[jira] [Created] (SINGA-97) SINGA-97 Add HDFS support

["Anh Dinh (JIRA)" <ji...@apache.org>]

Anh Dinh created SINGA-97:
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             Summary: SINGA-97 Add HDFS support 
                 Key: SINGA-97
                 URL: https://issues.apache.org/jira/browse/SINGA-97
             Project: Singa
          Issue Type: New Feature
            Reporter: Anh Dinh
            Assignee: Anh Dinh


This ticket implements HDFS Store for reading data from HDFS. It complements the existing CSV Store which reads data from CSV file. HDFS is the popular distributed file system with high (sequential) I/O throughputs, thus supporting it is necessary in order for SINGA to scale. 

The implementation will extend singa::io::Store class which is declared in `singa/io/store.h`. In particular, it will support the following I/O operations:

+ `bool Open(string& file, Mode mode)`
+ `bool Close()`
+ `bool Flush()`

+ `int Seek(int record_idx)`
+ `int Read(string *content)`
+ `int Write(string& content)`

HDFS usage in SINGA is different to that in standard MapReduce applications. Specifically, each SINGA worker may train on sequences of records which do not lie within block boundary, whereas in MapReduce  each Mapper process a number of complete blocks.  In MapReduce, the runtime engine may fetch and cache the entire block over the network, knowing that the block will be processed entirely. In SINGA, such pre-fetching and caching strategy will be sub-optimal because it wastes I/O and network bandwidth on data records which are not used. 

We defer I/O optimization to a future ticket. 

For implementation, we choose `libhdfs3` from Pivotal for HDFS implementation in C++. This library is built natively for C++, hence it is more optimized and easier to deploy than the original  `libhdfs` library that is shipped with Hadoop. Finally, we test the implementation in a distributed environment set up from a number of  Docker containers (see SINGA-11). 




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