异步如何在 Express 中工作？

Question

I found the following on the ExpressJS guide :

var mysql      = require('mysql');
var connection = mysql.createConnection({
  host     : 'localhost',
  user     : 'dbuser',
  password : 's3kreee7'
});

connection.connect();

connection.query('SELECT 1 + 1 AS solution', function(err, rows, fields) {
  if (err) throw err;
  console.log('The solution is: ', rows[0].solution);
});

connection.end();

Isn't this supposed to be bad practice? The way I see it, it is possible for the connection to end before the query can be executed. Wouldn't that give an error?

Answer 1

As stated here :

• Every method you invoke on a connection is queued and executed in sequence.

• Closing the connection is done using end() which makes sure all remaining queries are executed before sending a quit packet to the mysql server.

So even though the call to the end() method can be made before the query has completed, it won't actually be executed until the query has finished executing.

This has to do more with the mysql package than NodeJS itself.

Answer 2

Your question How does async work in Express? and Isn't this supposed to be bad practice? can be answered in many ways, but for clarity I would like to explain that It depends !!!!

It generally is very bad practice, assuming you don't know the actual implementation.

If the the implementation is really simple, where it does exactly what you ask -- ie closes or ends the connection when end is executed then it could lead to rather ugly race conditions where it may or may not work depending on the load of the machines.

However, a clever implementation that does reference counting -- that is the end does not actually close the connection but just sets a flag to say -- " when last callback is done then close " -- then it may work.

If the mysql connector it implemented using reference counting then this may well work fine -- but that is not the same as saying that it is good practice for everything you find as a plugin.