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https://issues.apache.org/jira/browse/HBASE-22149?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16808753#comment-16808753
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Sean Mackrory commented on HBASE-22149:
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So the rename will not be atomic in the sense that it's an O(1) metadata-only
operation, but it will be atomic in the sense that it will have either happened
or not happened, and no other calls to the FileSystem will see partial results
because of the locking. This guarantee only holds true if everything accessing
the storage is using this FileSystem implementation, but as far as I know that
shouldn't be an issue for HBase use cases.
{quote}Lock release must be bullet proof{quote}
Good catch, thanks - that is indeed the point of the AutoLock class in the
first place :) I've also moved the lock releases in RemoteIterator.hasNext()
under finally.
A couple of other things on my immediate to-do list: I'd like to add FS
contract tests to get better coverage of the rest of the FS API in addition to
the 2 main use cases I'm testing. Certainly more tests needed in general. I'd
also like to add a way to mock S3 so you don't need AWS credentials to run the
tests. Shouldn't be too tricky - the problems my tests currently reproduce
depend on client behavior, and not subtle server-side implementation details.
> HBOSS: A FileSystem implementation to provide HBase's required semantics
> ------------------------------------------------------------------------
>
> Key: HBASE-22149
> URL: https://issues.apache.org/jira/browse/HBASE-22149
> Project: HBase
> Issue Type: New Feature
> Components: Filesystem Integration
> Reporter: Sean Mackrory
> Assignee: Sean Mackrory
> Priority: Critical
> Attachments: HBASE-22149-hadoop.patch
>
>
> (Have been using the name HBOSS for HBase / Object Store Semantics)
> I've had some thoughts about how to solve the problem of running HBase on
> object stores. There has been some thought in the past about adding the
> required semantics to S3Guard, but I have some concerns about that. First,
> it's mixing complicated solutions to different problems (bridging the gap
> between a flat namespace and a hierarchical namespace vs. solving
> inconsistency). Second, it's S3-specific, whereas other objects stores could
> use virtually identical solutions. And third, we can't do things like atomic
> renames in a true sense. There would have to be some trade-offs specific to
> HBase's needs and it's better if we can solve that in an HBase-specific
> module without mixing all that logic in with the rest of S3A.
> Ideas to solve this above the FileSystem layer have been proposed and
> considered (HBASE-20431, for one), and maybe that's the right way forward
> long-term, but it certainly seems to be a hard problem and hasn't been done
> yet. But I don't know enough of all the internal considerations to make much
> of a judgment on that myself.
> I propose a FileSystem implementation that wraps another FileSystem instance
> and provides locking of FileSystem operations to ensure correct semantics.
> Locking could quite possibly be done on the same ZooKeeper ensemble as an
> HBase cluster already uses (I'm sure there are some performance
> considerations here that deserve more attention). I've put together a
> proof-of-concept on which I've tested some aspects of atomic renames and
> atomic file creates. Both of these tests fail reliably on a naked s3a
> instance. I've also done a small YCSB run against a small cluster to sanity
> check other functionality and was successful. I will post the patch, and my
> laundry list of things that still need work. The WAL is still placed on HDFS,
> but the HBase root directory is otherwise on S3.
> Note that my prototype is built on Hadoop's source tree right now. That's
> purely for my convenience in putting it together quickly, as that's where I
> mostly work. I actually think long-term, if this is accepted as a good
> solution, it makes sense to live in HBase (or it's own repository). It only
> depends on stable, public APIs in Hadoop and is targeted entirely at HBase's
> needs, so it should be able to iterate on the HBase community's terms alone.
> Another idea [[email protected]] proposed to me is that of an inode-based
> FileSystem that keeps hierarchical metadata in a more appropriate store that
> would allow the required transactions (maybe a special table in HBase could
> provide that store itself for other tables), and stores the underlying files
> with unique identifiers on S3. This allows renames to actually become fast
> instead of just large atomic operations. It does however place a strong
> dependency on the metadata store. I have not explored this idea much. My
> current proof-of-concept has been pleasantly simple, so I think it's the
> right solution unless it proves unable to provide the required performance
> characteristics.
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