I'd like to discuss clarification of part of our compatibility policy. Here is a link to the compatibility documentation for release 2.3.0:
http://hadoop.apache.org/docs/r2.3.0/hadoop-project-dist/hadoop-common/Compatibility.html#Wire_compatibility For convenience, here are the specific lines in question: Client-Server compatibility is required to allow users to continue using the old clients even after upgrading the server (cluster) to a later version (or vice versa). For example, a Hadoop 2.1.0 client talking to a Hadoop 2.3.0 cluster. Client-Server compatibility is also required to allow upgrading individual components without upgrading others. For example, upgrade HDFS from version 2.1.0 to 2.2.0 without upgrading MapReduce. Server-Server compatibility is required to allow mixed versions within an active cluster so the cluster may be upgraded without downtime in a rolling fashion. Notice that there is no specific mention of upgrading the client ahead of the server. (There is no clause for "upgraded client + old server".) Based on my experience, this is a valid use case when a user wants to pick up a client-side bug fix ahead of the cluster administrator's upgrade schedule. Is it our policy to maintain client compatibility with old clusters within the same major release? I think many of us have assumed that the answer is yes and coded our new features accordingly, but it isn't made explicit in the documentation. Do we all agree that the answer is yes, or is it possibly up for debate depending on the change in question? In RFC 2119 lingo, is it a MUST or a SHOULD? Either way, I'd like to update the policy text to make our decision clear. After we have consensus, I can volunteer to file an issue and patch the text of the policy. This discussion started initially in MAPREDUCE-4052, which involved changing our scripting syntax for MapReduce YARN container submissions. We settled the question there by gating the syntax change behind a configuration option. By default, it will continue using the existing syntax currently understood by the pre-2.4.0 NodeManager, thus preserving compatibility. We wanted to open the policy question for wider discussion though. Thanks, everyone. Chris Nauroth Hortonworks http://hortonworks.com/ -- CONFIDENTIALITY NOTICE NOTICE: This message is intended for the use of the individual or entity to which it is addressed and may contain information that is confidential, privileged and exempt from disclosure under applicable law. If the reader of this message is not the intended recipient, you are hereby notified that any printing, copying, dissemination, distribution, disclosure or forwarding of this communication is strictly prohibited. If you have received this communication in error, please contact the sender immediately and delete it from your system. Thank You.
