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From "Marcel Reutegger (JIRA)" <j...@apache.org>
Subject [jira] Created: (JCR-1820) Ensure queries are not blocked during large updates
Date Fri, 17 Oct 2008 16:08:44 GMT
Ensure queries are not blocked during large updates

                 Key: JCR-1820
                 URL: https://issues.apache.org/jira/browse/JCR-1820
             Project: Jackrabbit
          Issue Type: Improvement
          Components: jackrabbit-core
            Reporter: Marcel Reutegger
            Priority: Minor

The index currently guarantees that long running queries do not block updates. In addition
a query *may* run during an update, but there is not guarantee because it depends on the availability
of an index reader being available when the update starts. The index reader is invalidated
at the end of the update, which will force the creation of a new index reader when the next
query is executed.

Consider the following scenario:

1) update index -> transaction id T1
2) potential index reader is invalidated
3) execute query -> creates index reader R1, which includes changes up to T1
4) update index -> transaction id T2
5) index reader R1 is invalidated
6) update index (large transaction) -> transaction id T3
7) while previous update is running execute query -> thread is blocked because no reader
is available

The improvement should detect the large transaction and prepare an index reader for potential
queries during the update. That is, 6) should be split into:

6a) detect large transaction and prepare index reader R2, which includes changes up to T2
6b) update index -> transaction id T3

While the update is running a query will use index reader R2.

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