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"})}
})
})
}
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:
Google and previous SO questions haven't been able to help so far.
Thanks.
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.