Node.js MySQL ERR_HTTP_HEADERS_SENT - mysql

Question

This is the function in question:
castVote: function (req, res, ip, mysql) {
// POST return codes
const ALREADY_VOTED = '100';
const SUCCESS = '200';
const FAILURE = '300';
// Create connection to db
// Keep connection while the client is connected
var con = mysql.createConnection({
host: sqlHOST,
user: sqlUSER,
password: sqlPASS,
database: dbNAME
});
// Connect to db
con.connect(function (err) {
// MySQL error, return
if (err) {
res.send(FAILURE);
return;
}
});
// Get link voted for
var link = req.body.song_voted;
// Check if user has already voted
con.query(`SELECT * FROM VotedUsers WHERE ip = '${ip}'`, function (err, result, fields) {
if (err) {
res.send(FAILURE);
return;
}
// User hasn't voted
if (!result.length) {
// Cast vote here
// ...
// Add user's IP to voted list
con.query(`INSERT INTO VotedUsers (ip) VALUES ('${ip}')`, function (err, result) {
if (err) {
res.send(FAILURE);
return;
}
});
res.send(SUCCESS);
return;
}
// User already voted
else {
res.send(ALREADY_VOTED);
return;
}
});
}
I call the function like this, every time the user clicks a button
mysql_backend.castVote(req, res, uipv4, mysql);
(mysql is the mysql module. uipv4 is the user's ip).
Every time I click said button, This is the error output:
Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
I've got 2 questions:
What's wrong with my code?
Is there a better way of managing a mysql connection for each client (not opening a new one every time the user clicks the button)

Answer 1

1. What is wrong with your code
I can see a couple of things.
Firstly, the most obvious problem is your con.connect and con.query functions.
You have them implemented incorrectly. con.connect takes a callback that all your implementation should be in i.e. all your query logic, once you have created a connection.
Anyway the con.connect function is not required. You can remove it as con.query will create a connection for you.
Secondly, and this is causing the error that you are seeing. You need to check your application to ensure that the res.send (or equivalent) is only being invoked once per request.
So for example, this is going to throw the error you are seeing.
// User hasn't voted
if (!result.length) {
// Cast vote here
// ...
// Add user's IP to voted list
con.query(`INSERT INTO VotedUsers (ip) VALUES ('${ip}')`, function (err, result) {
if (err) {
res.send(FAILURE);
return;
}
});
res.send(SUCCESS);
return;
}
If the insert fails, res.send(SUCCESS) has already being executed, then the con.query callback is executed and the res.send(FAILURE) will be called.
This is just one example. You will need to check your code through the entire request to ensure that a response is only ever sent once. If you send more than once you will see the ERR_HTTP_HEADERS_SENT (or a similar error - depending on the function you invoke on the response object). The key takeaway is that you should only send a response once and only once! If the below is not fixing your error you need to check the rest of your application to make sure that you are not sending a response again e.g. after you call mysql_backend.castVote(req, res, uipv4, mysql);
To fix the above problem depends on the desired result.
If you want to return to the caller that the error has occurred, you must wait until the callback has returned. e.g.
if (!result.length) {
// Cast vote here
// ...
// Add user's IP to voted list
con.query(`INSERT INTO VotedUsers (ip) VALUES ('${ip}')`, function (err, result) {
if (err) {
res.send(FAILURE);
return;
}
res.send(SUCCESS);
return;
});
}
2. Is there a better way of Managing Connections
Yes - use connection pools. Connection pooling allows you to limit the number of connections that can be created, e.g. if you have 100 parallel requests and a maximum pool size of 10, you will only ever create 10 connections.