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| <h5>YARN Service</h5> |
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| <h5>Hadoop Compatible File Systems</h5> |
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| <a href="../../hadoop-yarn/hadoop-yarn-common/yarn-default.xml">yarn-default.xml</a> |
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| <h1>C API libhdfs</h1> |
| <ul> |
| <li><a href="#Overview">Overview</a></li> |
| <li><a href="#The_APIs">The APIs</a></li> |
| <li><a href="#A_Sample_Program">A Sample Program</a></li> |
| <li><a href="#How_To_Link_With_The_Library">How To Link With The Library</a></li> |
| <li><a href="#Common_Problems">Common Problems</a></li> |
| <li><a href="#Thread_Safe">Thread Safe</a></li></ul> |
| |
| <div class="section"> |
| <h2><a name="Overview"></a>Overview</h2> |
| <p>libhdfs is a JNI based C API for Hadoop’s Distributed File System (HDFS). It provides C APIs to a subset of the HDFS APIs to manipulate HDFS files and the filesystem. libhdfs is part of the Hadoop distribution and comes pre-compiled in <tt>$HADOOP_HDFS_HOME/lib/native/libhdfs.so</tt> . libhdfs is compatible with Windows and can be built on Windows by running <tt>mvn compile</tt> within the <tt>hadoop-hdfs-project/hadoop-hdfs</tt> directory of the source tree.</p></div> |
| <div class="section"> |
| <h2><a name="The_APIs"></a>The APIs</h2> |
| <p>The libhdfs APIs are a subset of the <a href="../../api/org/apache/hadoop/fs/FileSystem.html">Hadoop FileSystem APIs</a>.</p> |
| <p>The header file for libhdfs describes each API in detail and is available in <tt>$HADOOP_HDFS_HOME/include/hdfs.h</tt>.</p></div> |
| <div class="section"> |
| <h2><a name="A_Sample_Program"></a>A Sample Program</h2> |
| |
| <div> |
| <div> |
| <pre class="source">#include "hdfs.h" |
| |
| int main(int argc, char **argv) { |
| |
| hdfsFS fs = hdfsConnect("default", 0); |
| const char* writePath = "/tmp/testfile.txt"; |
| hdfsFile writeFile = hdfsOpenFile(fs, writePath, O_WRONLY |O_CREAT, 0, 0, 0); |
| if(!writeFile) { |
| fprintf(stderr, "Failed to open %s for writing!\n", writePath); |
| exit(-1); |
| } |
| char* buffer = "Hello, World!"; |
| tSize num_written_bytes = hdfsWrite(fs, writeFile, (void*)buffer, strlen(buffer)+1); |
| if (hdfsFlush(fs, writeFile)) { |
| fprintf(stderr, "Failed to 'flush' %s\n", writePath); |
| exit(-1); |
| } |
| hdfsCloseFile(fs, writeFile); |
| } |
| </pre></div></div> |
| </div> |
| <div class="section"> |
| <h2><a name="How_To_Link_With_The_Library"></a>How To Link With The Library</h2> |
| <p>See the CMake file for <tt>test_libhdfs_ops.c</tt> in the libhdfs source directory (<tt>hadoop-hdfs-project/hadoop-hdfs/src/CMakeLists.txt</tt>) or something like: <tt>gcc above_sample.c -I$HADOOP_HDFS_HOME/include -L$HADOOP_HDFS_HOME/lib/native -lhdfs -o above_sample</tt></p></div> |
| <div class="section"> |
| <h2><a name="Common_Problems"></a>Common Problems</h2> |
| <p>The most common problem is the <tt>CLASSPATH</tt> is not set properly when calling a program that uses libhdfs. Make sure you set it to all the Hadoop jars needed to run Hadoop itself as well as the right configuration directory containing <tt>hdfs-site.xml</tt>. Wildcard entries in the <tt>CLASSPATH</tt> are now supported by libhdfs.</p></div> |
| <div class="section"> |
| <h2><a name="Thread_Safe"></a>Thread Safe</h2> |
| <p>libdhfs is thread safe.</p> |
| <ul> |
| |
| <li> |
| |
| <p>Concurrency and Hadoop FS “handles”</p> |
| <p>The Hadoop FS implementation includes an FS handle cache which caches based on the URI of the namenode along with the user connecting. So, all calls to <tt>hdfsConnect</tt> will return the same handle but calls to <tt>hdfsConnectAsUser</tt> with different users will return different handles. But, since HDFS client handles are completely thread safe, this has no bearing on concurrency.</p> |
| </li> |
| <li> |
| |
| <p>Concurrency and libhdfs/JNI</p> |
| <p>The libhdfs calls to JNI should always be creating thread local storage, so (in theory), libhdfs should be as thread safe as the underlying calls to the Hadoop FS.</p> |
| </li> |
| </ul></div> |
| </div> |
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