Francesco Mari created OAK-4083:
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Summary: Simplify concurrency when loading data from the primary
Key: OAK-4083
URL: https://issues.apache.org/jira/browse/OAK-4083
Project: Jackrabbit Oak
Issue Type: Bug
Components: tarmk-standby
Reporter: Francesco Mari
Assignee: Francesco Mari
Fix For: 1.4
The cold standby client is overly complicated and potentially buggy when
segments are loaded from the primary.
In particular:
- when the {{StandbyClientHandler}} is removed from the pipeline, its
{{EventExecutorGroup}} is passed to the {{SegmentLoaderHandler}} to signal a
surrogate of the "channel close" event. It would be more sensible for
{{SegmentLoaderHandler}} to listen to the proper "channel close" event instead.
- after the {{SegmentLoaderHandler}} is added to the pipeline, the "channel
active" callback method is called directly using the {{ChannelHandlerContext}}
of the {{StandbyClientHandler}}. It would be better for
{{SegmentLoaderHandler}} to use a proper initialisation point like, in example,
the "handler added" callback method. The {{SegmentLoaderHandler}} should also
use its own {{ChannelHandlerContext}}, instead of owning one from another
handler.
- {{SegmentLoaderHandler}} saves and uses the {{ChannelHandlerContext}} of the
{{StandbyClientHandler}}. The only purpose for it is to use the
{{EventExecutorGroup}} of the {{StandbyClientHandler}} to issue "write
messages" events. The {{SegmentLoaderHandler}} should use Netty's event loop to
issue I/O operations, and instead use a different thread for business logic -
in this case, the synchronization process between primary and standby instances.
- The {{StandbyClientHandler}} is registered with its own
{{EventExecutorGroup}} with four threads. This is overkill, since the
synchronization process should run anyway in a separate thread (see below). The
{{StandbyClientHandler}} should use the default event loop.
- The {{SegmentLoaderHandler}} is registered with its own
{{EventExecutorGroup}} with four threads. This is overkill, since
request/response cycle between the standby and the primary instances is
synchronous. The {{SegmentLoaderHandler}} should instead use the default event
loop for I/O events and run another (single) thread to execute the
synchronization process.
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