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https://issues.apache.org/jira/browse/DERBY-2763?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12503925
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V.Narayanan commented on DERBY-2763:
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>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: 10.3.0.0
> Reporter: V.Narayanan
> Assignee: V.Narayanan
> Fix For: 10.3.0.0
>
> 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,
> UpdateSensitiveStreamsForClient_v1.stat
>
>
> 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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