Node.js and MySQL “Too many connections” error

Question

I'm using Node.js to run a web-server for my web application. I'm also using the node-mysql module to interface with a MySQL server for all my persistent database needs.

Whenever there is a critical error within my Node.js application that crashes my app's process I get an email sent to me. So, I keep getting this email with an error saying "Too many connections". Here's an example of the error:

Error: Too many connections
    at Function.Client._packetToUserObject (/apps/x/node_modules/mysql/lib/client.js:394:11)
    at Client._handlePacket (/apps/x/node_modules/mysql/lib/client.js:307:43)
    at Parser.EventEmitter.emit (events.js:96:17)
    at Parser.write.emitPacket (/apps/x/node_modules/mysql/lib/parser.js:71:14)
    at Parser.write (/apps/x/node_modules/mysql/lib/parser.js:576:7)
    at Socket.EventEmitter.emit (events.js:96:17)
    at TCP.onread (net.js:396:14)

As you can see all it tells me is that the error is coming from the mysql module, but it doesn't tell me where in my application code the issue is occurring.

My application opens a db connection anytime I need to run one or more queries. I immediately close the connection after all my queries and data has been collected. So, I don't understand how I could be exceeding the 151 max_connections limit.

Unless there is a place in my code where I forgot to call db.end() to close the connection, I don't see how my app would leak like this. Even if there was such a mistake, I wouldn't get these emails sent by the dozens. Yesterday, I received almost 100 emails with roughly the same error. How could this be happening? If my application had leaked and allocated connections over time, as soon as the first error occurred the app process would crash and all connections would be lost, preventing the app to crash again. Since I received ~100 emails, this means the app crashed ~100 times, and all within a short period of time. This could only mean that somewhere in my application a lot of connections where established in a short period of time, right?

How could I avoid this problem? This is very discouraging. All help is highly appreciated. Thanks

Answer 1

MySQL has a default MAX_CONNECTIONS = '100' not 151 unless you changed it. Also, in truth you have MAX_CONNECTIONS + 1. The plus 1 allows a root user to logon even after you have maxed out the conenctions in order to figure out what is actually being used. When your connections are maxed out try logging on as root and running the following command from MySQL.

mysql> SHOW FULL PROCESSLIST

Post the output of this command above. Once you actually know what is consuming your resources you can go about fixing it.It could easily be your code that is leaving open connections.

You should take a look at the follwoing documentation: Show Processlist

Answer 2

+1 for question. Investigations showed us that node-mysql opens the connections and doesn't close them. Because of that at one moment be reach the limit of max connections. The question is why node-mysql doesn't close the connections?