Concurrent Requests - Why's the database connection crashing the Node process - node.js

Question

Overview
I'm developing an MVC application with NodeJS. When the application loads for the first time, the database object (using a pool) is created.
var pool = mysql.createPool({connectionLimit: 150, host: __host,
user: __user, password: __password,
database: __database})
module.exports = pool
When a request is received, a Controller object is created, which creates a Model to perform actions. The model gets a connection from the pool, performs the action, and releases the connection back to the pool.
//router snippet
router.get('/post_data', function(req, res){
router.setRequestAndResponse(req, res)
var post_data = new Post_Data()
post_data.processDataFromGet(router)
})
//controller code snippet
Post_Data_Controller.prototype.processDataFromGet = function(router){
var controller_obj = this
var data_array = {}
var req = router.req, res = router.res
//retrieving data from request and passing to the data_array
controller_obj.model.create(data_array, function(result){
var xml = xmlbuilder.create("response")
if (result.code == "error"){
xml.e("code", "error")
xml.e("message", result.error_message)
}else if (result.code == "success"){
xml.e("code", "success")
}
controller_obj.sendResponse(router.res, xml, "xml")
})
}
Post_Data_Controller.prototype.sendResponse = function(res, response, type){
if (type == "json"){
res.set({"Content-Type": "application/json", "Content-Length": JSON.stringify(response).length})
res.send(response)
}else{ /* Default type is XML */
res.set({"Content-Type": "application/xml", "Content-Length": response.end({pretty: true}).length})
res.send(response.end({pretty: true}))
}
}
//Model snippet
Post_Data.prototype.create = function(data_array, callback){
/* data validation */
var fail = false, error_data = {}
if (fail) {callback({code: "fail", cause: error_data}); return;}
//the next 2 lines do not throw an error when uncommented
//callback({code: "fail", cause: "how's it going"});
//return;
__db_pool.getConnection(function(err, db_conn){
// the next two lines throw an error for two or more requests coming in at the same time
callback({code: "fail", cause: "how's it going"});
return;
if (err) { callback({code: "error", error_message: err}); return;}
callback({code: "fail", cause: "how's it going"});
return;
db_conn.query("sql command", [data_array],
function(err, result){
if (err){ callback({code: "error", error_message: err}); return;}
if (result && result.length > 0){ //affiliate and listing exist
data_array.listing_id = result[0].listings_id
var data = [data_to_insert]
db_conn.query("sql command here", data,
function(err, result){
db_conn.release()
if (err){ callback({code: "error", error_message: err}); return;}
if (result && result.affectedRows > 0) {
callback({code: "success", data: {data_to_be_returned}})
}else {callback({code: "error", error_message:"Error inserting data"}); return}
})
}else{
callback({code: "fail", cause: "error to send back"})}
})
})
}
Problem
These requests are web service requests. If I send one GET request, no error happens; however, when I send two or more concurrent requests, I receive this error:
/project_path/node_modules/mysql/lib/protocol/Parser.js:82
throw err;
^
Error: Can't set headers after they are sent.
at ServerResponse.OutgoingMessage.setHeader (http.js:689:11)
at ServerResponse.res.set.res.header (/project_path/node_modules/express/lib/response.js:549:10)
I traced the culprit to the specific line in the Model code pasted above. It seems that for some reason, once the model obtains a connection from the pool for the second request, it somehow interferes with the first request. Both requests still insert the proper data into the database; however, the second and subsequent requests can't send a response without throwing an error anymore.
I have performed the requests with GET, POST, and PUT content-types; only GET throws the error. All the other content-types don't throw any error, even with over one thousand concurrent requests.
Here's the web service code for the GET requests; it's the same for the other content-types except for the content-type changes and the data being put in the body.
for(var i=0; i less than 5; i++){
sendAsGet()
i++
}
function sendAsGet(){
try{
var data = "?data_to_be_sent"
var uri =url.parse("http://localhost:4000/post_data")
var options = {hostname: uri.hostname, port: uri.port, method: "GET",
path: uri.path + data, agent: false}
request = (uri.protocol == "https")? https : http
var req = request.request(options, function(res){
var result = ""
console.log("STATUS: " + res.statusCode)
console.log("HEADERS: " + JSON.stringify(res.headers))
res.setEncoding("utf8")
res.setTimeout(50, null)
res.on("data", function(chunk){
result += chunk
})
res.on("end", function(){
console.log(result)
})
})
req.end()
}catch(err){
console.log(err.message)
}
}
I would like to know 2 things:
Why is getting the database connection causing this problem?
Why does it happen only on GET requests and not on POST and PUT?
Google and previous SO questions haven't been able to help so far.
Thanks.

Answer 1

The reason you are seeing the error is because you're placing request/response instances on the router itself. Don't do that. The router object is a "static" object, it's not a per-request thing. So currently this is what's happening (in order):
Request #1 comes in and sets req/res on router and starts the asynchronous model.create().
Meanwhile, request #2 comes in and overwrites req/res on router and starts its own asynchronous model.create().
Request #1's model.create() callback is called, sending the response to request #2's socket instead.
Request #2's model.create() callbacks is called, where it attempts to send a response to the same res that was just responded to just a moment ago. Trying to write headers to a response that has already been sent then results in the error you are seeing.