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Date: Mon, 22 Oct 2007 08:08:30 -0700 (PDT)
From: Christiaan <christiaan_db@hotmail.com>
To: users@openjpa.apache.org
Subject: Use of List increases chance on concurrency contention
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Hi,
I am writing this since this may be useful to others. I=E2=80=99ve quite so=
me
classes which have a List attribute. OpenJPA create a relationship table
with an order column to maintain the order of the elements. Recently we
encountered quite some blocking scenarios when using it in a multi-user
environment. The reason was that when List.add(index, object) is being used
OpenJPA sends the following sql statement, so the order of the elements can
be built up from scratch:
=E2=80=9CDelete from RELATION_TABLE where ID =3D =E2=80=9C

In ms sql server this causes an exclusive lock on the entire table which of
course easily introduces blocking issues (long running transactions having =
a
shared lock or other client who like to insert objects in their list as
well).=20

List.add(Object) does not have this behaviour so when adding objects to the
end of the list it is always better to use this method. My suggestions:
1)=09In the proxy list openjpa creates add a check to List.add(index, objec=
t)
whether index is equal to List.size() so it can use the same logic as add()
2)=09May be provide some other mode for updating the order of elements when
elements are being inserted, eg. execute update statements instead of a
deletion?

Kind regards,
Christiaan

--=20
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