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>Explicit Subtransactions</TITLE
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><A
NAME="PLPYTHON-SUBTRANSACTION"
>42.8. Explicit Subtransactions</A
></H1
><P
> Recovering from errors caused by database access as described in
<A
HREF="plpython-database.html#PLPYTHON-TRAPPING"
>Section 42.7.2</A
> can lead to an undesirable
situation where some operations succeed before one of them fails,
and after recovering from that error the data is left in an
inconsistent state. PL/Python offers a solution to this problem in
the form of explicit subtransactions.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN59861"
>42.8.1. Subtransaction Context Managers</A
></H2
><P
> Consider a function that implements a transfer between two
accounts:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION transfer_funds() RETURNS void AS $$
try:
plpy.execute("UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'")
plpy.execute("UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'")
except plpy.SPIError, e:
result = "error transferring funds: %s" % e.args
else:
result = "funds transferred correctly"
plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"])
plpy.execute(plan, [result])
$$ LANGUAGE plpythonu;</PRE
><P>
If the second <TT
CLASS="LITERAL"
>UPDATE</TT
> statement results in an
exception being raised, this function will report the error, but
the result of the first <TT
CLASS="LITERAL"
>UPDATE</TT
> will
nevertheless be committed. In other words, the funds will be
withdrawn from Joe's account, but will not be transferred to
Mary's account.
</P
><P
> To avoid such issues, you can wrap your
<TT
CLASS="LITERAL"
>plpy.execute</TT
> calls in an explicit
subtransaction. The <TT
CLASS="LITERAL"
>plpy</TT
> module provides a
helper object to manage explicit subtransactions that gets created
with the <TT
CLASS="LITERAL"
>plpy.subtransaction()</TT
> function.
Objects created by this function implement the
<A
HREF="http://docs.python.org/library/stdtypes.html#context-manager-types"
TARGET="_top"
> context manager interface</A
>. Using explicit subtransactions
we can rewrite our function as:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION transfer_funds2() RETURNS void AS $$
try:
with plpy.subtransaction():
plpy.execute("UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'")
plpy.execute("UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'")
except plpy.SPIError, e:
result = "error transferring funds: %s" % e.args
else:
result = "funds transferred correctly"
plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"])
plpy.execute(plan, [result])
$$ LANGUAGE plpythonu;</PRE
><P>
Note that the use of <TT
CLASS="LITERAL"
>try/catch</TT
> is still
required. Otherwise the exception would propagate to the top of
the Python stack and would cause the whole function to abort with
a <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> error, so that the
<TT
CLASS="LITERAL"
>operations</TT
> table would not have any row
inserted into it. The subtransaction context manager does not
trap errors, it only assures that all database operations executed
inside its scope will be atomically committed or rolled back. A
rollback of the subtransaction block occurs on any kind of
exception exit, not only ones caused by errors originating from
database access. A regular Python exception raised inside an
explicit subtransaction block would also cause the subtransaction
to be rolled back.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN59876"
>42.8.2. Older Python Versions</A
></H2
><P
> Context managers syntax using the <TT
CLASS="LITERAL"
>with</TT
> keyword
is available by default in Python 2.6. If using PL/Python with an
older Python version, it is still possible to use explicit
subtransactions, although not as transparently. You can call the
subtransaction manager's <TT
CLASS="LITERAL"
>__enter__</TT
> and
<TT
CLASS="LITERAL"
>__exit__</TT
> functions using the
<TT
CLASS="LITERAL"
>enter</TT
> and <TT
CLASS="LITERAL"
>exit</TT
> convenience
aliases. The example function that transfers funds could be
written as:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION transfer_funds_old() RETURNS void AS $$
try:
subxact = plpy.subtransaction()
subxact.enter()
try:
plpy.execute("UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'")
plpy.execute("UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'")
except:
import sys
subxact.exit(*sys.exc_info())
raise
else:
subxact.exit(None, None, None)
except plpy.SPIError, e:
result = "error transferring funds: %s" % e.args
else:
result = "funds transferred correctly"
plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"])
plpy.execute(plan, [result])
$$ LANGUAGE plpythonu;</PRE
><P>
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> Although context managers were implemented in Python 2.5, to use
the <TT
CLASS="LITERAL"
>with</TT
> syntax in that version you need to
use a <A
HREF="http://docs.python.org/release/2.5/ref/future.html"
TARGET="_top"
>future
statement</A
>. Because of implementation details, however,
you cannot use future statements in PL/Python functions.
</P
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