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https://issues.apache.org/jira/browse/HBASE-21062?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16581607#comment-16581607
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Mingliang Liu commented on HBASE-21062:
---------------------------------------
The existing code implicitly employs the fact that {{defaultProvider}} is
having {{AsyncFSWALProvider}} class value. It makes sense as the enum
{{defaultProvider}} can not have a different value at runtime unless we change
code. This implementation pattern is not ideal to me either - because when we
change the {{defaultProvider}} we may forget to change {{getProviderClass}} -
but it's tolerable. Meanwhile, the newly added test in the patch does not
actually override the {{defaultProvider}} value and {{getDefaultProvider}} is
not used anywhere. So the test will pass anyway: w/ or w/o the fix in
{{getProviderClass}}, w/ or w/o the {{getDefaultProvider}}. Correct me if I'm
wrong.
Another (orthogonal?) problem in {{getProviderClass}} is that, the try-catch
scope is not implemented well. The reason is that, in the
{{catch(IllegalArgumentException exception)\{...\}}} clause, we end up with
returning the {{AsyncFSWALProvider}}. However, this time we would skip the
process that load {{AsyncFSWALProvider}} and then falls back to
{{FSHLogProvider}}.
Last, for any error case, it merits to have logging message.
Overall, a fix in my mind is like - NOT TESTED:
{code:java}
@VisibleForTesting
public Class<? extends WALProvider> getProviderClass(String key, String
defaultValue) {
Providers provider;
try {
provider = Providers.valueOf(conf.get(key, defaultValue));
} catch (IllegalArgumentException exception) {
// Fall back to them specifying a class name
// Note that the passed default class shouldn't actually be used, since
the above only fails
// when there is a config value present.
LOG.warn("Unrecognized WALProvider class. Falling back to default
AsyncFSWALProvider.");
provider = Providers.defaultProviderdr;
}
if (provider.clazz == AsyncFSWALProvider.class &&
!AsyncFSWALProvider.load()) {
// AsyncFSWAL has better performance in most cases, and also uses less
resources, we will try
// to use it if possible. But it deeply hacks into the internal of
DFSClient so will be easily
// broken when upgrading hadoop. If it is broken, then we fall back to
use FSHLog.
LOG.warn("Failed to load AsyncFSWALProvider. Falling back to
FSHLogProvider.");
return FSHLogProvider.class;
}
return provider.clazz;
}
{code}
Thanks!
> WALFactory has misleading notion of "default"
> ---------------------------------------------
>
> Key: HBASE-21062
> URL: https://issues.apache.org/jira/browse/HBASE-21062
> Project: HBase
> Issue Type: Bug
> Components: wal
> Reporter: Josh Elser
> Assignee: Josh Elser
> Priority: Major
> Fix For: 3.0.0, 2.0.2, 2.2.0, 2.1.1
>
> Attachments: HBASE-21062.001.branch-2.0.patch
>
>
> In WALFactory, there is an enum {{Providers}} which has a list of supported
> WALProvider implementations. In addition to list this, there is also a
> {{defaultProvider}} (which the Configuration defaults to), that is meant to
> be our "advertised" default WALProvider.
> However, the implementation of {{getProviderClass}} in WALFactory doesn't
> actually adhere to the value of this enum, instead *always* returning
> AsyncFSWal if it can be loaded.
> Having the default value in the enum but then overriding it in the
> implementation of {{getProviderClass}} is silly and misleading.
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