MySQL ERROR 1040: Too many connections

As the title , This chapter mainly talks about when the server appears ERROR 1040: Too many connections Some experience in dealing with mistakes .

max_connections see

##  See the maximum number of connections 
SHOW VARIABLES LIKE "max_connections";
+-----------------+-------+
| Variable_name   | Value |
+-----------------+-------+
| max_connections | 512   |
+-----------------+-------+

##  View the maximum number of connections used 
SHOW VARIABLES LIKE 'Max_used_connections';
show global status like 'Max_used_connections';
+----------------------+-------+
| Variable_name        | Value |
+----------------------+-------+
| Max_used_connections | 499   |
+----------------------+-------+

Treatment scheme
There are generally two solutions to this problem , The solution is very easy , We just need to add max_connections The number of connections is enough .

Add to the current session mysql maximum connection

##  modify 
max_connections = 1000

above mysql The connection value is temporarily increased to 1000, But only for the current session . Once we restart mysql Service or restart the system , The value will be reset to the default value .

Permanently increase mysql maximum connection
In order to permanently increase mysql The number of connections , We need editors mysql The configuration file , namely /etc/my.cnf.

sudo vim /etc/my.cnf

How much expansion is appropriate ？
Max_connextions It's not the bigger the better , So how to configure ？
Mode one
For improving MySQL Concurrent , A lot depends on memory , The official provided a report on innodb Of memory Calculation method ：

innodb_buffer_pool_size
+ key_buffer_size
+ max_connections * (sort_buffer_size + read_buffer_size + binlog_cache_size)
+ max_connections * 2MB

Mode two
Installation scale expansion ：

max_used_connections / max_connections * 100% = [85, 90]%

Maximum number of connections used / maximum connection Reached 80%～90% Section , Optimize or expand the proposed capacity .

Expand
Here are also several common effects MySQL Performance conditions ：

Threads

SHOW STATUS LIKE  'Threads%';
+-------------------+--------+
| Variable_name     | Value  |
+-------------------+--------+
| Threads_cached    | 1      | 
| Threads_connected | 217    |
| Threads_created   | 29     |
| Threads_running   | 88     |
+-------------------+--------+

SHOW VARIABLES LIKE 'thread_cache_size';
+-------------------+-------+
| Variable_name     | Value |
+-------------------+-------+
| thread_cache_size | 10     |
+-------------------+-------+

• Threads_cached The number of threads in the cache
• Threads_connected The current number of open connections
• Threads_created Number of threads created to handle connections .
• Threads_running Number of threads not sleeping

If Threads_created Big , You may have to add thread_cache_size value . The cache miss ratio can be calculated as Threads_created / Connections
Check the table lock

SHOW GLOBAL STATUS LIKE 'table_locks%';
+-----------------------+-------+
| Variable_name         | Value |
+-----------------------+-------+
| Table_locks_immediate | 90    |
| Table_locks_waited    | 0     |
+-----------------------+-------+

• Table_locks_immediate The number of times a table lock request was immediately obtained
• Table_locks_waited The number of times a request for a table lock could not be obtained immediately , Need to wait . If this value is too high, there may be a performance problem , And affect the release of connections

The slow query

show variables like '%slow%';

+---------------------------+----------------------------------------------+
| Variable_name             | Value                                        |
+---------------------------+----------------------------------------------+
| slow_launch_time          | 2                                            |
| slow_query_log            | On                                           |
+---------------------------+----------------------------------------------+

Thread details

##  View the details of each thread 
SHOW PROCESSLIST;
+--------+----------+------------------+--------------+---------+-------+-------------+------------------+
| Id     | User     | Host             | db           | Command | Time  | State       | Info             |
+--------+----------+------------------+--------------+---------+-------+-------------+------------------+
|      3 | xxxadmin | localhost        | NULL         | Sleep   |     1 | cleaning up | NULL             |
|      4 | xxxadmin | localhost        | NULL         | Sleep   |     0 | cleaning up | NULL             |
|      5 | xxxadmin | localhost        | NULL         | Sleep   |     6 | cleaning up | NULL             |
+--------+----------+------------------+--------------+---------+-------+-------------+------------------+

summary
Of course , The above is just a general solution , Either way , Both need to be expanded in combination with actual business scenarios .

in addition , For production environment , Set the appropriate alarm threshold , It's also necessary .

Last , In programming , Because of the use of MySQL Statement calls the database to execute SQL, Will assign a thread operation MySQL, So at the end of the call , Need to recycle connection , Avoid leakage .

Reference resources

1. max_connections Max_used_connections
2. database MySQL- Optimize configuration parameters