我们正在运行 Cascading，其中一个 Sink Tap 被配置为存储在 Amazon S3 中，并且遇到了一些 FileAlreadyExistsException(请参阅 [1])。 这只是偶尔发生(大约 100 次有 1 次)并且不可重现。
深入研究级联编码器，我们发现 Hfs.deleteResource() 被 BaseFlow.deleteSinksIfNotUpdate() 调用(除其他外)。 顺便说一句，我们对沉默的 NPE 很感兴趣(评论是“hack to get around npe thrown when fs reaches root directory”)。
从那里开始，我们使用自己的 Tap 扩展了 Hfs Tap，以在 deleteResource() 方法(参见 [2])中添加更多操作，并使用直接调用 getFileSystem(conf).delete 的重试机制。
重试机制似乎带来了改进，但我们仍然有时会面临失败(见[3]中的示例):听起来HDFS返回isDeleted=true，但直接询问文件夹是否存在后，我们收到exists=true ，这不应该发生。当流成功时，日志还会随机显示 isDeleted true 或 false，这听起来像是返回值无关紧要或不可信。
任何人都可以通过这种行为带来自己的 S3 体验:“文件夹应该被删除，但实际上没有”？我们怀疑是 S3 问题，但它是否也存在于 Cascading 或 HDFS 中？
我们在 Hadoop Cloudera-cdh3u5 和 Cascading 2.0.1-wip-dev 上运行。
[1]
org.apache.hadoop.mapred.FileAlreadyExistsException: Output directory s3n://... already exists
at org.apache.hadoop.mapreduce.lib.output.FileOutputFormat.checkOutputSpecs(FileOutputFormat.java:132)
at com.twitter.elephantbird.mapred.output.DeprecatedOutputFormatWrapper.checkOutputSpecs(DeprecatedOutputFormatWrapper.java:75)
at org.apache.hadoop.mapred.JobClient$2.run(JobClient.java:923)
at org.apache.hadoop.mapred.JobClient$2.run(JobClient.java:882)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:396)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1278)
at org.apache.hadoop.mapred.JobClient.submitJobInternal(JobClient.java:882)
at org.apache.hadoop.mapred.JobClient.submitJob(JobClient.java:856)
at cascading.flow.hadoop.planner.HadoopFlowStepJob.internalNonBlockingStart(HadoopFlowStepJob.java:104)
at cascading.flow.planner.FlowStepJob.blockOnJob(FlowStepJob.java:174)
at cascading.flow.planner.FlowStepJob.start(FlowStepJob.java:137)
at cascading.flow.planner.FlowStepJob.call(FlowStepJob.java:122)
at cascading.flow.planner.FlowStepJob.call(FlowStepJob.java:42)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.j
[2]
@Override
public boolean deleteResource(JobConf conf) throws IOException {
LOGGER.info("Deleting resource {}", getIdentifier());
boolean isDeleted = super.deleteResource(conf);
LOGGER.info("Hfs Sink Tap isDeleted is {} for {}", isDeleted,
getIdentifier());
Path path = new Path(getIdentifier());
int retryCount = 0;
int cumulativeSleepTime = 0;
int sleepTime = 1000;
while (getFileSystem(conf).exists(path)) {
LOGGER
.info(
"Resource {} still exists, it should not... - I will continue to wait patiently...",
getIdentifier());
try {
LOGGER.info("Now I will sleep " + sleepTime / 1000
+ " seconds while trying to delete {} - attempt: {}",
getIdentifier(), retryCount + 1);
Thread.sleep(sleepTime);
cumulativeSleepTime += sleepTime;
sleepTime *= 2;
} catch (InterruptedException e) {
e.printStackTrace();
LOGGER
.error(
"Interrupted while sleeping trying to delete {} with message {}...",
getIdentifier(), e.getMessage());
throw new RuntimeException(e);
}
if (retryCount == 0) {
getFileSystem(conf).delete(getPath(), true);
}
retryCount++;
if (cumulativeSleepTime > MAXIMUM_TIME_TO_WAIT_TO_DELETE_MS) {
break;
}
}
if (getFileSystem(conf).exists(path)) {
LOGGER
.error(
"We didn't succeed to delete the resource {}. Throwing now a runtime exception.",
getIdentifier());
throw new RuntimeException(
"Although we waited to delete the resource for "
+ getIdentifier()
+ ' '
+ retryCount
+ " iterations, it still exists - This must be an issue in the underlying storage system.");
}
return isDeleted;
}
[3]
INFO [pool-2-thread-15] (BaseFlow.java:1287) - [...] at least one sink is marked for delete
INFO [pool-2-thread-15] (BaseFlow.java:1287) - [...] sink oldest modified date: Wed Dec 31 23:59:59 UTC 1969
INFO [pool-2-thread-15] (HiveSinkTap.java:148) - Now I will sleep 1 seconds while trying to delete s3n://... - attempt: 1
INFO [pool-2-thread-15] (HiveSinkTap.java:130) - Deleting resource s3n://...
INFO [pool-2-thread-15] (HiveSinkTap.java:133) - Hfs Sink Tap isDeleted is true for s3n://...
ERROR [pool-2-thread-15] (HiveSinkTap.java:175) - We didn't succeed to delete the resource s3n://... Throwing now a runtime exception.
WARN [pool-2-thread-15] (Cascade.java:706) - [...] flow failed: ...
java.lang.RuntimeException: Although we waited to delete the resource for s3n://... 0 iterations, it still exists - This must be an issue in the underlying storage system.
at com.qubit.hive.tap.HiveSinkTap.deleteResource(HiveSinkTap.java:179)
at com.qubit.hive.tap.HiveSinkTap.deleteResource(HiveSinkTap.java:40)
at cascading.flow.BaseFlow.deleteSinksIfNotUpdate(BaseFlow.java:971)
at cascading.flow.BaseFlow.prepare(BaseFlow.java:733)
at cascading.cascade.Cascade$CascadeJob.call(Cascade.java:761)
at cascading.cascade.Cascade$CascadeJob.call(Cascade.java:710)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:619)
最佳答案
首先，仔细检查级联兼容性页面以了解支持的发行版。
http://www.cascading.org/support/compatibility/
请注意，Amazon EMR 会定期运行兼容性测试并报告结果。
其次，S3 是一个最终一致的文件系统。 HDFS 不是。因此，关于 HDFS 行为的假设不会延续到针对 S3 存储数据。例如，重命名实际上是复制和删除。副本可能需要几个小时。亚马逊已经修补了他们的内部发行版以适应许多差异。
第三，S3中没有目录。这是一个 hack，不同的 S3 接口(interface)支持不同(jets3t vs s3cmd vs ...)。考虑到前面的观点，这必然会出现问题。
第四，网络延迟和可靠性至关重要，尤其是在与 S3 通信时。从历史上看，我发现在使用 EMR 与标准 EC2 实例相比，Amazon 网络在 S3 上操作海量数据集时表现更好。我也相信他们是 EMR 中的一个补丁，也可以改善这里的问题。
因此，我建议尝试运行 EMR Apache Hadoop 发行版，看看您的问题是否得到解决。
关于amazon-s3 - 未使用 SinkMode.REPLACE 删除级联 S3 Sink Tap，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/13421593/