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From Chris Nauroth <cnaur...@hortonworks.com>
Subject Re: [DISCUSS] Clarification on Compatibility Policy: Upgraded Client + Old Server
Date Mon, 24 Mar 2014 21:03:17 GMT
Adding back all *-dev lists to make sure everyone is covered.

Chris Nauroth
Hortonworks
http://hortonworks.com/



On Mon, Mar 24, 2014 at 2:02 PM, Chris Nauroth <cnauroth@hortonworks.com>wrote:

> Thank you, everyone, for the discussion.  There is general agreement, so I
> have filed HADOOP-10423 with a patch to update the compatibility
> documentation.
>
> Chris Nauroth
> Hortonworks
> http://hortonworks.com/
>
>
>
> On Thu, Mar 20, 2014 at 11:24 AM, Colin McCabe <cmccabe@alumni.cmu.edu>wrote:
>
>> +1 for making this guarantee explicit.
>>
>> It also definitely seems like a good idea to test mixed versions in
>> bigtop.
>>
>> HDFS is not immune to "new client, old server" scenarios because the HDFS
>> client gets bundled into a lot of places.
>>
>> Colin
>> On Mar 20, 2014 10:55 AM, "Chris Nauroth" <cnauroth@hortonworks.com>
>> wrote:
>>
>> > Our use of protobuf helps mitigate a lot of compatibility concerns, but
>> > there still can be situations that require careful coding on our part.
>> >  When adding a new field to a protobuf message, the client might need
>> to do
>> > a null check, even if the server-side implementation in the new version
>> > always populates the field.  When adding a whole new RPC endpoint, the
>> > client might need to consider the possibility that the RPC endpoint
>> isn't
>> > there on an old server, and degrade gracefully after the RPC fails.  The
>> > original issue in MAPREDUCE-4052 concerned the script commands passed
>> in a
>> > YARN container submission, where protobuf doesn't provide any validation
>> > beyond the fact that they're strings.
>> >
>> > Forward compatibility is harder than backward compatibility, and
>> testing is
>> > a big challenge.  Our test suites in the Hadoop repo don't cover this.
>> >  Does anyone know if anything in Bigtop tries to run with mixed
>> versions?
>> >
>> > I agree that we need to make it clear in the language that upgrading
>> client
>> > alone is insufficient to get access to new server-side features,
>> including
>> > new YARN APIs.  Thanks for the suggestions, Steve.
>> >
>> > Chris Nauroth
>> > Hortonworks
>> > http://hortonworks.com/
>> >
>> >
>> >
>> > On Thu, Mar 20, 2014 at 5:53 AM, Steve Loughran <stevel@hortonworks.com
>> > >wrote:
>> >
>> > > I'm clearly supportive of this, though of course the testing costs
>> needed
>> > > to back up the assertion make it more expensive than just a statement.
>> > >
>> > > Two issues
>> > >
>> > > -we'd need to make clear that new cluster features that a client can
>> > invoke
>> > > won't be available. You can't expect snapshot or symlink support
>> running
>> > > against a -2.2.0 cluster, even if the client supports it.
>> > >
>> > > -in YARN, there are no guarantees that an app compiled against later
>> YARN
>> > > APIs will work in old clusters. Because YARN apps upload themselves to
>> > the
>> > > server, and run with their hadoop, hdfs & yarn libraries. We have to
>> do a
>> > > bit of introspection in our code already to support this situation.
>> The
>> > > compatibility doc would need to be clear on that too: "YARN apps that
>> use
>> > > new APIs (including new fields in datastructures) can expect link
>> > > exceptions"
>> > >
>> > >
>> > >
>> > >
>> > >
>> > > On 20 March 2014 04:25, Vinayakumar B <vinayakumar.b@huawei.com>
>> wrote:
>> > >
>> > > > +1, I agree with your point Chris. It depends on the client
>> application
>> > > > how they using the hdfs jars in their classpath.
>> > > >
>> > > > As implementation already supports the compatibility (through
>> > protobuf),
>> > > > No extra code changes required to support new Client + old server.
>> > > >
>> > > > I feel it will be good to explicitly mention about the
>> compatibility of
>> > > > existing APIs in both versions.
>> > > >
>> > > > Anyway this is not applicable for the new APIs in latest client and
>> > this
>> > > > is understood. We can make it explicit in the document though.
>> > > >
>> > > >
>> > > > Regards,
>> > > > Vinayakumar B
>> > > >
>> > > > -----Original Message-----
>> > > > From: Chris Nauroth [mailto:cnauroth@hortonworks.com]
>> > > > Sent: 20 March 2014 05:36
>> > > > To: common-dev@hadoop.apache.org
>> > > > Cc: mapreduce-dev@hadoop.apache.org; hdfs-dev@hadoop.apache.org;
>> > > > yarn-dev@hadoop.apache.org
>> > > > Subject: Re: [DISCUSS] Clarification on Compatibility Policy:
>> Upgraded
>> > > > Client + Old Server
>> > > >
>> > > > I think this kind of compatibility issue still could surface for
>> HDFS,
>> > > > particularly for custom applications (i.e. something not executed
>> via
>> > > > "hadoop jar" on a cluster node, where the client classes ought to
be
>> > > > injected into the classpath automatically).  Running DistCP between
>> 2
>> > > > clusters of different versions could result in a 2.4.0 client
>> calling a
>> > > > 2.3.0 NameNode.  Someone could potentially pick up the 2.4.0 WebHDFS
>> > > > client as a dependency and try to use it to make HTTP calls to a
>> 2.3.0
>> > > HDFS
>> > > > cluster.
>> > > >
>> > > > Chris Nauroth
>> > > > Hortonworks
>> > > > http://hortonworks.com/
>> > > >
>> > > >
>> > > >
>> > > > On Wed, Mar 19, 2014 at 4:28 PM, Vinod Kumar Vavilapalli <
>> > > > vinodkv@apache.org
>> > > > > wrote:
>> > > >
>> > > > > It makes sense only for YARN today where we separated out the
>> > clients.
>> > > > > HDFS is still a monolithic jar so this compatibility issue is
>> kind of
>> > > > > invalid there.
>> > > > >
>> > > > > +vinod
>> > > > >
>> > > > > On Mar 19, 2014, at 1:59 PM, Chris Nauroth <
>> cnauroth@hortonworks.com
>> > >
>> > > > > wrote:
>> > > > >
>> > > > > > 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/
>> > > > > >
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