`conan_config_install` command to automatically install it in Conan clients.
+
+
+To deactivate the download cache, you can remove the entry ``download_cache`` from the *conan.conf* with the command:
+
+.. code-block:: bash
+
+    $ conan config rm storage.download_cache
+
+
+Concurrency, multiple caches and CI
+-----------------------------------
+
+The downloads cache implements exclusive locks for concurrency, so it can be shared among different concurrent Conan instances.
+This is a typical scenario in CI servers, in which each job uses a different Conan package cache (defined by ``CONAN_USER_HOME`` environment
+variable). Every different Conan instance could configure its download cache to share the same storage. The download cache implements interprocess
+exclusive locks, so only 1 process will access at a time to a given cached artifact. If other processes needs the same artifact, they will wait
+until it is released, avoiding multiple downloads of the same file, even if they were requested almost simultaneously.
+
+For Continuous Integration processes, it is recommended to have a different Conan package cache (``CONAN_USER_HOME``) for each job, in most of the cases,
+because the Conan package cache is not concurrent, and it might also have old dependencies, stale packages, etc. It is better to run CI jobs in a clean
+environment.
+
+
+Removing cached files
+---------------------
+
+The download cache will store a lot of artifacts, for all recipes, packages, versions and configurations that are used. This can grow and consume
+a lot of storage. If you are using this feature, provide for a sufficiently large and fast download cache folder.
+
+At the moment, it is only a folder. You can clean the cached artifacts just by removing that folder and its contents. You might also be able to 
+run scripts and jobs that remove old artifacts only. If you do such operations, please make sure that there are not other Conan processes using
+it simultaneously, or they might fail.

index.rst

@@ -24,6 +24,7 @@ Contents:
    systems_cross_building
    extending
    integrations
+   configuration
    howtos
    reference
    videos

reference/config_files/conan.conf.rst

@@ -66,6 +66,7 @@ The typical location of the **conan.conf** file is the directory ``~/.conan/``:
     # path beginning with "~" (if the environment var CONAN_USER_HOME is specified, this directory, even
     # with "~/", will be relative to the conan user home, not to the system user home)
     path = ./data
+    # download_cache = /path/to/my/cache
 
     [proxies]
     # Empty (or missing) section will try to use system proxies.
@@ -218,6 +219,13 @@ On Windows:
     If you want to change the default "conan home" (directory where ``conan.conf`` file is) you can adjust
     the environment variable ``CONAN_USER_HOME``.
 
+
+The ``storage.download_cache`` variable defines the path to a folder that can be used to cache the different file downloads from Conan servers but
+also from user downloads via the ``tools.get()`` and ``tools.download()`` methods that provide a checksum. Defining this variable will both configure
+the path and activate the download cache. If it is not defined, the download cache will not be used.
+
+Read more about the :ref:`download cache here <download_cache>`.
+
 .. _proxys:
 
 Proxies