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https://issues.apache.org/jira/browse/HBASE-6009?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13277810#comment-13277810
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David S. Wang commented on HBASE-6009:
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I looked at the total size field option for this, starting from the write case.
To calculate total size written, you have to know how many bytes were written
for each write() call on ClusterStatus, including any objects contained inside
it.
The DataOutput interface for Writables doesn't have a way to return how many
bytes were written to the stream. This is not a problem for primitive types as
we can figure that out trivially. Even for somewhat more complicated
situations such as modified UTF-8s written with the writeUTF call, the number
of written bytes for a String can at least be calculated based on the formula
for modified UTF-8 conversion.
However, for calls to Object's write functions (e.g. for HRegionLoad), this
becomes somewhat more problematic as there is no obvious answer as to how many
bytes were written. We could use reflection to grab the fields, but then there
is no guarantee that all of the fields of the Object are actually written to
the stream when write() is called. So you'd have to introduce some hardcoded
way of knowing how much was written for each Object, which is Bad.
I'm tempted to say that we shouldn't add any more fields to ClusterStatus or
similar APIs until 0.96, when hopefully our wire compatibility efforts will
kick in and we can do this in a compatible way without having to jump through
hoops.
> Changes for HBASE-5209 are technically incompatible
> ---------------------------------------------------
>
> Key: HBASE-6009
> URL: https://issues.apache.org/jira/browse/HBASE-6009
> Project: HBase
> Issue Type: Bug
> Components: master
> Affects Versions: 0.92.1, 0.94.0
> Reporter: David S. Wang
>
> The additions to add backup masters to ClusterStatus are technically
> incompatible between clients and servers. Older clients will basically not
> read the extra bits that the newer server pushes for the backup masters, thus
> screwing up the serialization for the next blob in the pipe.
> For the Writable, we can add a total size field for ClusterStatus at the
> beginning, or we can have start and end markers. I can make a patch for
> either approach; interested in whatever folks have to suggest. Would be good
> to get this in soon to limit the damage to 0.92.1 (don't know if we can get
> this in in time for 0.94.0).
> Either change will make us forward-compatible starting with when the change
> goes in, but will not fix the backwards incompatibility, which we will have
> to mark with a release note as there have already been releases with this
> change.
> Hopefully we can do this in a cleaner way when wire compat rolls around in
> 0.96.
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