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#------------------------------------------------------------------------------

# session_callback.py

# Demonstrate how to use a connection pool session callback written in

# Python. The callback is invoked whenever a newly created session is acquired

# from the pool, or when the requested tag does not match the tag that is

# associated with the session. It is generally used to set session state, so

# that the application can count on known session state, which allows the

# application to reduce the number of round trips made to the database.

# If all your connections should have the same session state, you can simplify

# the session callback by removing the tagging logic.

# This script requires cx_Oracle 7.1 or higher.

# Also see session_callback_plsql.py

#------------------------------------------------------------------------------

import cx_Oracle as oracledb

import sample_env

# define a dictionary of NLS_DATE_FORMAT formats supported by this sample

SUPPORTED_FORMATS = {

"SIMPLE" : "'YYYY-MM-DD HH24:MI'",

"FULL" : "'YYYY-MM-DD HH24:MI:SS'"

}

# define a dictionary of TIME_ZONE values supported by this sample

SUPPORTED_TIME_ZONES = {

"UTC" : "'UTC'",

"MST" : "'-07:00'"

}

# define a dictionary of keys that are supported by this sample

SUPPORTED_KEYS = {

"NLS_DATE_FORMAT" : SUPPORTED_FORMATS,

"TIME_ZONE" : SUPPORTED_TIME_ZONES

}

# define session callback

def init_session(conn, requested_tag):

# display the requested and actual tags

print("init_session(): requested tag=%r, actual tag=%r" % \

(requested_tag, conn.tag))

# tags are expected to be in the form "key1=value1;key2=value2"

# in this example, they are used to set NLS parameters and the tag is

# parsed to validate it

if requested_tag is not None:

state_parts = []

for directive in requested_tag.split(";"):

parts = directive.split("=")

if len(parts) != 2:

raise ValueError("Tag must contain key=value pairs")

key, value = parts

value_dict = SUPPORTED_KEYS.get(key)

if value_dict is None:

raise ValueError("Tag only supports keys: %s" % \

(", ".join(SUPPORTED_KEYS)))

actual_value = value_dict.get(value)

if actual_value is None:

raise ValueError("Key %s only supports values: %s" % \

(key, ", ".join(value_dict)))

state_parts.append("%s = %s" % (key, actual_value))

sql = "alter session set %s" % " ".join(state_parts)

cursor = conn.cursor()

cursor.execute(sql)

# assign the requested tag to the connection so that when the connection

# is closed, it will automatically be retagged; note that if the requested

# tag is None (no tag was requested) this has no effect

conn.tag = requested_tag

# create pool with session callback defined

pool = oracledb.SessionPool(user=sample_env.get_main_user(),

password=sample_env.get_main_password(),

dsn=sample_env.get_connect_string(), min=2, max=5,

increment=1, session_callback=init_session)

# acquire session without specifying a tag; since the session returned is

# newly created, the callback will be invoked but since there is no tag

# specified, no session state will be changed

print("(1) acquire session without tag")

with pool.acquire() as conn:

cursor = conn.cursor()

cursor.execute("select to_char(current_date) from dual")

result, = cursor.fetchone()

print("main(): result is", repr(result))

# acquire session, specifying a tag; since the session returned has no tag,

# the callback will be invoked; session state will be changed and the tag will

# be saved when the connection is closed

print("(2) acquire session with tag")

with pool.acquire(tag="NLS_DATE_FORMAT=SIMPLE") as conn:

cursor = conn.cursor()

cursor.execute("select to_char(current_date) from dual")

result, = cursor.fetchone()

print("main(): result is", repr(result))

# acquire session, specifying the same tag; since a session exists in the pool

# with this tag, it will be returned and the callback will not be invoked but

# the connection will still have the session state defined previously

print("(3) acquire session with same tag")

with pool.acquire(tag="NLS_DATE_FORMAT=SIMPLE") as conn:

cursor = conn.cursor()

cursor.execute("select to_char(current_date) from dual")

result, = cursor.fetchone()

print("main(): result is", repr(result))

# acquire session, specifying a different tag; since no session exists in the

# pool with this tag, a new session will be returned and the callback will be

# invoked; session state will be changed and the tag will be saved when the

# connection is closed

print("(4) acquire session with different tag")

with pool.acquire(tag="NLS_DATE_FORMAT=FULL;TIME_ZONE=UTC") as conn:

cursor = conn.cursor()

cursor.execute("select to_char(current_date) from dual")

result, = cursor.fetchone()

print("main(): result is", repr(result))

# acquire session, specifying a different tag but also specifying that a

# session with any tag can be acquired from the pool; a session with one of the

# previously set tags will be returned and the callback will be invoked;

# session state will be changed and the tag will be saved when the connection

# is closed

print("(4) acquire session with different tag but match any also specified")

with pool.acquire(tag="NLS_DATE_FORMAT=FULL;TIME_ZONE=MST", matchanytag=True) \

as conn:

cursor = conn.cursor()

cursor.execute("select to_char(current_date) from dual")

result, = cursor.fetchone()

print("main(): result is", repr(result))

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