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From "V.Narayanan (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-2763) In the Network Client InputStreams and Readers returned from LOB's should be sensitive to underlying LOB data changes.
Date Tue, 12 Jun 2007 16:58:26 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2763?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12503925

V.Narayanan commented on DERBY-2763:

>It seems to me that the input stream's position is an offset from the start of the LOB.

By InputStreams I guess the UpdateSensitiveStreams (UpdateSensitiveLOBInputStream, 
UpdateSensitiveClobLocatorReader) are meant.

>It appears to me that the recreation of the streams supports some other use-case, 
>in which the writer is writing bytes further on in the LOB, and not in
>the porition that has already been read. 

These streams internally wrap inside a Buffered Streams

UpdateSensitiveLOBInputStream - Has a BlobLocatorInputStream or a ClobLocatorInputStream
                                                               inside a BufferedInputStream

UpdateSensitiveClobLocatorReader - Has a BlobLocatorReader inside a BufferedReader.

The Buffered streams pre-fetch larger data than we request(4k = 4096 bytes).

So if a request for 1000 bytes is made initially from the InputStream the BufferedInputStream
would fetch
4096 bytes. 

Now if the user decides to update from the 2000 position or byte onwards, a subsequent read,
unless re-creation of the streams that are wrapped inside the Buffered streams is done, will
return already
pre-feched data. 

So upto 4096 bytes you would get the pre-fetched or old data and from 4096 onwards you would
get the new
data. This would be inconsistent.

So while re-creating the streams they would then re-create with the original position and
the reading would
resume with the updated data. If we remove the BufferedStreams the process of reads becomes
very slow.

> In the Network Client InputStreams and Readers returned from LOB's should be sensitive
to underlying LOB data changes.
> ----------------------------------------------------------------------------------------------------------------------
>                 Key: DERBY-2763
>                 URL: https://issues.apache.org/jira/browse/DERBY-2763
>             Project: Derby
>          Issue Type: Bug
>          Components: Network Client
>    Affects Versions:
>            Reporter: V.Narayanan
>            Assignee: V.Narayanan
>             Fix For:
>         Attachments: Approach_2.diff, Approach_2.stat, Approach_2.txt, Approach_3.diff,
Approach_3.stat, Approach_4.diff, Approach_4.stat, LOBLengthPersists.java, UpdateSensitiveStreamsForClient_v1.diff,
> Currently the Embedded and Network Client would differ 
> in behaviour when the following series of steps is 
> followed.
> a) Create an empty Blob
> b) get an InputStream using Blob.getBinaryStream()
> c) write data into this Blob
>    c.1) Get an OutputStream
>    c.2) Use OutputStream.write(byte [] b) to write
>         into this Blob.
> d) Now read from the InputStream obtained in step b)
>    and print the number of bytes read as output.
> The output of step d) differs in the client and in the Embedded side.
> In the Client
> -------------
> The number of bytes read would always be -1.
> In the Embedded
> ---------------
> The number of bytes would be the number of bytes we
> reflected.
> The above behaviour in the NetworkClient is because
> the length of the Blob is read once and stored in the 
> constructor of the locator Stream returned (in the 
> attribute maxPos).
> This instead should be read each time we use the streams.
> A similar issue exists for Clobs also.
> I will raise a seperate JIRA issue for this.

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