tests/multiprocessing.py
author Radek Brich <radek.brich@devl.cz>
Thu, 15 Dec 2011 18:21:41 +0100
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PgManager: Add partial support for multiprocessing.
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#!/usr/bin/env python3
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'''Multiprocessing test.
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PgManager is partially safe against process forking.
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You can create connection credentials (create_conn) in main process, then fork
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and continue working with pgmanager in children processes. You can call create_conn again
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in children, but new connection credentials will be accessible only from process
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where it was created -- no sharing between processes.
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get_conn and cursor will still work after fork. Connections from parent process will
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be forgotten, that cannot be used un child process. 
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Connections cannot be shared between processes, DO NOT:
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 * get_conn, then fork, then put_conn
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 * get_conn, then fork, then use the connection, then .close()
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Good usage:
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 * create_conn, fork, get_conn, cursor, etc.
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Basically, you can transfer only information from create_conn(), nothing else.
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'''
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import multiprocessing
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from config import Config
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from pgtoolkit import pgmanager
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def sub1(id):
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    with pgm.cursor() as curs:
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        print('[%d] pgconn: %d' % (multiprocessing.current_process().pid, curs.connection.get_backend_pid()))
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        print('[%d] update' % multiprocessing.current_process().pid)
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        curs.execute('''UPDATE test SET name = 'multi-1' WHERE id=%s''', [id])
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        print('[%d] commit' % multiprocessing.current_process().pid)
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        curs.connection.commit()
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def sub2(id):
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    pgm = pgmanager.get_instance()
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    with pgm.cursor() as curs:
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        print('[%d] pgconn: %d' % (multiprocessing.current_process().pid, curs.connection.get_backend_pid()))
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        print('[%d] update' % multiprocessing.current_process().pid)
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        curs.execute('''UPDATE test SET name = 'multi-2' WHERE id=%s''', [id])
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        print('[%d] commit' % multiprocessing.current_process().pid)
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        curs.connection.commit()
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cfg = Config('tests.conf')
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pgm = pgmanager.get_instance()
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pgm.create_conn(**cfg)
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with pgm.cursor() as curs:
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    print('[%d] pgconn: %d' % (multiprocessing.current_process().pid, curs.connection.get_backend_pid()))    
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    print('[%d] insert' % multiprocessing.current_process().pid)
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    curs.execute('''INSERT INTO test (name) VALUES ('multi') RETURNING id''')
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    id = curs.fetchone()[0]
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    curs.connection.commit()
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    print('[%d] update' % multiprocessing.current_process().pid)
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    curs.execute('''UPDATE test SET name = 'multi-main' WHERE id=%s''', [id])
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    # not committed
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p1 = multiprocessing.Process(target=sub1, args=[id])
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p1.start()
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p2 = multiprocessing.Process(target=sub2, args=[id])
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p2.start()
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with pgm.cursor() as curs:
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    print('[%d] commit' % multiprocessing.current_process().pid)
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    curs.connection.commit()
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print('join')
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p1.join()
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p2.join()
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print('done')