use multiprocessing to query in same mysqldb connect , block? - mysql

Question

abort MySQLdb.
I know many process not use same connect, Because this will be a problem .
BUT, run the under code , mysql request is block , then many process start to query sql at the same time , the sql is "select sleep(10)" , They are one by one.
I not found code abort lock/mutux in MySQLdb/mysql.c , Why there is no problem ? I think there will be problems with same connection fd in general . But pass my test, only block io, problems not arise . Where is the lock ？
import time
import multiprocessing
import MySQLdb
db = MySQLdb.connect("127.0.0.1","root","123123","rui" )
def func(*args):
while 1:
cursor = db.cursor()
cursor.execute("select sleep(10)")
data = cursor.fetchall()
print len(data)
cursor.close()
print time.time()
time.sleep(0.1)
if __name__ == "__main__":
task = []
for i in range(20):
p = multiprocessing.Process(target = func, args = (i,))
p.start()
task.append(p)
for i in task:
i.join()
result log, we found each request interval is ten seconds.
1
1464325514.82
1
1464325524.83
1
1464325534.83
1
1464325544.83
1
1464325554.83
1
1464325564.83
1
1464325574.83
1
1464325584.83
1
1464325594.83
1
1464325604.83
1
1464325614.83
1
1464325624.83
tcpdump log:
we found each request interval is ten seconds two.
13:07:04.827304 IP localhost.44281 > localhost.mysql: Flags [.], ack 525510411, win 513, options [nop,nop,TS val 2590846552 ecr 2590846552], length 0
0x0000: 4508 0034 23ad 4000 4006 190d 7f00 0001 E..4#.#.#.......
0x0010: 7f00 0001 acf9 0cea fc09 7cf9 1f52 a70b ..........|..R..
0x0020: 8010 0201 ebe9 0000 0101 080a 9a6d 2e58 .............m.X
0x0030: 9a6d 2e58 .m.X
13:07:04.928106 IP localhost.44281 > localhost.mysql: Flags [P.], seq 0:21, ack 1, win 513, options [nop,nop,TS val 2590846653 ecr 2590846552], length 21
0x0000: 4508 0049 23ae 4000 4006 18f7 7f00 0001 E..I#.#.#.......
0x0010: 7f00 0001 acf9 0cea fc09 7cf9 1f52 a70b ..........|..R..
0x0020: 8018 0201 fe3d 0000 0101 080a 9a6d 2ebd .....=.......m..
0x0030: 9a6d 2e58 1100 0000 0373 656c 6563 7420 .m.X.....select.
0x0040: 736c 6565 7028 3130 29 sleep(10)
13:07:14.827526 IP localhost.44281 > localhost.mysql: Flags [.], ack 65, win 513, options [nop,nop,TS val 2590856553 ecr 2590856552], length 0
0x0000: 4508 0034 23af 4000 4006 190b 7f00 0001 E..4#.#.#.......
0x0010: 7f00 0001 acf9 0cea fc09 7d0e 1f52 a74b ..........}..R.K
0x0020: 8010 0201 9d73 0000 0101 080a 9a6d 5569 .....s.......mUi
0x0030: 9a6d 5568 .mUh
13:07:14.927960 IP localhost.44281 > localhost.mysql: Flags [P.], seq 21:42, ack 65, win 513, options [nop,nop,TS val 2590856653 ecr 2590856552], length 21
0x0000: 4508 0049 23b0 4000 4006 18f5 7f00 0001 E..I#.#.#.......
0x0010: 7f00 0001 acf9 0cea fc09 7d0e 1f52 a74b ..........}..R.K
0x0020: 8018 0201 fe3d 0000 0101 080a 9a6d 55cd .....=.......mU.
0x0030: 9a6d 5568 1100 0000 0373 656c 6563 7420 .mUh.....select.
0x0040: 736c 6565 7028 3130 29 sleep(10)
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Answer 1

It works contingency.
MySQL is request-rensponse protocol.
When two process sends query, it isn't mixed unless the query is large.
MySQL server (1) receive one query, (2) send response of (1), (3) receive next query, (4) send response of (3).
When first response was send from MySQL server, one of two processes receives it. Since response is small enough, it is received by atomic.
And next response is received by another process.
Try sending "SELECT 1+2" from one process and "SELECT 1+3" from another process. "1+2" may be 4 by chance and "SELECT 1+3" may be 3 by chance.