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From Owen O'Malley <omal...@apache.org>
Subject Re: 0.20.2 HDFS incompatible with 0.20.1
Date Tue, 05 Jan 2010 22:06:33 GMT

On Jan 5, 2010, at 11:29 AM, Todd Lipcon wrote:

> 1) Although we certainly do not guarantee wire compatibility between  
> minor
> versions (0.20 -> 0.21) have we previously implied wire compatibility
> between bugfix releases?

Correction. Pre-1.0, the 0.N to 0.N+1 is a major upgrade. After 1.0,  
1.N to 1.N+1 is a minor. In both cases, X.Y.z to X.Y.z+1 is a patch  

I thought we had it documented somewhere, but can't find it. There is  
some discussion of compatibility in HADOOP-5071 that should be pulled  
out into a wiki page.

The standing rules are that you don't incompatibly break APIs or wire  
protocols in patch release. So, this patch violates the rule and  
should have had a vote called before it was applied to branch-0.20.  
(And arguably branch-0.21, although since it hasn't been released, it  
isn't nearly the same level or problem.

> 2) Is the above something we *should* be guaranteeing already?

Patch releases:
   1. Must be backwards API compatible without a client recompile.
   2. Must be on the wire compatible.

Exceptions require a vote of the committers. We should also put a  
notice of any exceptions at the top of the release notes.

> 4) If the above are problems, would it be worth considering a patch  
> for
> branch-20 that provides a client that is compatible with either,  
> based on
> the datanode protocol version number of the server? It seems like a  
> bit of
> scary complexity, but wanted to throw it out there.

I would like Hairong to consider if she could fix the issue in 0.20  
without the incompatible change. If it can not be done (or no one  
wants to do the work), we should vote whether the change should be made.

-- Owen

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