我们正在运行 Cascading，其中 Sink Tap 被配置为存储在 Amazon S3 中，并且面临一些 FileAlreadyExistsException（请参阅 [1]）。这只是不时的（大约 100 次有 1 次）并且不可重现。
深入研究 Cascading codem，我们发现 Hfs.deleteResource() 被 BaseFlow.deleteSinksIfNotUpdate() 调用（其中包括）。顺便说一句，我们对沉默的 NPE 非常感兴趣（评论“当 fs 到达根目录时，hack to get around npe throw away”）。
从那里，我们用我们自己的 Tap 扩展了 Hfs Tap，以在 deleteResource() 方法中添加更多操作（参见 [2]），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，但我们仍然有时会遇到失败（参见[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，我们收到exists=true，应该不会发生。当流程成功时，日志还会随机显示 isDeleted true 或 false，这听起来像是返回的值无关紧要或不可信。
任何人都可以通过这样的行为带来他自己的 S3 体验：“文件夹应该被删除，但不是”？我们怀疑是 S3 问题，但它也可能出现在 Cascading 或 HDFS 中吗？
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)