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Date: Sun, 28 Feb 2010 19:22:05 +0000 (UTC)
From: "Jonathan Ellis (JIRA)" <jira@apache.org>
To: cassandra-commits@incubator.apache.org
Subject: [jira] Issue Comment Edited: (CASSANDRA-836)
 CommitLogSegment::seekAndWriteCommitLogHeader assumes header size doesn't
 change.
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    [ https://issues.apache.org/jira/browse/CASSANDRA-836?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12839479#action_12839479 ] 

Jonathan Ellis edited comment on CASSANDRA-836 at 2/28/10 7:20 PM:
-------------------------------------------------------------------

it's not a bug, because we never change the size of the bitset.

if you want to add an assertion to that effect, fine, but making the serialization handle a situation that would be a horrible bug, is bad design.

      was (Author: jbellis):
    it's not a bug, because we never change the size of the bitset
  
> CommitLogSegment::seekAndWriteCommitLogHeader assumes header size doesn't change.
> ---------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-836
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-836
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>         Environment: n/a - all
>            Reporter: Ross M
>            Priority: Minor
>         Attachments: BitSetSerializer.java
>
>
> CommitLogSegment::seekAndWriteCommitLogHeader assumes header size doesn't grow. there are pieces of the header (BitSet) that are serialized with java serialization which makes no such promises. 
> the following code:
>     /** writes header at the beginning of the file, then seeks back to current position */
>     void seekAndWriteCommitLogHeader(byte[] bytes) throws IOException
>     {
>         long currentPos = logWriter.getFilePointer();
>         logWriter.seek(0);
>         writeCommitLogHeader(bytes);
>         logWriter.seek(currentPos);
>     }
> works fine as long as the header size doesn't change, but if it grows the new header will over write the beginning of the data segment. the bit-set being written in the header happens to serialize to the same size, but there is no guarantee of this.
> i found this when looking at optimizing the serialization of data to disk (thus improving write throughput/performance.) i removed the ObjectOutputStream serialization in BitSetSerializer and replaced it with a custom serialization that omits the generic java serialization/ObjectOutputStream stuff and just writes on the "true" bits. the custom serialization worked fine, but broke other parts of the code when the header bitset had new bits turned on, thus growing the header's size, data segment bytes were overwritten.
> the serialized version of a BitSet can grow in a similar manner, no pomises of size/consistency are made, but with current use it luckily doesn't seem to happen.
> a good fix is unclear. without forcing the header to be a fixed/constant size in some manner this problem could pop up at any point. it's generally not safe to rewrite headers like this without custom code that ensures the size doesn't change. one fix would be to manually write all of the header data out (rather than relying on java serialization and serialization code in other parts of cassandra not to change.) another might be to pad the size of the header so that the data inside can grow, but that seems fraught with (potential) problems. (i've played around with padding the header length, but that seems to cause other things to break, which i haven't been able to track down yet.)

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