This example shows how to write a stored procedure that will wait on multiple events and that will finish the wait when any one of those events is posted.
This example contains two scripts. Script 1 waits on more than one events. Script 2 is a stored procedure.
============================= SCRIPT 1============================= -- SCRIPT NAME: MultiWaitExamplePart1.sql -- PURPOSE: -- This code shows how to wait on more than one event. -- If you run this demonstration, you will see that a "wait" lasts only -- until one of the events is received. Thus a wait on multiple events -- is like an "OR" (rather than an "AND"); you wait until event1 OR -- event2 OR ... occurs. -- -- This demo uses 2 scripts, one of which waits for an event(s) and one -- of which posts an event. -- To run this example, you will need 2 consoles. -- 1) Run this script (MultiWaitExamplePart1.sql) in one window. After -- this script reaches the point where it is waiting for the event, then -- start Step 2. -- 2) Run the script MultiWaitExamplePart2.sql in the other window. -- This will post one of the events. -- After the event is posted, the first script will finish. -- Create the 3 different events on which we will wait. CREATE EVENT event1; CREATE EVENT event2(i INTEGER); CREATE EVENT event3(i INTEGER, c CHAR(4)); -- When an event is received, the process that is waiting on the event -- will insert a record into this table. That lets us see which events -- were received. CREATE TABLE event_records(event_name CHAR(10)); -- This procedure inserts a record into the event_records table. -- This procedure is called when an event is received. "CREATE PROCEDURE insert_a_record(event_name_param CHAR(10)) BEGIN EXEC SQL PREPARE insert_cursor INSERT INTO event_records (event_name) VALUES (?); EXEC SQL EXECUTE insert_cursor USING (event_name_param); EXEC SQL CLOSE insert_cursor; EXEC SQL DROP insert_cursor; END"; -- This procedure has a single "WAIT" command that has 3 subsections; -- each subsection waits on a different event. -- The "WAIT" is finished when ANY of the events occur, and so the -- event_records table will hold only one of the following: -- "event1", -- "event2", or -- "event3". "CREATE PROCEDURE event_wait(i1 INTEGER) RETURNS (eventresult CHAR(10)) BEGIN DECLARE i INTEGER; DECLARE c CHAR(4); -- The specific values of i and c are irrelevant in this example. i := i1; c := 'mark'; -- Set eventresult to an empty string. eventresult := ''; -- Will we exit after any of these 3 events are posted, or must -- we wait until all of them are posted? The answer is that -- we will exit after any one event is posted and received. WAIT EVENT -- When the event named "event1" is received... WHEN event1 BEGIN eventresult := 'event1'; -- Insert a record into the event_records table showing that -- this event was posted and received. EXEC SQL PREPARE call_cursor CALL insert_a_record(?); EXEC SQL EXECUTE call_cursor USING (eventresult); EXEC SQL CLOSE call_cursor; EXEC SQL DROP call_cursor; RETURN; END EVENT WHEN event2(i) BEGIN eventresult := 'event2'; EXEC SQL PREPARE call_cursor2 CALL insert_a_record(?); EXEC SQL EXECUTE call_cursor2 USING (eventresult); EXEC SQL CLOSE call_cursor2; EXEC SQL DROP call_cursor2; RETURN; END EVENT WHEN event3(i, c) BEGIN eventresult := 'event3'; EXEC SQL PREPARE call_cursor3 CALL insert_a_record(?); EXEC SQL EXECUTE call_cursor3 USING (eventresult); EXEC SQL CLOSE call_cursor3; EXEC SQL DROP call_cursor3; RETURN; END EVENT END WAIT END"; COMMIT WORK; -- Call the procedure that waits until one of the events is posted. CALL event_wait(1); -- See which event was posted. SELECT * FROM event_records;
=========================== SCRIPT 2 =================================== -- SCRIPT NAME: MultiWaitExamplePart2.sql -- PURPOSE: -- This is script 2 of 2 scripts that show how to wait for multiple -- events. See the instructions at the top of MultiWaitExamplePart1.sql. -- Create a stored procedure to post an event. "CREATE PROCEDURE post_event1 BEGIN -- Post the event. POST EVENT event1; END"; --Create a stored procedure to post the event. "CREATE PROCEDURE post_event2(param INTEGER) BEGIN -- Post the event. POST EVENT event2(param); END"; --Create a stored procedure to post the event. "CREATE PROCEDURE post_event3(param INTEGER, s CHAR(4)) BEGIN -- Post the event. POST EVENT event3(param, s); END"; COMMIT WORK; -- Notice that to finish the "wait", only one event needs to be posted. -- You may execute any one of the following 3 CALL commands to post an -- event. -- We've commented out 2 of them; you may change which one is de -- commented. CALL post_event1(); --CALL post_event2(2); --CALL post_event3(3, 'mark');