How I prevent deadlock occurrence in my application?

I am developing an LMS application in PHP framework(Codeigniter 2.1.0). I am using MySQL database.
All the tables in the database have innodb engine. I also created indexes on each tables.
Now I am doing load testing using Jmeter version 2.9 locally for 200 users concurrently. During the load testing, in a specific page action I got Deadlock Found error. I changed my original query to the new one but again same error is occurring.

I have written save_interactions function which takes four parameters interaction array,module_id,course_id,user_id & is been called so many times by the AJAX script.
The following script inserts the record if the specific interaction_id is not present in that table otherwise the update query will get fire.

``
public function save_interactions($interaction_array,$modid,$cid,$uid)
{
foreach($interaction_array as $key=>$interact_value)
{
$select_query = $this->db->query("SELECT COUNT(*) AS total FROM scorm_interactions WHERE mod_id='".$modid."' AND course_id='".$cid."' AND user_id='".$uid."' AND interaction_id`='".$interact_value[0]."'");
$fetchRow = $select_query->row_array();

if($fetchRow['total']==1)
{
$update_data = array(
"interaction_type"=>$interact_value[1],
"time"=>$interact_value[2],
"weighting"=>$interact_value[3],
"correct_response"=>$interact_value[4],
"learner_response"=>$interact_value[5],
"result"=>$interact_value[6],
"latency"=>$interact_value[7],
"objectives"=>$interact_value[8],
"description"=>$interact_value[9]
);
$this->db->where('mod_id', $modid);
$this->db->where('course_id', $cid);
$this->db->where('user_id', $uid);
$this->db->where('interaction_id', $interact_value[0]);

DISCLAIMER : Not a Codeigniter Person, Just a MySQL DBA

Looking at the flow of control in the PHP. It says to me:

• Check the following mod_id,course_id,user_id,interaction_id

• If not found, INSERT the new row

• If found, UPDATE all the other columns

There are three(3) mechanisms provided for INSERT and UPDATE scenarios

MECHANISM #1 : REPLACE INTO

REPLACE INTO `scorm_interactions` SET
    `mod_id` =  '4',
    `course_id` =  '5',
    `user_id` =  '185',
    `interaction_id` =  'Question2_1',
    `interaction_type` = 'choice',
    `time` = '10:45:31',
    `weighting` = '1',
    `correct_response` = 'Knees*',
    `learner_response` = 'Knees*',
    `result` = 'correct',
    `latency` = '0000:00:02.11',
    `objectives` = 'Question2_1',
    `description` = ''
;

or

REPLACE INTO `scorm_interactions`
(`mod_id`,`course_id`,`user_id`,`interaction_id`,
`interaction_type`,`time`,`weighting`,`correct_response`,
`learner_response`,`result`,`latency`,`objectives`,`description`) VALUES
('4','5','185','Question2_1','choice','10:45:31','1','Knees*',
'Knees*','correct','0000:00:02.11','Question2_1','');

This mechanically performs DELETE and INSERT on mod_id, course_id , user_id, interaction_id

MECHANISM #2 : INSERT ... ON DUPLICATE KEY

INSERT INTO `scorm_interactions`
(`mod_id`,`course_id`,`user_id`,`interaction_id`,
`interaction_type`,`time`,`weighting`,`correct_response`,
`learner_response`,`result`,`latency`,`objectives`,`description`) VALUES
('4','5','185','Question2_1',
'choice','10:45:31','1','Knees*',
'Knees*','correct','0000:00:02.11','Question2_1','')
ON DUPLICATE KEY UPDATE
    `interaction_type` = 'choice',
    `time` = '10:45:31',
    `weighting` = '1',
    `correct_response` = 'Knees*',
    `learner_response` = 'Knees*',
    `result` = 'correct',
    `latency` = '0000:00:02.11',
    `objectives` = 'Question2_1',
    `description` = ''
;

MECHANISM #3 : SELECT ... FOR UPDATE

Did you know you can perform an exclusive lock on rows you intend to update with precision?

According the MySQL Documentation, you can run a SELECT query on the very row and issue a row lock along the way. For example:

SELECT counter_field FROM child_codes FOR UPDATE;
UPDATE child_codes SET counter_field = counter_field + 1;

This will lock the entire table and allow an UPDATE of the counter_field column.

In your case, simply run the two queries back-to-back

SELECT * FROM `scorm_interactions` 
WHERE `mod_id` =  '4',
AND `course_id` =  '5',
AND `user_id` =  '185',
AND `interaction_id` =  'Question2_1'
FOR UPDATE ;
UPDATE `scorm_interactions` SET
    `interaction_type` = 'choice',
    `time` = '10:45:31',
    `weighting` = '1',
    `correct_response` = 'Knees*',
    `learner_response` = 'Knees*',
    `result` = 'correct',
    `latency` = '0000:00:02.11',
    `objectives` = 'Question2_1',
    `description` = ''
WHERE `mod_id` =  '4'
AND `course_id` =  '5',
AND `user_id` =  '185',
AND `interaction_id` =  'Question2_1'
;

EPILOGUE

As a safety cushion, try increasing innodb_lock_wait_timeout if the traffic is write-heavy. See my StackOverflow post : How to debug Lock wait timeout exceeded?

Give it a Try !!!

StackExchange Database Administrators Q#36481, answer score: 5