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From "Catalin Alexandru Zamfir (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (AVRO-1090) DataFileWriter should expose "sync marker" to allow concurrent writes to same .avro file
Date Mon, 14 May 2012 13:09:46 GMT

    [ https://issues.apache.org/jira/browse/AVRO-1090?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13274584#comment-13274584
] 

Catalin Alexandru Zamfir commented on AVRO-1090:
------------------------------------------------

>From the source-code of DatumFileWriter, the "appendTo" method seems to do what it's intended.
But only accepts a (File) argument. In the case where one writer connects over the network
to Hadoop and needs to write a "FSDataOutputStream" instead of a file, the advantages of the
appendTo method cannot be used. So it seems it is possible to retrieve the sync marker from
an existing .avro file and write forward with the same marker.

Can this be done here also? Can an appendTo (FSDataOutputStream) method be created? This would
allow concurrent writers to create or append on the same output stream using the same marker,
thus enabling the data to be read back.
                
> DataFileWriter should expose "sync marker" to allow concurrent writes to same .avro file
> ----------------------------------------------------------------------------------------
>
>                 Key: AVRO-1090
>                 URL: https://issues.apache.org/jira/browse/AVRO-1090
>             Project: Avro
>          Issue Type: Bug
>    Affects Versions: 1.6.3
>            Reporter: Catalin Alexandru Zamfir
>
> We're writing to Hadoop via DataFileWriter (FSDataOutputStream). We're doing this with
two threads per node, on 8 nodes. Some of the nodes share the same path. For example, our:
TimestampedWriter class, takes a path argument and appends the timestamp to it (ex: SomePath/2012/05/14).
Thus, two threads or two nodes can access the same path. The "race" condition when these streams
are written, is resolved with a check to see if the file exists (has been created) by a faster
thread. If that's so, it appends, instead of creating the file on the HDFS.
> The problem is that DataFileWriter, generates a 16-byte, random string for each instance.
So, two threads with 2 different writer instances, have a different sync marker. That means
that data, when trying to read it back, will get an IOException ("Invalid sync!").
> There's a big performance penalty here. Because only one writer can write at once to
one given path, it becomes a bottleneck. For 1B (billion) rows, it took us 4 hours to generate
& load. With 20 concurrent threads, it took only 12.5 minutes. 
> If DataFileWriter would expose the "sync" marker, a developer could read that and make
sure that the next thread that appends to the file, uses the same sync marker. Don't know
if it's even possible to expose the sync marker so as other instances of "DataFileWriter"
can share the sync marker, from the file. We have a fix for this, making sure each writer
is an "unique" instance and generating a path based on that uniqueness. But instead of having
"SomePath/2012/05/14/Shard.avro" we'd now have "SomePath/2012/05/14/Shard-some-random-UUID.avro"
for each of the writers that write the data in.
> If it can be done, it would be a huge fix for a bottleneck problem. The bottleneck being
the single writer that can write to a single path.

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