Arpit Agarwal created HDFS-5751:
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Summary: Remove the FsDatasetSpi and FsVolumeImpl interfaces
Key: HDFS-5751
URL: https://issues.apache.org/jira/browse/HDFS-5751
Project: Hadoop HDFS
Issue Type: Improvement
Components: datanode, test
Affects Versions: 3.0.0
Reporter: Arpit Agarwal
The in-memory block map and disk interface portions of the DataNode have been
abstracted out into an {{FsDatasetpSpi}} interface, which further uses
{{FsVolumeSpi}} to represent individual volumes.
The abstraction is useful as it allows DataNode tests to use a
{{SimulatedFSDataset}} which does not write any data to disk. Instead it just
stores block metadata in memory and returns blank data for all reads. This is
useful for both unit testing and for simulating arbitrarily large datanodes
without having to provision real disk capacity.
A 'real' DataNode uses {{FsDataSetImpl}}. Both {{FsDatasetImpl}} and
{{SimulatedFSDataset}} implement {{FsDatasetSpi}}.
However there are a few problems with this approach:
# Using the factory class significantly complicates the code flow for the
common case. This makes the code harder to understand and debug.
# There is additional burden of maintaining two different dataset
implementations.
# Fidelity between the two implementations is poor.
Instead we can get eliminate the SPIs and just hide the disk read/write
routines with a dependency injection framework like Google Guice.
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