node.js - Node JS Mysql PROTOCOL ENQUEUE AFTER FATAL ERROR

Question

What does the error mean?

{ [Error: Cannot enqueue Query after fatal error.] code: 'PROTOCOL_ENQUEUE_AFTER_FATAL_ERROR', fatal: false }

This code works in my test file:

function handleDisconnect() {
    objConn = mysql.createConnection(db_config);    // Recreate the connection, since
                                                    // the old one cannot be reused.
    objConn.connect(function(err) {                 // The server is either down
        if(err) {                                   // or restarting (takes a while sometimes).
            console.log('error when connecting to db:', err.code);
            setTimeout(handleDisconnect, 2000);     // We introduce a delay before attempting to reconnect,
        }else{
            console.log('Connected to db!');
        }                                           // to avoid a hot loop, and to allow our node script to
    });                                             // process asynchronous requests in the meantime.
                                                    // If you're also serving http, display a 503 error.
    objConn.on('error', function(err) {
        if(err.code === 'PROTOCOL_CONNECTION_LOST') { // Connection to the MySQL server is usually
            handleDisconnect();                       // lost due to either server restart, or a
        }else{
            throw err;
        }
    });
}

handleDisconnect();
megaLoop();

function megaLoop(){
    objConn.query('SELECT u.`email` FROM `users` as u', function(err, rows) {
        console.log(err);
        console.log(rows);
    });
    setTimeout(megaLoop, 100); 
}

But when I use the function in my Express App I get the error:

{ [Error: Cannot enqueue Query after fatal error.] code: 'PROTOCOL_ENQUEUE_AFTER_FATAL_ERROR', fatal: false }

Why does it work in my test and not my app?

Answer 1

I had similar issue connecting with MySQL. I solved this issue using connection pool. The concept is to get a connection from a connection pool only when required and release it after use. That way the connection will not be in unstable state. You can get implementation details here.