Mysql max connections and event scheduler

Question

Is there a limit to how many "instance" of a scheduled event MySQL can run? I have set this up:

CREATE TABLE IF NOT EXISTS t (c1 INT UNSIGNED NOT NULL) ENGINE=INNODB;

SELECT * FROM t;

DELIMITER $$

DROP EVENT IF EXISTS e_second;
CREATE EVENT e_second ON SCHEDULE EVERY 1 SECOND
DO BEGIN
    INSERT INTO t (c1) VALUES (UNIX_TIMESTAMP());
    SELECT SLEEP(1000);
    INSERT INTO t (c1) VALUES (UNIX_TIMESTAMP());
END$$

DELIMITER ;

SELECT * FROM t;

SHOW PROCESSLIST;

SHOW VARIABLES LIKE '%connect%';

This creates a dumb event started every second, that runs for 1000 seconds. In my query log, I indeed have:

2020-02-18T15:47:00.289050Z      1016 Query     INSERT INTO t (c1) VALUES (UNIX_TIMESTAMP())
2020-02-18T15:47:00.430801Z      1216 Query     INSERT INTO t (c1) VALUES (UNIX_TIMESTAMP())
2020-02-18T15:47:00.430950Z      1216 Query     SELECT SLEEP(200)
2020-02-18T15:47:01.290518Z      1017 Query     INSERT INTO t (c1) VALUES (UNIX_TIMESTAMP())
2020-02-18T15:47:01.430943Z      1217 Query     INSERT INTO t (c1) VALUES (UNIX_TIMESTAMP())
...

So the event is run every second, and hangs (on purpose). Now, the processlist shows (after a while):

"559"   "root-heidisql" "127.0.0.1" "test_sql"  "Connect"   "702"   "User sleep"    "SELECT SLEEP(1000)"
"560"   "root-heidisql" "127.0.0.1" "test_sql"  "Connect"   "701"   "User sleep"    "SELECT SLEEP(1000)"
"561"   "root-heidisql" "127.0.0.1" "test_sql"  "Connect"   "700"   "User sleep"    "SELECT SLEEP(1000)"
"562"   "root-heidisql" "127.0.0.1" "test_sql"  "Connect"   "699"   "User sleep"    "SELECT SLEEP(1000)"
"563"   "root-heidisql" "127.0.0.1" "test_sql"  "Connect"   "698"   "User sleep"    "SELECT SLEEP(1000)"
...

So I have about 528 rows in this processlist now. But, my connections limit is 151 as shown by showing variables:

"max_connections"   "151"

So clearly, the running events are not part of the connections limit. Then, what is the limit? Is there one?

It feels to me that at some point, some mysql queue/list/whatever will be full of the pending events... Is this case documented somewhere?

Answer 1

the running events are not part of the connections limit. Then, what is the limit? Is there one?

Connection limit means outer connections, event scheduler is not counted. But there is a lot of another limited resources - limited by some settings or by some physical conditions. And it is too problematic to find what resource limits the server.

there is no direct limitation of the number of currently running events?

Yes, such limitation not exists. MySQL treates an event as a trigger which fires on a timer event - and there is no limitation on the number of triggers in the system.

In general the event procedure must check does its previous instance have already finished its work... by using some flag field/value in a service table, for example, or by some another mark.