Using Postgres in a web application: transaction errors

Question

I recently translated a web application that I am developing from MySQL to PostgreSQL for performance reasons (I need PostGIS functionality). Now quite often the following error occurs:

current transaction is aborted, commands ignored until end of transaction block

The server application uses mod_python. The error occurs in the welcome function (i.e., the one that creates a new session for this particular client). Here comes the corresponding code fragment (an exception occurs in the line where sessionAppId is called:

def hello(req):
req.content_type = "text/json"
req.headers_out.add('Cache-Control', "no-store, no-cache, must-revalidate")
req.headers_out.add('Pragma', "no-cache")
req.headers_out.add('Expires', "-1")
instance = req.hostname.split(".")[0]

cookieSecret = '....' # whatever :-)
receivedCookies = Cookie.get_cookies(req, Cookie.SignedCookie, secret = cookieSecret)
sessionList = receivedCookies.get('sessions', None)
sessionId = str(uuid.uuid4())
if sessionList:
    if type(sessionList) is not Cookie.SignedCookie:
        return "{status: 'error', errno:1, errmsg:'Permission denied.'}"
    else:
        sessionList = sessionList.value.split(",")
        for x in sessionList[:]:
            revisionCookie = receivedCookies.get('rev_' + str(sessionAppId(x, instance)), None)
            # more processing here....
# .....
cursors[instance].execute("lock revision, app, timeout IN SHARE MODE")
cursors[instance].execute("insert into app (type, active, active_revision, contents, z) values ('session', true, %s, %s, 0) returning id", (cRevision, sessionId))
sAppId = cursors[instance].fetchone()[0]
cursors[instance].execute("insert into revision (app_id, type) values (%s, 'active')", (sAppId,))
cursors[instance].execute("insert into timeout (app_id, last_seen) values (%s, now())", (sAppId,))
connections[instance].commit()
# .....

And here is sessionAppId itself:

def sessionAppId(sessionId, instance):
cursors[instance].execute("select id from app where type='session' and contents = %s", (sessionId, ))
row = cursors[instance].fetchone()
if row == None:
    return 0
else:
    return row[0]

Some clarifications and additional questions:

• cursors [instance] [instance] - -, . example1.com example2.com , .
• hello()?
• hello() . , , -, cookie. ?

.

Answer 1

. :

>>> import psycopg2
>>> conn = psycopg2.connect('')
>>> cur = conn.cursor()
>>> cur.execute('select current _date')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
psycopg2.ProgrammingError: syntax error at or near "_date"
LINE 1: select current _date
                       ^

>>> cur.execute('select current_date')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
psycopg2.InternalError: current transaction is aborted, commands ignored until end of transaction block

>>> conn.rollback()
>>> cur.execute('select current_date')
>>> cur.fetchall()
[(datetime.date(2010, 2, 5),)]
>>> 

, twisted.enterprise.adbapi , . :

try:
    cur.execute("...")
    cur.fetchall()
    cur.close()
    connection.commit()
except:
    connection.rollback()