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From "J. Rottinghuis" <jrottingh...@gmail.com>
Subject Re: symlink support in Hadoop 2 GA
Date Thu, 19 Sep 2013 00:11:25 GMT
However painful protobuf version changes are at build time for Hadoop
developers, at runtime with multiple clusters and many Hadoop users this is
a total nightmare.
Even upgrading clusters from one protobuf version to the next is going to
be very difficult. The same users will run jobs on, and/or read&write to
multiple clusters. That means that they will have to fork their code, run
multiple instances? Or in the very least they have to do an update to their
applications. All in sync with Hadoop cluster changes. And these are not
doable in a rolling fashion.
All Hadoop and HBase clusters will all upgrade at the same time, or we'll
have to have our users fork / roll multiple versions ?
My point is that these things are much harder than just fix the (Jenkins)
build and we're done. These changes are massively disruptive.

There is a similar situation with symlinks. Having an API that lets users
create symlinks is very problematic. Some users create symlinks and as Eli
pointed out, somebody else (or automated process) tries to copy to / from
another (Hadoop 1.x?) cluster over hftp. What will happen ?
Having an API that people should not use is also a nightmare. We
experienced this with append. For a while it was there, but users were "not
allowed to use it" (or else there were large #'s of corrupt blocks). If
there is an API to create a symlink, then some of our users are going to
use it and others are going to trip over those symlinks. We already know
that Pig does not work with symlinks yet, and as Steve pointed out, there
is tons of other code out there that assumes that !isDir() means isFile().

I like symlink functionality, but in our migration to Hadoop 2.x this is a
total distraction. If the APIs stay in 2.2 GA we'll have to choose to:
a) Not uprev until symlink support is figured out up and down the stack,
and we've been able to migrate all our 1.x (equivalent) clusters to 2.x
(equivalent). Or
b) rip out the API altogether. Or
c) change the implementation to throw an UnsupportedOperationException
I'm not sure yet which of these I like least.



On Wed, Sep 18, 2013 at 9:48 AM, Arun C Murthy <acm@hortonworks.com> wrote:

> On Sep 16, 2013, at 6:49 PM, Andrew Wang <andrew.wang@cloudera.com> wrote:
> > Hi all,
> >
> > I wanted to broadcast plans for putting the FileSystem symlinks work
> > (HADOOP-8040) into branch-2.1 for the pending Hadoop 2 GA release. I
> think
> > it's pretty important we get it in since it's not a compatible change; if
> > it misses the GA train, we're not going to have symlinks until the next
> > major release.
> Just catching up, is this an incompatible change, or not? The above reads
> 'not an incompatible change'.
> Arun
> >
> > However, we're still dealing with ongoing issues revealed via testing.
> > There's user-code out there that only handles files and directories and
> > will barf when given a symlink (perhaps a dangling one!). See HADOOP-9912
> > for a nice example where globStatus returning symlinks broke Pig; some of
> > us had a conference call to talk it through, and one definite conclusion
> > was that this wasn't solvable in a generally compatible manner.
> >
> > There are also still some gaps in symlink support right now. For example,
> > the more esoteric FileSystems like WebHDFS, HttpFS, and HFTP need symlink
> > resolution, and tooling like the FsShell and Distcp still need to be
> > updated as well.
> >
> > So, there's definitely work to be done, but there are a lot of users
> > interested in the feature, and symlinks really should be in GA. Would
> > appreciate any thoughts/input on the matter.
> >
> > Thanks,
> > Andrew
> --
> Arun C. Murthy
> Hortonworks Inc.
> http://hortonworks.com/
> --
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