Title: Apache License v2.0 and GPL Compatibility
Notice:    Licensed to the Apache Software Foundation (ASF) under one
           or more contributor license agreements.  See the NOTICE file
           distributed with this work for additional information
           regarding copyright ownership.  The ASF licenses this file
           to you under the Apache License, Version 2.0 (the
           "License"); you may not use this file except in compliance
           with the License.  You may obtain a copy of the License at
           .
             http://www.apache.org/licenses/LICENSE-2.0
           .
           Unless required by applicable law or agreed to in writing,
           software distributed under the License is distributed on an
           "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
           KIND, either express or implied.  See the License for the
           specific language governing permissions and limitations
           under the License.

The [Free Software Foundation](http://www.fsf.org/) 
[considers](http://www.fsf.org/licensing/licenses/#apache2) the [Apache
License, Version 2.0](http://www.apache.org/licenses/LICENSE-2.0) to be a
[free software](http://www.gnu.org/philosophy/free-sw.html) license,
[compatible](http://www.gnu.org/licenses/license-list.html#GPLCompatibleLicenses) with
version 3 of the [GPL](http://www.fsf.org/licensing/licenses/gpl.html). The
[Software Freedom Law Center](http://www.softwarefreedom.org/) provides
[practical
advice](http://www.softwarefreedom.org/resources/2007/gpl-non-gpl-collaboration.html)
for developers about including permissively licensed source.

Apache 2 software can therefore be included in GPLv3 projects, because the
GPLv3 license accepts our software into GPLv3 works. However, GPLv3
software cannot be included in Apache projects. The licenses are
incompatible in one direction only, and it is a result of ASF's licensing
philosophy and the GPLv3 authors' interpretation of copyright law.

This licensing incompatibility applies *only* when some Apache project
software becomes a derivative work of some GPLv3 software, because then the
Apache software would have to be distributed under GPLv3. This would be
incompatible with ASF's requirement that all Apache software must be
distributed under the Apache License 2.0.

We avoid GPLv3 software because merely linking to it is considered by the
GPLv3 authors to create a derivative work. We want to honor their license.
Unless GPLv3 licensors relax this interpretation of their own license
regarding linking, our licensing philosophies are fundamentally
incompatible. This is an identical issue for both GPLv2 and GPLv3.

Despite our best efforts, the FSF has never considered the Apache License
to be compatible with GPL version 2, citing the patent termination and
indemnification provisions as restrictions not present in the older GPL
license. The Apache Software Foundation believes that you should always try
to obey the constraints expressed by the copyright holder when
redistributing their work.

The Apache Software Foundation is a nonprofit organization that is
dedicated to open source software development. We never knowingly
incorporate patented technology in our own products unless such technology
has been offered free for everyone.

Please note that this discussion only benefits third parties that produce
GPL-covered products. The Apache Software Foundation does not allow its own
projects to distribute software under licenses more restrictive than the
Apache License, and the Free Software Foundation does not distribute
software under the Apache License.

The ASF will not dual-license our software because such licenses make it
impossible to determine the conditions under which we have agreed to
collaborate on a collective product, and are thus contrary to the Apache
spirit of open, collaborative development among individuals, industry, and
nonprofit organizations.