Avro file - Seek to specific offset and read

[Venkat <vr...@ymail.com>]

Hi All,


Using DataFileReader, I'm trying to read data from a specific [start-offset] to an [end-offset]. Both the start and end offsets are marked with synchronization markers using DataFileWriter.sync()

The following is the snipped I use to read the data back:

        DataFileReader<GenericRecord> fileReader = new DataFileReader<GenericRecord>(input, reader);      
        fileReader.seek(startOffset);  // set to the start-offset       

        while(fileReader.hasNext() && !fileReader.pastSync(endOffset))
        {
            GenericRecord gr = fileReader.next();
        }

This, however, reads & returns more records than what I wrote between the two offsets. 


Appreciate your help regarding this.

Thanks

Re: Avro file - Seek to specific offset and read

[Venkat <vr...@ymail.com>]

Hi Doug,

Adjusting the start and end offsets (returned by DataFileWriter.sync()) back by 16 bytes (DataFileConstants.SYNC_SIZE) fixed the issue.


This assumption is based on looking at the DataFileReader.pastSync() implementation.

97   public boolean pastSync(long position) throws IOException {
98     return ((blockStart >= position+SYNC_SIZE)||(blockStart >= sin.length()));
99   }

Let me know if this assumption is correct.

Thanks
Venkat


________________________________
 From: Doug Cutting <cu...@apache.org>
To: user@avro.apache.org; Venkat <vr...@ymail.com> 
Sent: Friday, February 22, 2013 12:01 PM
Subject: Re: Avro file - Seek to specific offset and read
 
Venkat,

That should work.  It's hard for me to guess what's going wrong,
whether there's a bug in Avro, in your program, or perhaps just
unclear documentation.  Could you post a complete program that
demonstrates the issue?

Thanks,

Doug

On Wed, Feb 20, 2013 at 12:23 PM, Venkat <vr...@ymail.com> wrote:
> Hi All,
>
> Using DataFileReader, I'm trying to read data from a specific [start-offset]
> to an [end-offset]. Both the start and end offsets are marked with
> synchronization markers using DataFileWriter.sync()
>
> The following is the snipped I use to read the data back:
>
>         DataFileReader<GenericRecord> fileReader = new
> DataFileReader<GenericRecord>(input, reader);
>         fileReader.seek(startOffset);  // set to the start-offset
>         while(fileReader.hasNext() && !fileReader.pastSync(endOffset))
>         {
>             GenericRecord gr = fileReader.next();
>         }
>
> This, however, reads & returns more records than what I wrote between the
> two offsets.
>
> Appreciate your help regarding this.
>
> Thanks
>

Re: Avro file - Seek to specific offset and read

[Doug Cutting <cu...@apache.org>]

Venkat,

That should work.  It's hard for me to guess what's going wrong,
whether there's a bug in Avro, in your program, or perhaps just
unclear documentation.  Could you post a complete program that
demonstrates the issue?

Thanks,

Doug

On Wed, Feb 20, 2013 at 12:23 PM, Venkat <vr...@ymail.com> wrote:
> Hi All,
>
> Using DataFileReader, I'm trying to read data from a specific [start-offset]
> to an [end-offset]. Both the start and end offsets are marked with
> synchronization markers using DataFileWriter.sync()
>
> The following is the snipped I use to read the data back:
>
>         DataFileReader<GenericRecord> fileReader = new
> DataFileReader<GenericRecord>(input, reader);
>         fileReader.seek(startOffset);  // set to the start-offset
>         while(fileReader.hasNext() && !fileReader.pastSync(endOffset))
>         {
>             GenericRecord gr = fileReader.next();
>         }
>
> This, however, reads & returns more records than what I wrote between the
> two offsets.
>
> Appreciate your help regarding this.
>
> Thanks
>