Hdfs之DistributedCache

在利用mapred分析大数据时，程序中常常会引入hdfs上一些“辅助数据”，通常的做法在mr的启动前加载这些数据形成cache以提高效率，而mr在大并发下去访问hdfs的同一文件可能存在性能瓶颈，DistributedCache可以帮助解决。

       DistributedCache</code> is a facility provided by the Map-Reduce framework to cache files (text, archives, jars etc.) needed by applications.

添加cacheFile：

                    DistributedCache.addFileToClassPath(new Path(args[2]), conf);

cacheFile通过hadoop命令参数提供，args[2]是/group/tlog/resources/ipAppMapping.txt

在mapper或reducer中使用：

        @Override
        protected void setup(Context context) throws IOException, InterruptedException {
            Path[] localArchives = DistributedCache.getFileClassPaths(context.getConfiguration());
            FileSystem fs = FileSystem.get(context.getConfiguration());
            InputStream in;
            if (localArchives == null) {
                System.out.println("Load refources file form system class loader.");
                in = ClassLoader.getSystemResourceAsStream("ipAppMapping.txt");
            } else {
                in = fs.open(localArchives[0]);
            }
            if (in == null) {
                throw new RuntimeException("Resource file is not exist.");
            }
            BufferedReader reader = new BufferedReader(new InputStreamReader(in));
            //加载辅助数据
            reader.close();
        }