added a definition of UoW

git-svn-id: https://svn.jboss.org/repos/hibernate/core/trunk@16744 1b8cb986-b30d-0410-93ca-fae66ebed9b2

documentation/manual/src/main/docbook/en-US/content/transactions.xml

@@ -91,6 +91,23 @@
         <sect2 id="transactions-basics-uow" revision="1">
             <title>Unit of work</title>
 
+            <para>
+                First, let's define a unit of work.  A unit of work is a
+                design pattern described by Martin Fowler as
+                <quote>
+                    [maintaining] a list of objects affected by a business
+                    transaction and coordinates the writing out of changes
+                    and the resolution of concurrency problems.
+                </quote><citation>PoEAA</citation>
+                In other words, its a series of operations we wish to carry out
+                against the database together.  Basically, it is a transaction,
+                though fulfilling a unit of work will often span multiple
+                physical database transactions (see <xref linkend="transactions-basics-apptx"/>).
+                So really we are talking about a more abstract notion of a
+                transaction.  The term "business transaction" is also sometimes
+                used in lieu of unit of work.
+            </para>
+
             <para>
                 Do not use the <emphasis>session-per-operation</emphasis> antipattern:
                 do not open and close a <literal>Session</literal> for every simple database call in