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https://issues.apache.org/jira/browse/HBASE-4050?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13411693#comment-13411693
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Alex Baranau commented on HBASE-4050:
-------------------------------------
So, about the shim. Haven't done that kind of thing before, so please correct
me where/if I'm wrong.
Background:
There are quite a lot (>10) classes we want to use in HBase code which are
different in Hadoop 1.0+ and 2.0+ from metrics2 framework.
There are two options basically.
*1. Create shim classes for every class we want to use from Hadoop metrics2*
and which are different in Hadoop 1.0+ and 2.0+ codebase.
Shim class is basically consists of:
* interface (may be abstract class) similar (or exact same, depending on which
methods we want to use from it)
* multiple implementations of this interface, each uses reflection to
dynamically load the class from hadoop (based on which hadoop version we should
use) and propagate methods invocation to it
_Pros_: no extra/external jars to be loaded, all code is in main HBase maven
module
_Cons_: a lot of code which uses reflection (i.e. ugly code, hard to maintain,
+ bad performance)
*2. Create shim classes for "central/factory" classes*, *add only interfaces
for classes which we want to use from Hadoop metrics2* and which are different
in Hadoop 1.0+ and 2.0+ codebase, *extract actual implementation of these
interfaces into shim jars* and load them depending on which Hadoop version we
need to use.
In other words:
* add _only_ interfaces in HBase codebase (in new metrics2 package) for classes
of metrics2 framework we want to use
* create shim classes for "central/factory" classes, such as MetricsRegistry:
interface + code to load the actual implementation by classname
* create two shim jars (e.g. "metrics2-hadoop-1.0-shim.jar" and
"metrics2-hadoop-2.0-shim.jar") each with implementation of the interfaces
above using classes from the corresponded hadoop version
* add ShimLoader class which, at runtime finds out which hadoop version is used
and which shim jar should be loaded, and loads the correspondent jar
_Pros_: minimum reflection (just for initializing "central/factory" classes,
once)
_Cons_: needs loading jars (not sure this is a con), probably we'll have to add
separate maven modules to develop two shim jars
> Update HBase metrics framework to metrics2 framework
> ----------------------------------------------------
>
> Key: HBASE-4050
> URL: https://issues.apache.org/jira/browse/HBASE-4050
> Project: HBase
> Issue Type: New Feature
> Components: metrics
> Affects Versions: 0.90.4
> Environment: Java 6
> Reporter: Eric Yang
> Assignee: Alex Baranau
> Priority: Critical
> Fix For: 0.96.0
>
> Attachments: 4050-metrics-v2.patch, 4050-metrics-v3.patch,
> HBASE-4050.patch
>
>
> Metrics Framework has been marked deprecated in Hadoop 0.20.203+ and 0.22+,
> and it might get removed in future Hadoop release. Hence, HBase needs to
> revise the dependency of MetricsContext to use Metrics2 framework.
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