[Xapian-tickets] [Xapian] #284: occasional DatabaseModifiedErrors

[Xapian]

#284: occasional DatabaseModifiedErrors
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 Reporter:  mrks           |        Owner:  olly     
     Type:  defect         |       Status:  assigned 
 Priority:  normal         |    Milestone:  1.1.1    
Component:  Backend-Flint  |      Version:  SVN trunk
 Severity:  normal         |   Resolution:           
 Keywords:                 |    Blockedby:           
 Platform:  Linux          |     Blocking:           
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Comment(by richard):

 I believe that our algorithm is at fault.  Here's my reasoning:

 Suppose we start at revision 7.  All readers are open at revision 7.

 The writer, as soon as any modifications are made, writes these modified
 blocks with a revision number of 8.  This is fine, since only blocks which
 are not used in revision 7 are modified.
 When the commit happens, the writer writes all modified blocks as revision
 8.

 At this point, any new readers will open revision 8.  However, old readers
 (even if they called reopen() just before the commit), will still be on
 revision 7.

 After the commit, any further modifications will be written as revision 9.
 If a revision 7 reader sees one of these, it will raise the error we've
 seen.  However, the writer is now only guaranteeing not to modify blocks
 used in revision 8, so this is quite possible.

 In other words, the algorithm only guarantees that reopen() will prevent
 DatabaseModifiedErrors being raised if the writer holds off writing blocks
 to the database for a period of time equal to, at least, the longest
 length that readers are reading from the database after calling reopen().

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Ticket URL: <http://trac.xapian.org/ticket/284#comment:15>
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