Mysql Events Execution cycle - mysql

Question

I created an event in mysql to gather some data from different tables which will repeat itself in every 5 minutes. Let's say the event may take more than 5 minutes to complete in some scenario(maybe the db is running slow or needs a restart). Many other events gets fired simultaneously so to handle this I read locks can be used as per mysql manual.
If a repeating event does not terminate within its scheduling interval, the result may be multiple instances of the event executing simultaneously. If this is undesirable, you should institute a mechanism to prevent simultaneous instances. For example, you could use the GET_LOCK() function, or row or table locking.
But simply putting a lock didn't resolved my issue as the events were still getting executed in a queue and unpredicted data getting dumped, what I wanted was simply if the lock is there don't do anything and wait.
In locks I read if one named lock is assigned to a session another same name lock can be used until earlier lock get released.
if GET_LOCK('ev_test',-1) is not TRUE then
SIGNAL SQLSTATE '45000' set MESSAGE_TEXT = 'failed to obtain lock; not continuing; ';
end if;
some_event_body
RELEASE_LOCK('ev_test');
so I used this statement in mysql event body. and later releasing this lock manually on completion of event
My question is what happens when event some_event_body triggers some other exception like if there is select query and some columns were removed used by event body?
will the lock gets released automatically? will the lock be there always?
mysql manual says locks stays there until the session terminates. But I don't know if event lives inside a session or every event creates a new session?
Externally without above code simply using GET_LOCK I encountered this kind of situation.
+------+-----------------+-----------+-------------+---------+------+-----------------------------+-----------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+------+-----------------+-----------+-------------+---------+------+-----------------------------+-----------------------------+
| 5 | event_scheduler | localhost | NULL | Daemon | 30 | Waiting for next activation | NULL |
| 8 | root | localhost | logi_test_2 | Query | 0 | init | show processlist |
| 1330 | root | localhost | logi_test_2 | Connect | 2 | User sleep | SELECT SLEEP(30) |
| 1331 | root | localhost | logi_test_2 | Connect | 4974 | User lock | SELECT GET_LOCK('test', -1) |
| 1332 | root | localhost | logi_test_2 | Connect | 4969 | User lock | SELECT GET_LOCK('test', -1) |
| 1333 | root | localhost | logi_test_2 | Connect | 4964 | User lock | SELECT GET_LOCK('test', -1) |
| 1334 | root | localhost | logi_test_2 | Connect | 4959 | User lock | SELECT GET_LOCK('test', -1) |
| 1335 | root | localhost | logi_test_2 | Connect | 4953 | User lock | SELECT GET_LOCK('test', -1) |
| 1338 | root | localhost | logi_test_2 | Connect | 4949 | User lock | SELECT GET_LOCK('test', -1) |
| 1339 | root | localhost | logi_test_2 | Connect | 4944 | User lock | SELECT GET_LOCK('test', -1) |
| 1340 | root | localhost | logi_test_2 | Connect | 4939 | User lock | SELECT GET_LOCK('test', -1) |
| 1341 | root | localhost | logi_test_2 | Connect | 4934 | User lock | SELECT GET_LOCK('test', -1) |
| 1342 | root | localhost | logi_test_2 | Connect | 4929 | User lock | SELECT GET_LOCK('test', -1) |
| 1343 | root | localhost | logi_test_2 | Connect | 4924 | User lock | SELECT GET_LOCK('test', -1) |
| 1344 | root | localhost | logi_test_2 | Connect | 4919 | User lock | SELECT GET_LOCK('test', -1) |
| 1345 | root | localhost | logi_test_2 | Connect | 4914 | User lock | SELECT GET_LOCK('test', -1) |
| 1346 | root | localhost | logi_test_2 | Connect | 4909 | User lock | SELECT GET_LOCK('test', -1) |
| 1347 | root | localhost | logi_test_2 | Connect | 4904 | User lock | SELECT GET_LOCK('test', -1) |
| 1348 | root | localhost | logi_test_2 | Connect | 4899 | User lock | SELECT GET_LOCK('test', -1) |
| 1349 | root | localhost | logi_test_2 | Connect | 4894 | User lock | SELECT GET_LOCK('test', -1) |
| 1352 | root | localhost | logi_test_2 | Connect | 4889 | User lock | SELECT GET_LOCK('test', -1) |
| 1353 | root | localhost | logi_test_2 | Connect | 4884 | User lock | SELECT GET_LOCK('test', -1) |
why locks are getting duplicated here when only one named lock is allowed regardless of session?
I tried finding results on stackoverflow and reading mysql manual to but couldn't find anything.

Answer 1

This is a classic problem with cron, EVENT, etc.
I like to recommend this solution:
Instead of repeatedly firing off a potentially slow process, have a single process that loops. It would do the task, then repeat.
Embellishments
Add a "sleep" between iterations.
Add a calculated sleep to pause 'the rest of 5 minutes'.
Do something to observe that the system is busy and sleep longer.
Add a cron/EVENT as a "keepalive". This would restart the looping task if it dies. This might also be the way to get initially fired up after any type of crash or graceful outage.
I would also look at the queries -- 5 minutes is a looooong time for an SQL task.