[jira] [Commented] (NIFI-4715) ListS3 produces duplicates in frequently updated buckets

["Andrew Burkard (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/NIFI-4715?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16361129#comment-16361129 ] 

Andrew Burkard commented on NIFI-4715:
--------------------------------------

An additional problem with the processor is that it can also skip files if the bucket is being written to while being listed. Consider the following scenario:

 

We are in the middle of listing a bucket with millions of objects. Simultaneously we write two objects to the bucket:

aaaaaaaaa.txt,    at t=1

zzzzzzzzzz.txt,    at t=2

 

Because files are listed in lexographical order, the processor will eventually get to the 'z' file and persist state as t=2. It will never see the 'a' file. As a result, we can only be certain that the processor will list all files with lastModified < the time at which the processor starts. This implies that to avoid duplicates, we also need to store all keys seen during the loop with lastModified > the processor start time. As an added complication, I'm not sure how we resolve potential differences between S3 timestamps and the system's time to enable this.

> ListS3 produces duplicates in frequently updated buckets
> --------------------------------------------------------
>
>                 Key: NIFI-4715
>                 URL: https://issues.apache.org/jira/browse/NIFI-4715
>             Project: Apache NiFi
>          Issue Type: Bug
>          Components: Core Framework
>    Affects Versions: 1.2.0, 1.3.0, 1.4.0
>         Environment: All
>            Reporter: Milan Das
>            Priority: Major
>         Attachments: List-S3-dup-issue.xml, screenshot-1.png
>
>
> ListS3 state is implemented using HashSet. HashSet is not thread safe. When ListS3 operates in multi threaded mode, sometimes it  tries to list  same file from S3 bucket.  Seems like HashSet data is getting corrupted.
> currentKeys = new HashSet<>(); // need to be implemented Thread Safe like currentKeys = //ConcurrentHashMap.newKeySet();
> *{color:red}+Update+{color}*:
> This is not a HashSet issue:
> Root cause is: 
> When the file gets uploaded to S3 simultaneously  when List S3 is in progress.
> onTrigger-->  maxTimestamp is initiated as 0L.
> This is clearing keys as per the code below
> When lastModifiedTime on S3 object is same as currentTimestamp for the listed key it should be skipped. As the key is cleared, it is loading the same file again. 
> I think fix should be to initiate the maxTimestamp with currentTimestamp not 0L.
> {code}
>  long maxTimestamp = currentTimestamp;
> {code}
> Following block is clearing keys.
> {code:title=org.apache.nifi.processors.aws.s3.ListS3.java|borderStyle=solid}
>  if (lastModified > maxTimestamp) {
>                     maxTimestamp = lastModified;
>                     currentKeys.clear();
>                     getLogger().debug("clearing keys");
>                 }
> {code}
> Update: 01/03/2018
> There is one more flavor of same defect.
> Suppose: file1 is modified at 1514987611000 on S3 and currentTimestamp = 1514987311000 on state.
> 1. File will be picked up time current state will be updated to currentTimestamp=1514987311000 (but OS System time is 1514987611000)
> 2. next cycle for file2 with lastmodified: 1514987611000 : keys will be cleared because lastModified > maxTimeStamp (=currentTimestamp=1514987311000). 
> CurrentTimeStamp will saved as 1514987611000
> 3. next cycle: currentTimestamp=1514987611000 , "file1 modified at 1514987611000" will be picked up again because file1 is no longer in the keys.
> I think solution is currentTimeStamp need to persisted current system time stamp.



--
This message was sent by Atlassian JIRA
(v7.6.3#76005)