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From "Kristian Waagan (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-2762) Document, verify and fix synchronization issues related to Clob in the embedded driver
Date Fri, 08 Jun 2007 14:29:25 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2762?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12502825
] 

Kristian Waagan commented on DERBY-2762:
----------------------------------------

Here's my first take at defining a synchronization policy for Clob-objects.
There are two paths that must be synchronized:
 a) Use through the EmbedClob object.
 b) Reads/writes through one of the available streams.

For streams, the scenarios below must be handled. My initial propositions for
what to do is listed as well. I think they are in agreement with the current
efforts in the community.
  i) A read-only Clob is changed to a read-write Clob.
     Streams must be updated to take data from the new representation.
 ii) Clob is closed/invalidated by EmbedClob.free() or Connection.commit().
     Next read/write must throw IOException with a Derby SQLState message, i.e.
     XJ215/XJ073. The streams should not return EOF.
iii) Clob is truncated to a position after the current stream position.
     Nothing happens, stream is updated to reflect the new length/content.
 iv) Clob is truncated to a position before the current stream position.
     Throw IOException with some kind of error message.
  v) Clob is truncated to a position at the current stream position.
     Next read() will return EOF (-1). Next write() will append Clob.
 vi) Clob is updated through EmbedClob.setString().
     Streams are updated with the new content. If the stream is positioned in
     the middle of the updated portion, the user would see some data from the
     old content and some data from the new content. Users not accepting this
     should be able to enforce consistency themselves by controlling access to
     the Clob-object and stream objects.

In general the synchronization must ensure only one operation on the Clob, both
through the Clob-object itself and through the streams, can happen at a time.

Let's say you get two character streams from a Clob and read repeatedly from them, one at
a time but in various orders.
Should both streams retain their own position, or should reading from one stream advance the
position in the other stream?
My take on this is that they retain their own position, but that this access pattern might
be very ineffective (reposition from first position every time).

What do people think about these statements?
Are they incorrect?
Do they describe a sensible behavior?
Since no behavior is dictated by the JDBC spec (?), I don't think it will be
wise to implement highly sophisticated concurrency guarantees. Is my proposal
already crossing this line?
Should we simply say concurrent access is undefined, or that all access must be
single threaded? Note that even if it is single threaded, we have to decide what happens when
the user mixes calls to Clob and the streams.

Implementation-wise I think we have a few alternatives, but the current
implementation of EmbedClob also imply a few limitations. For instance I hoped
all streams could operate on InternalClob instead of EmbedClob, but that does
not seem feasible since the InternalClob object in EmbedClob is replaced when
the Clob goes from read-only to read-write. At least the streams would need a
reference to both.


Does anyone have strong opinions on what we should do with this issue?
This comment is an invitation to get a discussion started...

> Document, verify and fix synchronization issues related to Clob in the embedded driver
> --------------------------------------------------------------------------------------
>
>                 Key: DERBY-2762
>                 URL: https://issues.apache.org/jira/browse/DERBY-2762
>             Project: Derby
>          Issue Type: Bug
>          Components: JDBC
>    Affects Versions: 10.3.0.0
>            Reporter: Kristian Waagan
>
> Synchronization with respect to Clobs is a bit tricky.
> A full review of synchronization should be performed, and documentation and actual behavior
must be made in agreement with each other.
> The synchronization wrt. Clob is made tricker due to the many streams that can be used
to read or write its value.
> The main classes to review will be (there might be more):
>  a) EmbedClob
>  b) StoreStreamClob
>  c) ClobStreamControl (may be renamed to TemporaryClob)
>  d) ClobUtf8Writer
>  e) ClobAsciiStream
>  f) ClobUpdateableReader
> We should also clarify and document what is supposed to be allowed. Can
> you read from one stream and write to another one at the same time, both
> from the same Clob?
> Can you expect the ascii stream and the character stream to be in sync
> if you read from both of them?
> A related issue is that of garbage collection of underlying resources
> before the streams are closed. DERBY-2734 has already been filed for
> this.
> I would like to try a little experiment by using a few simple
> annotations to document intended synchronization policies. These are the
> annotations defined in the book "Java Concurrency in Practice" by Brian
> Goetz et al, and the JavaDoc for them can be found here: 
> http://javaconcurrencyinpractice.com/annotations/doc/index.html
> Since we are still using Java 1.4, the annotations must be used as
> comments. I still think they are valuable, as we do not use any tools to
> document/check synchronization anyway.
> Briefly, the following four annotations are defined:
>  @GuardedBy
>  @Immutable
>  @NotThreadSafe
>  @ThreadSafe

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