问题描述

我正在尝试找到登录 MapReduce 作业的最佳方法.我在我的其他 Java 应用程序中使用带有 log4j appender 的 slf4j，但是由于 MapReduce 作业在集群中以分布式方式运行，我不知道应该在哪里设置日志文件位置，因为它是一个访问受限的共享集群特权.

I'm trying to find the best approach for logging in MapReduce jobs. I'm using slf4j with log4j appender as in my other Java applications, but since MapReduce job runs in a distributed manner across the cluster I don't know where should I set the log file location, since it is a shared cluster with limited access privileges.

是否有任何记录 MapReduce 作业的标准做法，以便您可以在作业完成后轻松查看整个集群的日志?

Is there any standard practices for logging in MapReduce jobs, so you can easily be able to look at the logs across the cluster after the job completes?

推荐答案

您可以使用 log4j，它是 hadoop 使用的默认日志框架.因此，从您的 MapReduce 应用程序中，您可以执行以下操作:

You could use log4j which is the default logging framework that hadoop uses. So, from your MapReduce application you could do something like this:

import org.apache.log4j.Logger;
// other imports omitted

public class SampleMapper extends Mapper<LongWritable, Text, Text, Text> {
    private Logger logger = Logger.getLogger(SampleMapper.class);

    @Override
    protected void setup(Context context) {
        logger.info("Initializing NoSQL Connection.")
        try {
            // logic for connecting to NoSQL - ommitted
        } catch (Exception ex) {
            logger.error(ex.getMessage());
        }
    }

    @Override
    protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
        // mapper code ommitted
    }
}        

此示例代码将使用 log4j 记录器将事件记录到继承的 Mapper 记录器.所有日志事件都将记录到它们各自的任务日志中.您可以从 JobTracker(MRv1)/ResourceManager(MRv2) 网页访问任务日志.

This sample code will user log4j logger to log events to the inherited Mapper logger. All the log events will be logged to their respective task log's. You could visit the task logs from either JobTracker(MRv1)/ResourceManager(MRv2) webpage.

如果您使用的是 yarn，您可以使用以下命令从命令行访问应用程序日志:

If you are using yarn you could access the application logs from command line using the following command:

yarn logs -applicationId <application_id>

如果您使用的是 mapreduce v1，则无法从命令行进行单点访问；因此，您必须登录每个 TaskTracker 并查看配置的路径，通常是 ${hadoop.log.dir} 中指定的 /var/log/hadoop/userlogs/attempt_<job_id>/syslog/userlogs 包含 log4j 输出.

While if you are using mapreduce v1, there is no single point of access from command line; hence you have to log into each TaskTracker and look in the configured path generally /var/log/hadoop/userlogs/attempt_<job_id>/syslog specified in ${hadoop.log.dir}/userlogs contains log4j output.

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