MySQL server has gone away – but no connect error

Consider the following scenario: We have an AWS worker instance (SQS based) which opens persistent MySQL connections to our RDS.

$this->connectRegistry[ $host ][ 'connect' ] = mysqli_connect(
    "p:" . $host ,
    $this->hostCredentials[ $host ][ 'username' ] ,
    $this->hostCredentials[ $host ][ 'password' ]
);

Now sometimes – not on a regular basis – this call genertes the following warning:

PHP Warning: mysqli_connect(): MySQL server has gone away

As AWS worker instances are handling requests with a web-server, the setup basically identical to a usual instance which serves a website. We use the same class in multiple projects but in not a single website this error has even occurred once.

To get an idea where these warnings originates from we have tried to print a stack trace if a connection error occurs but – and now the awkward thing starts to come in – there is no connect error. Please take a look at the whole connect function:

private function connect ( $host )
{
    $this->connectRegistry[ $host ][ 'connect' ] = mysqli_connect(
        "p:" . $host ,
        $this->hostCredentials[ $host ][ 'username' ] ,
        $this->hostCredentials[ $host ][ 'password' ]
    );
    $error = mysqli_connect_error();
    if( $error ) {
        $this->raiseError( "Connect (" . mysqli_connect_errno() . ") " . $error );
        return false;
    }
    return true;
}

The method raiseError builds a complete error message with a stack trace and so on and then calls trigger_error. However, this method is not called if the above-mentioned warning is triggered.

Our first goal is to know in which cronjobs these warnings are triggered – maybe there are some bad performing queries.

Answer

There would not be any connection error if you get

mysqli_connect(): MySQL server has gone away

This message means that the existing connection is not usable anymore. Something must have happened and the connection was closed by MySQL server. There could be many reasons for this. The most common one is calling mysqli_close() somewhere else in your code, but as you are using persistent connections here the root cause could be something different altogether. You need to debug what is causing the connection to be dropped.

However, I would strongly recommend to stop using persistent connections altogether. It is highly unlikely that you have a valid technical reason to use them and debugging them can be very problematic.