Gary Helmling created HBASE-7460:
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Summary: Cleanup client connection layers
Key: HBASE-7460
URL: https://issues.apache.org/jira/browse/HBASE-7460
Project: HBase
Issue Type: Improvement
Components: Client, IPC/RPC
Reporter: Gary Helmling
This issue originated from a discussion over in HBASE-7442. We currently have
a broken abstraction with {{HBaseClient}}, where it is bound to a single
{{Configuration}} instance at time of construction, but then reused for all
connections to all clusters. This is combined with multiple, overlapping
layers of connection caching.
Going through this code, it seems like we have a lot of mismatch between the
higher layers and the lower layers, with too much abstraction in between. At
the lower layers, most of the {{ClientCache}} stuff seems completely unused. We
currently effectively have an {{HBaseClient}} singleton (for {{SecureClient}}
as well in 0.92/0.94) in the client code, as I don't see anything that calls
the constructor or {{RpcEngine.getProxy()}} versions with a non-default socket
factory. So a lot of the code around this seems like built up waste.
The fact that a single Configuration is fixed in the {{HBaseClient}} seems like
a broken abstraction as it currently stands. In addition to cluster ID, other
configuration parameters (max retries, retry sleep) are fixed at time of
construction. The more I look at the code, the more it looks like the
{{ClientCache}} and sharing the {{HBaseClient}} instance is an unnecessary
complication. Why cache the {{HBaseClient}} instances at all? In
{{HConnectionManager}}, we already have a mapping from {{Configuration}} to
{{HConnection}}. It seems to me like each {{HConnection(Implementation)}}
instance should have it's own {{HBaseClient}} instance, doing away with the
{{ClientCache}} mapping. This would keep each {{HBaseClient}} associated with a
single cluster/configuration and fix the current breakage from reusing the same
{{HBaseClient}} against different clusters.
We need a refactoring of some of the interactions of
{{HConnection(Implementation)}}, {{HBaseRPC/RpcEngine}}, and {{HBaseClient}}.
Off hand, we might want to expose a separate {{RpcEngine.getClient()}} method
that returns a new {{RpcClient}} interface (implemented by {{HBaseClient}}) and
move the {{RpcEngine.getProxy()}}/{{stopProxy()}} implementations into the
client. So all proxy invocations can go through the same client, without
requiring the static client cache. I haven't fully thought this through, so I
could be missing other important aspects. But that approach at least seems like
a step in the right direction for fixing the client abstractions.
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